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 The Ice Doctor 27 Jun 2018

Do new cars no longer carry a spare wheel?

I don't own a car, and only hire them on the odd occasion, I was shocked to discover the Fiat I used on the continent did not come with a spare.

 TMM 27 Jun 2018
In reply to The Ice Doctor:

It all depends.

Some have a full size spare

Some have a space saver spare which is narrower than the standard wheel and limited to 55km/h

Some have a tube of sealant and a compressor

Manufacturers have been making these changes for a long time as they seek to optimise fuel efficiency, interior packaging and profitability.

Post edited at 14:18
1
Ferret 27 Jun 2018
In reply to The Ice Doctor:

Mixed bag - some come with full size spare, some with a 'spacesaver' that is smaller than normal and has reduced speed limit when fitted... and some come with nothing bar a can of emergency goop to seal small puncture and re-inflate tyre - if you are lucky!

Personally I don't trust goop so if I was buying a car from new with that as standard I'd be ticking eth option box you usually get to specify a spare wheel instead.... rental fleets probably don't bother as a fair few of their clients wouldn't try to change a wheel in any case and would simply call the emergency number no matter what.

Some cars have self sealing tyres that make punctures rare.... but you are stuffed if you have a proper blow out/broken wheel incident with either emergency goop or self sealing tyres. That said - in around 30 years driving I've had about 3 punctures proper and a fair few slow leaks/valve/tyre not sealed to rim properly issues so I may be over cautious in my distaste for the thought of not having a genuine spare. But sods law says you get a blow out, in middle of nowhere with no mobile signal so I'd still rather have it. I guess city dwellers are probably less concerned than outdoorsy types like us lot who are more likely to have issues in remote places I guess. For your average biddy doing 5k miles a year in a micra in their home town is a spare really required? They dial the AA or limp to kwik fit and wouldn't dream of changing it..... Or like my own mother, probably don't even notice when it's flat as a pancake......

 Martin W 27 Jun 2018
In reply to The Ice Doctor:

> Do new cars no longer carry a spare wheel?

> I don't own a car, and only hire them on the odd occasion, I was shocked to discover the Fiat I used on the continent did not come with a spare.

Very often they don't.  On my last two cars (ie previous and current) a spare was an optional extra.  That said, it could be retro-fitted if you forgot to add it to the factory order, or bought a used one which didn't have it.  You just had to find a helpful parts department who can order of the necessary bits for you (IIRC the cost worked out pretty much the same as having it as a factory fitted option).  The bits were required to reconfigure the underfloor area in the boot to accommodate the spare - and the spare itself, of course.

I've seen many reasons suggested as to why a spare is often not supplied as standard equipment these days, including:

  • They take up space that could otherwise be used for carrying luggage;
  • As a factory option, it doesn't have to be in the car when it's going through the regulatory emissions tests.  In marginal cases the reduced weight could mean that the car scrapes in to a lower VED bracket (under the old VED regime).  It helps manufacturers to meet their emissions reduction commitments, anyway;
  • Lots of people don't like to change a wheel themselves, preferring to rely on recovery services to get them out of trouble.  (Changing a wheel at the side of the road can be a risky business is there is heavy passing traffic.  I wouldn't enjoy doing it on the hard shoulder of a busy motorway.  There are multiple instances of people being hit and killed or injured while trying to fix their car at the side of the road.  It happened to a colleague of mine: a lorry ran at full speed in to the back of his caravan while he was on the hard shoulder checking a worrying noise which appeared to be coming from the area of the tow hitch.  He was killed instantly.)

Given that I frequently make trips to remote areas where (a) the roads, while usually not very busy, can have poor surfaces which can damage tyres, and (b) recovery services can be several hours away, I prefer to carry a spare.  Most rental companies would probably prefer you call up their recovery service rather trying to fix the car yourself, whatever the problem, so they probably don't mind if the vehicles they buy for their fleet don't come with a spare.

Removed User 27 Jun 2018
In reply to The Ice Doctor:

Expanding on the general concept of modern cars, I’ve recently converted from having an old sports car fetish to all out worship of modern car design. Two weeks ago I had a combined 90mph head on with some poor Spaniards who’d forgotten which side of the road we drive on. I didn’t think anyone survived impacts like that let alone walked away with mere bruising and neck strain. Ok, the Spaniards were quite a bit worse but they will thankfully recover. The medics and the cops both said that if we’d been in cars from even 20 years ago we’d probably all be dead.

Anyway I no longer ponder about old 911s, Sebring MGBs or TVR Vixens. Give me an 09 plate Octavia any day (or a new Qashquai, which didn’t fare so well but still, nobody died).

 LastBoyScout 27 Jun 2018
In reply to The Ice Doctor:

My last car came as standard with a space-saver spare wheel, but the previous owner had purchased a full-sized one to avoid the limitations on speed if he ever got a puncture and needed it.

My current car has a compressor and sealant, as did my wife's last car. As TMM said, it's a move to reduce weight to optimise fuel efficiency and space - not sure about profitability, as the compressor is quite expensive. Keep toying with the idea of getting a spare wheel, but it's really not a priority and the boot is only designed to take a space-saver, rather than my preferred full-size one.

Wife's new car comes with a space-saver spare, which looks pathetic compared to the normal ones - it's an SUV with 19" alloys, but that's another argument. We had cause to use it a couple of weeks ago when we got a puncture.

Fortunately, my local tyre specialist was able to repair it, but, looking inside the tyre, I was surprised to see the back of the tread coated with liquid latex, all ready to seal any minor punctures. Never seen that before. The screw that caused the puncture was bloody big and had just missed this layer, hence the loss of pressure.

 LastBoyScout 27 Jun 2018
In reply to Ferret:

Just seen your comment about self-sealing tyres. Your assessment of puncture frequency and other issues also sounds similar to mine - in once case, I actually managed to gash the sidewall and no amount of sealant was going to fix that!

My current car has been doing ~3k miles a year as a runabout for work and shops, as my wife had a fully expensed company car, so we used that all the time as the family workhorse - hence no real need for a spare in my car, as long as I can limp to either home or a garage. Now that we've opted out of the company scheme, my car is being used more and for longer trips.

You're right about fixing punctures roadside - neither my wife, my Mum or my sisters would have the faintest idea how to go about it and would phone me, Dad or the AA. There's no way my wife would be able to lift the full-sized wheel on her car into the boot, even if she could get it off - it's bloody heavy!

Post edited at 16:14
 TMM 27 Jun 2018
In reply to Removed UserStuart en Écosse:

Sounds nasty, Glad you all walked away.

A point of pedantry, it is not a combined 90mph impact. If you are both travelling at 45mph it is the equivalent of hitting a stationary object at your given speed. 

Check our Mythbusters on the subject!

 kathrync 27 Jun 2018
In reply to LastBoyScout:

> You're right about fixing punctures roadside - neither my wife, my Mum or my sisters would have the faintest idea how to go about it and would phone me, Dad or the AA. There's no way my wife would be able to lift the full-sized wheel on her car into the boot, even if she could get it off - it's bloody heavy!

I don't think this is only a problem for women -  a quick poll in my office suggests 3 of the 5 men in here wouldn't be confident to change a wheel either. 

I have changed wheels on smaller cars by myself, but on my van I don't have the physical mass to get the wheel nuts off - I can't shift them even by physically jumping up and down on an extended nut wrench, which is probably not what I want to be doing when the van is on a jack anyway.  Yes, the torques are correct...  

We have a proper spare wheel, but carry a tube of sealant and a compressor as well as that is a much easier option for me if I am out on my own.

 Jon Read 27 Jun 2018
In reply to kathrync:

> ... I can't shift them even by physically jumping up and down on an extended nut wrench, which is probably not what I want to be doing when the van is on a jack anyway. 

Having changed a wheel the other day (and only then looked up to see how I should have done it!), jumping/stomping on the wrench to loosen the nuts is best done before you jack up the vehicle. 

 

 

 kathrync 27 Jun 2018
In reply to Jon Read:

> Having changed a wheel the other day (and only then looked up to see how I should have done it!), jumping/stomping on the wrench to loosen the nuts is best done before you jack up the vehicle. 

Doesn't make much difference, I don't have the physical mass to shift them on the van whether it is jacked up or not.  With smaller cars I can generally get them off with a telescopic wrench, but I will bear this in mind if I happen to need to do this on something intermediate...thanks.

Rigid Raider 27 Jun 2018
In reply to The Ice Doctor:

It's all about technique, I used to own a Land Rover 90 with the big wheels and they were damned heavy but with the right technique and doing things in the right order the hardest bit was lifting the wheel back onto the back door studs.

 LastBoyScout 27 Jun 2018
In reply to kathrync:

Sorry - I didn't mean to imply it was just a problem for women, just going on my personal experience. My wife, sisters and Mum were all standing around watching Dad and I change the wheel a couple of weeks ago and all quite happy to admit they wouldn't have a clue about doing it themselves.

For future reference, you should loosen the nuts before jacking the vehicle and give them the final tighten once it is back on the ground. I'm sure I don't need to mention it, but jumping on the wrench can cause all sorts of damage if it comes off the nut mid-jump.

My Dad once stopped to help a girl changing a wheel in an awkward place (bit of a hairy bend) - she knew what she was doing, but was struggling because it wasn't her car and she didn't have the 3' pipe she kept in her own car to give her the leverage on the nuts.

 Hooo 27 Jun 2018
In reply to TMM:

If you're going to be pedantic...

That only applies if both vehicles have the same mass. A head-on with a significantly heavier vehicle will be worse than hitting a stationary object. 

 Rick Graham 27 Jun 2018
In reply to LastBoyScout:

Stumbling block last couples of wheel changes is getting the wheel off.

Lots of tips on line.

The only one that did the job for my vehicle was driving a few metres and jabbing hard on the brakes with slightly loosened wheel bolts before jacking up, or was it turning lock to lock a few times?

 kathrync 27 Jun 2018
In reply to LastBoyScout:

> Sorry - I didn't mean to imply it was just a problem for women...

No problem

> For future reference, you should loosen the nuts before jacking the vehicle and give them the final tighten once it is back on the ground. I'm sure I don't need to mention it, but jumping on the wrench can cause all sorts of damage if it comes off the nut mid-jump.

Yeah, I knew you were supposed to tighten them after lowering the car.  I don't know why it didn't occur to me to slacken them before raising the car although it makes a lot of sense.  In reality, on most cars I have been able to slacken them easily enough by hand so it hasn't been a problem.

> My Dad once stopped to help a girl changing a wheel in an awkward place (bit of a hairy bend) - she knew what she was doing, but was struggling because it wasn't her car and she didn't have the 3' pipe she kept in her own car to give her the leverage on the nuts.

Yeah, I carried a length of pipe around for this purpose for a while when I first got the van.  It has quite an impressive bend in it now and I still couldn't slacken the damn nuts!

I have resigned myself to sealant and compressor for simple punctures and AA for larger problems for this particular vehicle, sadly!

 

 DancingOnRock 27 Jun 2018
In reply to TMM:

I’m not sure about that. 

The energy in two same size cars both hitting at 45mph is mv^2. 2025m. The speed of a single car with the equivalent kinetic energy is 63mph.

So you’re on the right lines. It’s not double, it’s only root 2 times the speed. About 1.5 times. And only then if the car hits another car of the same mass. 

 wercat 27 Jun 2018
In reply to DancingOnRock:

However, it is quite correct to talk of a closing speed (and hence collision speed) of 90mph.

Very different from driving into a stationary vehicle or into the back of another vehicle travelling 5mph slower than you and in the same direction as you.

 TMM 27 Jun 2018
In reply to DancingOnRock:

Newton’s third law.

An equal and opposite reaction (assuming the same mass).

 

Removed User 27 Jun 2018
In reply to TMM:

Whatever. I was doing c.50, they were probably doing c.40. They got knocked back 15m, I spun off backwards quite an impressive distance. Anyway thanks for the kind thought. I, and the emergency services, still think we were all bloody lucky. All hail crumple zones. 

And I replaced it with a VW camper. You can see a long way round blind corners in those

Post edited at 19:30
1
 Dax H 27 Jun 2018
In reply to kathrync:

> I don't think this is only a problem for women -  a quick poll in my office suggests 3 of the 5 men in here wouldn't be confident to change a wheel either. 

Sorry but they are not men, some kind of metro sexual fop but not a man. 

On the spare wheel subject. I would never rely on sealing gunk. I stopped to help a guy on the road between Durness and Toung late one Saturday night, foreign guy on holiday who had a puncture that was actually a hand sized hole in the side wall. I ended up dropping him and his family in Durness to look for a hotel. 

 

11
 Mal Grey 27 Jun 2018
In reply to The Ice Doctor:

To continue the tyre ranting...

My (excellent) old Honda Accord had a tube of gloop. When I had a puncture in Lochinver, I had no idea how it worked, there were no instructions. I tried forcing it in, but could see the valve inner was in the way. However, the little tool to remove it (which I didn't know existed) wasn't there. Luckily, there was a great little garage in town who saved me a whole lot of hassle by fixing it for a tenner. I gave him £20. I bought a full sized spare second hand on return.

This experience led me to specify a full size spare on the next new car (company car) I had. I thought, great, no space saver and the associated slow speeds if you have to use it, which would be a pain on any of the long distance holiday trips to remote areas that I undertake as often as possible. I once had to use a space saver on a heavily laded car (having unloaded 200 boxes of catalogues all over the hard shoulder to get at it) and we simply couldn't get enough pressure into the tyre (with a fairly poor foot pump, to be fair) to stop it looking rather flat, so limped to a garage in the nearest town.

So I was a little miffed when the "full size" spare I ordered was, yes, the same overall diameter (tyre + wheel), but had a smaller inner wheel and higher profile tyre meaning that you're not supposed to drive at over 50mph....so I might as well have had the space saver...

I have just picked up a temporary lease car with a new job. It has a space saver. However, I have been told that company policy is that you don't change a tyre yourself, you call roadside assistance...

 

While I'm ranting about modern car changes, what is it with this whole "keyless entry" thing? For 30 years I have been in the habit of checking the doors are locked when walking away from the car, as I often have lots of stuff in it. Some of it is even worth money. Now I've no blinkin' idea if its locked or not, as I can't check it without it reopening, or me moving the keys away, putting them down, and walking back. Which looks a bit mad in the supermarket car park, even for me.

 

On safety, though, modern cars are absolutely remarkable, including how comfortable they are, thus making journeys easier and less tiring, and as the minor issues above give us oldies something to moan about, we all go away happy.

 wintertree 27 Jun 2018
In reply to Mal Grey:

> keyless entry

On our Nissan, it is keyless but you can check it is locked.  You manually lock/unlock if using a subtle  button embedded in the door and boot handles that only works in the presence of a key.  It’s badically perfect - you are in control of the locks, only the button near the key is enabled and only when you’re close enough to touch it, you hear the locks thunk, and you don’t have to touch the key, and you can try the handle to ensure the car is locked.  It also beeps if you tell to lock and it can’t/won’t.

I’d like it if my mobile or an RFID card could replace the expensive key, so I would never have to get it in the morning, have the big thing in my pocket by day, store it in the evening or risk loosing it.

 Mal Grey 27 Jun 2018
In reply to wintertree:

Cheers. This is a Ford, and doesn't seem to work quite like that. I can't even check the rear doors without holding the key at least 1.5m from the front handle. There's probably a way, but the manual ain't helped much!

 wintertree 27 Jun 2018
In reply to The Ice Doctor:

Our Leaf doesn’t have a spare or space for one.  It’s got special gunk that integrates with the compressor and is designed not to break the £££ active TPMS units.  The gunk has a short shelf life so I’ll need to replace it this winter.  

My 325d hardtop convertible doesn’t have a spare or space for one.  It barely has space for anything in the boot really.  I keep gunk and a compressor in the boot.  It used to have model standard runflats but shortly after I got the car I discovered they had - long ago - cracked both rear alloys through general pothole wear and tear, and the wheels were bodged with alloy sealant.  Couldn’t tell till the tyres came off for replacement.  Two new wheels and four non-runflats later and it was like a different car to drive.  With the runflats being so rigid, small bumps would send the back airborn enough to kick the TCS in under enthusiastic acceleration on the straight in the dry.  It turns out runflats wrecking alloys is a common BMW problem in the UK.  They’re like the automotive version of the solid tyres I had on my BMX as a kid.  

My old X-trail has a proper spare but I’ve never needed it.  

My two old cars had spares - one real and one space saver - and I had to use both. 

I wouldn’t have a problem waiting for the AA to come and change a wheel if needed.  On the rare day conditions are bad enough they’d not be with me in an hour I invariably use the X-Trail anyhow.

 

 wintertree 27 Jun 2018
In reply to Mal Grey:

> There's probably a way, but the manual ain't helped much!

I believe the standard OCD solution is to have a radio frequency shielded key pouch.  Always a good idea to prevent scum using RF repeaters to open and start your car without breaking in to your house.  

 Robert Durran 27 Jun 2018
In reply to DancingOnRock:

> The energy in two same size cars both hitting at 45mph is mv^2. 2025m. The speed of a single car with the equivalent kinetic energy is 63mph.

I think you are forgetting that the energy disipation is shared between the two vehicles.

 Robert Durran 27 Jun 2018
In reply to TMM:

> A point of pedantry, it is not a combined 90mph impact. If you are both travelling at 45mph it is the equivalent of hitting a stationary object at your given speed. 

That is a solid, immovable wall, not a stationary car. It would be equivalent to hitting a similar stationary car at 90mph.

 

 DancingOnRock 27 Jun 2018
In reply to Robert Durran:

Yep. But if one car is travelling at 90mph and one is zero. The energy is 4050m. If two cars are travelling at  45mph the energy is 2025m. 

The energy of one car travelling at 63mph and one car at rest is 2025m. 

1
 DancingOnRock 27 Jun 2018
In reply to wercat:

Yes. You’re correct. There’s still a huge amount of energy. The key is the crumple zones. Hitting another car at whatever speed is completely different to hitting a tree. 

 DancingOnRock 27 Jun 2018
In reply to Mal Grey:

Yes. We have the same policy. I tend to ignore it. They trust us to run multimillion point projects and look after and service mechanical equipment worth hundreds of thousands of pounds.

But changing a tyre is beyond our capabilities. 

Its nonsense. 

 DancingOnRock 27 Jun 2018
In reply to TMM:

Conservation of energy. 

Conservation of momentum. 

Post edited at 22:10
 Robert Durran 27 Jun 2018
In reply to DancingOnRock:

> Yep. But if one car is travelling at 90mph and one is zero. The energy is 4050m. If two cars are travelling at  45mph the energy is 2025m. 

But, assuming the cars stick together, if they are both travelling at 45mph then both stop dead and all the kinetic energy is disipated in the "crunch", but if one is travelling at 90mph and the other stationary, then both end up at 45mph (conservation of momentum) so not all the kinetic energy is disipated and a quick calculation shows that the same amount of energy per vehicle is disipated in the "crunch".

Edit: In fact it can be shown that the same amount of energy per vehicle is disipated as long as the sum of the vehicles' speeds is 90mph. This can also be explained by invoking the fact that all motion is relative.

Post edited at 22:46
 Robert Durran 27 Jun 2018
In reply to DancingOnRock:

> Yes. You’re correct. There’s still a huge amount of energy. The key is the crumple zones. Hitting another car at whatever speed is completely different to hitting a tree.

No, hitting another identical car with both going at 45mph is the same as hitting a completely solid tree at 45mph. If you don't want to consider energy, then just think of the symmetry of the collision - once your bumper hits the other car's bumper it won't move forward any further (just as when hitting the immovable tree). You still have crumple zone when hitting the tree!

 Blue Straggler 28 Jun 2018
In reply to The Ice Doctor:

I wrote a massive reply to this but it somehow never got posted. In summary:

Never change a wheel, not even a nearside one, on the hard shoulder of motorway or dual carriageway or even in some layby on a single carriageway with national speed limit. The wind draught of vehicles whizzing by, combines with the instability of your skinny feeble little scissor jack, to create a "car drops onto your arm" scenario. 

after a wheel change, get the wheel nut torque/tightness checked. It is common to overtighten. This once resulted in four sheared off wheel nuts and just one holding the wheel on, on a car of mine. 

1
 Jim Fraser 28 Jun 2018
In reply to The Ice Doctor:

The situation is rubbish. I do not think there is a new car wheel situation of any kind that I have not heard of going extremely badly wrong other than the traditional option of a full size spare wheel.

As for the reputedly appalling risk of changing a wheel at the roadside, 4min40s on the hard shoulder of the M90 is my record time, which I count as low risk compared to standing on a wet grassy bank in the rain for an hour and a half waiting for a breakdown truck.

Wobbly jack? You've got one wheel available as a prop at any stage in the procedure. 

We are being screwed over by the manufacturers who are cheating their way to fake fuel statistics and saving money on a wheel and tyre. Then we have a whole industry plus the police and the government prepared to lie through their teeth telling you that its not safe for you to be doing it anyway. Maybe we should do a thorough risk assessment for untrained people wiping your own 4r5e and have it all put out to contract. The country's full of 4r5e lickers anyway so it's only a small transfer of skills.

RANT ENDS

 

Post edited at 02:37
1
 colinakmc 28 Jun 2018
In reply to The Ice Doctor:

I’ll join the list of people who knew somebody (a work colleague in this case) who was killed instantly while changing a wheel. RIP wee Stevie.

 TMM 28 Jun 2018
In reply to tmm:

 

For the naysayers. Take a look at the pendulum test replicating these forces.

youtube.com/watch?v=-W937NM11o8&

 

Post edited at 07:48
 wintertree 28 Jun 2018
In reply to Blue Straggler:

> It is common to overtighten.

Not if you keep a torque wrench in the boot...

 Robert Durran 28 Jun 2018
In reply to The Ice Doctor:

I only discovered after buying my current car that it had no spare wheel at all (only the silly goo stuff). Really, really annoying - of course I want a spare wheel! I eventually managed to get a proper second hand one, so am now better off than with the space savers I had on my last two cars (one of which I upgraded to a proper one for a big trip to Norway). The idea of being unable to change a wheel and drive a long distance normally on it for people such as climbers who travel to remote places is crazy.

 wercat 28 Jun 2018
In reply to Robert Durran:

surely what you hit makes a huge difference?  If you hit a heavily laden articulated lorry front-on with a closing speed of 90mph you only have your car's crumple zone to dissipate the energy of the impact and the lorry will continue forwards whereas if it is another identical car it will have an identical crumple zone so there is twice as much "beneficial" dissipation possible as well as both vehicles rebounding rather than one continuing to crush the wreck of the other.

If you hit a vehicle pushing a large load of hay in front of it on a crushable cart I imagine that would have a better outcome at a closing speed of 90mph than hitting a car, even with a crumple zone.

 

Isn't this exactly why modern ropes reduce the injuries from the forces involved in falls, as well as reducing the chances of runners failing through destruction of the materials of which they are composed?

Post edited at 09:13
 wintertree 28 Jun 2018
In reply to TMM:

> For the naysayers. Take a look at the pendulum test replicating these forces.

Very good but it ignores the subtly of impulse vs force - due to different closing speeds in the two cases the impulse is applied over different times.  I think the collision forces are the same, but are applied for twice as long in one case as the other.  In terms of damage and injury mechanisms, force vs impulse is an important distinction.  Exceed the impulse rating of a crumple zone and you have problems...

 DancingOnRock 28 Jun 2018
In reply to Robert Durran:

Yes. That’s close to what I’m saying. 

Hitting an identical car both travelling at 45 is the same as one car hitting a tree at 63. It’s not the same as one car hitting at tree at 90. 

Purely looking in terms of the energy that needs to be dissipated in bringing all the bodies to rest. 

All crumple zones do is increase the time taken to come to rest and reduce the g forces that the occupants are subjected to. 

 Robert Durran 28 Jun 2018
In reply to wercat:

> Surely what you hit makes a huge difference?  If you hit a heavily laden articulated lorry front-on with a closing speed of 90mph you only have your car's crumple zone to dissipate the energy of the impact and the lorry will continue forwards.

Yes, of course. I was talking about the simplified situation of two similar cars. Obviously a big static lorry is closer to the car hitting an immovable tree situation, and a moving lorry even worse.

 Robert Durran 28 Jun 2018
In reply to DancingOnRock:

> Yes. That’s close to what I’m saying. 

No it's not - other wise I would not have posted disagreeing with you!

> Hitting an identical car both travelling at 45 is the same as one car hitting a tree at 63.

No, it's the same as hitting a tree at 45 or a static car at 90 (as I explained earlier by two methods - energy and symmetry)

> It’s not the same as one car hitting at tree at 90.

No, as I said 45mph.

> All crumple zones do is increase the time taken to come to rest.

Yes, work/time gives average force and it's force which is going to kill you.

Edit: I of course meant work/distance! The crumple zone increases the distance (and time) over which the energy is disipated.

Post edited at 10:09
 wintertree 28 Jun 2018
In reply to wintertree:

Considering both impulse and force is important, but some of what I wrote there is total crap...

 

 wintertree 28 Jun 2018
In reply to Robert Durran:

> Yes, work/time gives average force and it's force which is going to kill you.

You can survive a surprisingly high force (45 g - https://en.m.wikipedia.org/wiki/John_Stapp ).

What I think often kills people is crush and trauma injuries caused by the collapse of the car compacting their immediate environment.  For this once forces are sufficient to cause deformation yet moderated enough to be survivable (ie most crashes in modern cars) I think the total impulse is the big killer, not the force.

Its not just force on the driver reduced by the crumple zone.  It also absorbs energy that would otherwise shove the bulkhead through your knees and the steering wheel through your chest.

 

Post edited at 10:15
 DancingOnRock 28 Jun 2018
In reply to Robert Durran:

> No, as I said 45mph.

Well then you’re changing the problem, and the original statement. Which was two cars with a closing speed of 90mph is the same as a car hitting a stationary object at 90mph.

Even if you look at a moving car of mass m hitting a stationary object of mass 100m. That has to be different to a moving object of 100m hitting a stationary car of mass m. There’s different amounts of kinetic energy involved. You can’t just subtract one velocity from the other.

 

 Robert Durran 28 Jun 2018
In reply to DancingOnRock:

> Well then you’re changing the problem, and the original statement. Which was two cars with a closing speed of 90mph is the same as a car hitting a stationary object at 90mph.

No, you appear not to have read what I wrote. I said that two cars closing at 90mph (of which both at 45mph is a special case) is the same as a car hitting another stationary car at 90mph (which is just another special case). My point was that this is different to hitting an immovable wall or tree at 90mph (as I said it is the same as hitting it at 45mph). Your 63mph is wrong because you failed to take account of the fact that the kinetic energy disipated is shared between two cars when there are two cars!

> Even if you look at a moving car of mass m hitting a stationary object of mass 100m. That has to be different to a moving object of 100m hitting a stationary car of mass m. There’s different amounts of kinetic energy involved. You can’t just subtract one velocity from the other.

Thry will be the same (though I've not done the detailed momentum/KE calculation, symmetry/relative motion suggests so). In the first case the car more or less stops dead. In the second case, the lorry more or less keeps going as before and the car goes backwards at the same speed. So in each case the car has the same change in speed and therefore same acceleration and forces on it. In the first case almost all the KE is disipated, whereas in the second case, because the lorry just keeps going, only a small proportion of it is disipated, but this will be the same as the energy disipated in the first case.

I think it is obvious that a car hitting an immovable wall is the same as an "immovable" wall hitting a stationary car at the same speed!

Post edited at 10:34
 tlouth7 28 Jun 2018
In reply to DancingOnRock:

> Hitting an identical car both travelling at 45 is the same as one car hitting a tree at 63. It’s not the same as one car hitting at tree at 90. 

> Purely looking in terms of the energy that needs to be dissipated in bringing all the bodies to rest. 

Energy in collision with tree (wall would be a better analogy) = 1 car mass * 0.5 * its speed^2 which is all absorbed by the 1 car

Energy in head-on collision with car = 2 * (1 car mass * 0.5 * speed^2) shared between 2 cars

So 45 mph head-on collision with car = 45 mph collision with immovable wall. This is apparent from the symmetry of the head-on collision.

Energy in collision with stationary car (without its handbrake on, and assuming you become locked together) = energy in - energy out

= (1 car mass * 0.5 * speed^2) - (2 car mass * 0.5 * (0.5 * initial speed)^2) shared between 2 cars

(NB final speed comes from conservation of momentum, the case where you bounce off is left as an exercise for the reader)

 = 1 car mass * 0.25 * speed^2 shared between 2 cars

So because of the speed^2 term a collision with a stationary car at 90 mph is equivalent to a head-on collision at 45 mph. This is apparent from the fact that all motion is relative.

Therefore using closing speed is valid, but should be compared to a collision with a stationary car, not with an immovable wall. What you hit matters, and the closing speed, but not the absolute speeds (ignoring factors like getting pinned between two vehicles). This has the slightly counterintuitive result that hitting a stationary lorry at 50 mph is the same as a 50 mph lorry hitting you.

Edit: Crossed posts with Robert Durran, his words are better.

Post edited at 10:38
 Robert Durran 28 Jun 2018
In reply to wintertree:

Yes, I agree that average force, energy disipated etc are only ever going to be very rough indicators of survivability of a crash and I think you are right that what kills are probably sharp bits crushing or impaling you, so car design is crucial. However, given the same design, average forces and so on are probably reasonable things to consider.

 Michael Hood 28 Jun 2018
In reply to all:

Apologies if telling you things you already know...

Many years ago i used to work where they did crash testing (amongst other stuff I did some software for it). They used to crash a car into this rather large concrete block at 30mph, equivalent to a head-on at 30+30mph.

Unbelievably violent to watch.

Anyway, there are two things that kill you in a crash; intrusion of something into the passenger compartment (i.e. crushing) and decelerations incompatible with life. These are where the highest g force is very high; a spiky graph if you like.

Car design has done masses to reduce these, effective crumple zones reduce intrusion/crushing and also absorb lots of energy so that your personal deceleration is less. Also, soft materials etc. mean lower g forces if you bang your head on something.

I was told that going from 30 to zero in one foot was quite survivable, as long as the deceleration was evenly spread out. That was the tricky bit, avoiding a "spike".

Another interesting fact, apparently there have been no fatal accidents involving Volvo XC90s in the UK (more than ten years).

 

 Robert Durran 28 Jun 2018
In reply to Michael Hood:

> Another interesting fact, apparently there have been no fatal accidents involving Volvo XC90s in the UK (more than ten years).

I wonder if that says as much about Volvo drivers as it does about the cars; conservative, safe drivers are going to tend to buy conservative, safe cars.

 

 DancingOnRock 28 Jun 2018
In reply to tlouth7:

No. Because the energy in two cars travelling at 45mph is double the energy of one car travelling at 45mph. 

The energy in one car travelling at 90mph is 4x the energy of one car travelling at 45mph. 

 Toerag 28 Jun 2018
In reply to The Ice Doctor:

Use of goo - I had to help a damsel in distress whose Fiesta had compressor and goo. Following the instructions resulted in the goo going in the wheel no trouble, but not fully sealing the hole (a 2" sidewall gash) until I'd rolled the car until the gash was at the bottom of the wheel, at which point it gushed goo then sealed instantly. However, as soon as the car was driven out of the carpark down the kerb onto the road the gash opened up again. As the gash was irreparable it got driven the 3 miles home slowly, the tyre was completely ruined by that point - sidewalls flexed to death.

Top tip - after your car's had a service re-torque the wheelnuts - they're often put on with an air wrench and are too tight to get undone in an emergency.  Over-torqueing the nuts can also stretch the bolts and the nuts subsequently won't stay on because the threads are distorted enough to stop them being gripped properly.

 tlouth7 28 Jun 2018
In reply to DancingOnRock:

> No. Because the energy in two cars travelling at 45mph is double the energy of one car travelling at 45mph. 

> The energy in one car travelling at 90mph is 4x the energy of one car travelling at 45mph. 


Indeed, but in the first case each car only has to absorb half of that. Thus the total energy involved in the collision is double, but as far as each car and its passengers are concerned the effect is the same*.

I do not dispute the second case either, but again the energy that each car has to absorb comes out the same*.

*as a 45 mph car hitting an immovable wall

Removed User 28 Jun 2018
In reply to Michael Hood:

Yes the violence of the impact is quite something, I remember the whole thing quite vividly. The spike you mention is interesting. I’ve been told that a common cause of death in crashes like this is from heart ruptures; a friend who is a doctor told me of an acquaintance of hers who bled out with a ruptured aorta after a head on, no other injuries other than the usual bruising and whiplash.

Aside from force dissipation in the car, another factor is the ability of the occupant’s body to deal with the forces. One of the occupants of the other car was, without trying to be insensitive, very overweight and out of shape, which means lots of soft flesh to bruise and also means internal organs can deform a lot more. The medics said she would ultimately be ok but would have much worse bruising and a longer recovery as a result. I was told that a significant factor in me getting off lightly (I was perfectly functional afterwards albeit on an adrenaline high) was because I’m in reasonable shape so my organs were well contained and depth of bruising and flesh separation was limited.  

Post edited at 16:31
 LastBoyScout 28 Jun 2018
In reply to Toerag:

> Top tip - after your car's had a service re-torque the wheelnuts - they're often put on with an air wrench and are too tight to get undone in an emergency.  Over-torqueing the nuts can also stretch the bolts and the nuts subsequently won't stay on because the threads are distorted enough to stop them being gripped properly.

Good advice, but how many people actually own an accurate torque wrench.

These days, garages are not allowed to just use the airguns and leave it. My local preferred tyre dealers always do the final tightening with a torque wrench. The setting is much lower than you might expect - hence why I'm surpised kathrync has problems with her van and not her car.

 lithos 28 Jun 2018
In reply to DancingOnRock:

here's another approach

consider two cars on a head on collision course at 45mph, they both hit a very very thin 1 atom very stiff wall made of un-obtainium at the same time. Each car is going to experience hitting the wall at 45mph.

to the casual observer they have a head-on collision

the wall is probably quite sore.

 DancingOnRock 28 Jun 2018
In reply to tlouth7:

But they’re not hitting an immovable wall. They’re hitting another car. 

Two cars crashing head on at 45mph each is not the same as a car travelling at 90mph hitting a stationary car. 

 Robert Durran 29 Jun 2018
In reply to DancingOnRock:

> Two cars crashing head on at 45mph each is not the same as a car travelling at 90mph hitting a stationary car. 

The forces, decelerations and deformations will be the same. If you were blindfolded in one of the cars both collisions would be indistinguishable and would feel the same.

Several people have now explained it to you. What do you not accept?

 

 DancingOnRock 29 Jun 2018
In reply to Robert Durran:

There is twice as much energy in the second collision. Where does all that energy go?

1/2mv^2 + 1/2mv^2 = mv^2

1/2m(2v)^2 = 2mv^2

Post edited at 06:58
 Dr.S at work 29 Jun 2018
In reply to DancingOnRock:

Does v=v?

 

 wintertree 29 Jun 2018
In reply to DancingOnRock:

> There is twice as much energy in the second collision. Where does all that energy go?

Maybe KE isn’t as simple as you think it is?

Try thinking about the cars in the vacuum of space as seen from different reference frames (still classical mechanics).  One sees the two cars converging with equal speeds and one is co-moving with one car.

You will quickly see that kinetic energy is not some magic invariant but actually depends on your frame of reference.  

During an eleasic collision, KE is conserved within any given reference frame, but it changes between different reference frames. 

Now clearly the collision damage won’t depend on the observational choice of a reference frame and this is why kinetic energy can be different and the crash results the same.  Momentum is invariant across reference frames, and it is the change in momentum - the impulse - that produces the damage.

 

Post edited at 08:00
 Hooo 29 Jun 2018
In reply to DancingOnRock:

If two identical cars travelling at the same speed hit head on, they both stop dead. All the KE is dissipated in the crash.

If you hit a stationary car at 90mph, the stationary car will accelerate away from you, so some of your KE is transferred to it and not dissipated in the crash.

 DancingOnRock 29 Jun 2018
In reply to Hooo:

We are not in the vacuum of space and we are not taking about a frictionless elastic collision. 

 wintertree 29 Jun 2018
In reply to DancingOnRock:

> We are not in the vacuum of space

It doesn’t matter.  It was a thought excercise intended to help you understand how and why you are wrong.  Your objection would still apply two two cars in the vacuum of space.  Think it through, work out the total KE from each reference frame - they are not the same!  

>  we are not taking about a frictionless elastic collision. 

Elastic or not, Hooo’s example holds by conservation of momentum.

> There is twice as much energy in the second collision. Where does all that energy go?

Hooo explained this clearly.  In the reference frame where one car is stationary before the collision, conservation of momentum dictates that it has speed after the collision, and this means it has KE after the collision - that is where the KE goes in this reference frame.

 

Post edited at 08:56
 DancingOnRock 29 Jun 2018
In reply to wintertree:

Your frame of reference is the planet earth and the stationary tarmac (plane). Following the collision all vehicles end up at rest on that plane.

If you don’t consider the earth to be the sstationary plane then both vehicles end up with 1/2mv^2 kinetic energy, even though they are stationary on the plane. Which is odd, don’t you think? It also means the vehicles have an additional 1/2mv^2 energy when they start. 

Post edited at 09:01
 DancingOnRock 29 Jun 2018
In reply to Dr.S at work:

v=45mph in this case. 

So the 90mph car is travelling at 2v. 

 wintertree 29 Jun 2018
In reply to DancingOnRock:

> Your frame of reference is the planet earth and the stationary tarmac (plane). 

An inertial frame of reference can be anything you choose it to be and the laws of Physics still work.   For a person inside a moving car their local environment - the car - is stationary but planet earth is moving by outside the window.  If they drop a test mass inside the car it falls vertically down, it doesn’t fall stationary with respect to the earth.

> Following the collision all vehicles end up at rest on that plane.

Not if one vehicle was initially stationary and you believe in conservation of momentum.  Sure, ultimately after the crash friction will bring everything to rest with respect to the earth but that’s got nothing to do with the crash.

 

Post edited at 09:02
 DancingOnRock 29 Jun 2018
In reply to wintertree:

Yep. Have edited my post. It means that after the crash the vehicles have kinetic energy with reference to their original state, even though they appear stationary. You need to look at that scenario again. Where does that energy come from?

 wintertree 29 Jun 2018
In reply to DancingOnRock:

> If you don’t consider the earth to be the sstationary plane then both vehicles end up with 1/2mv^2 kinetic energy, even though they are stationary on the plane. Which is odd, don’t you think?

Indeed.  Odd, but not wrong.

Because kinetic energy depends on your choice of reference frame.  Whilst it’s conserved within any given reference frame, it’s not a true global invariant.

 Robert Durran 29 Jun 2018
In reply to DancingOnRock:

> There is twice as much energy in the second collision. Where does all that energy go?

> 1/2mv^2 + 1/2mv^2 = mv^2

> 1/2m(2v)^2 = 2mv^2


Because both cars can end up at 45mph (v) in the second collision, half the KE remains as KE and is not disipated in the collision.

When both cars are at 45mph, they both stop dead (change in speed both 45mph). When a car at 90mph hits a stationary car, it slows to 45mph and the other car is accelerated to 45mph. So in both scenarios, both cars change their speed by 45mph, so the collisions feel the same for both cars. It is easy to check that momentum is conserved in both scenarios and that the same amount of KE is disipated.

The extreme case is a really massive lorry hitting a stationary car - it hardly slows down at all and the same amount of KE is disipated as when the car hits the stationary lorry at the same speed and more or less stops dead.

 DancingOnRock 29 Jun 2018
In reply to Robert Durran:

They don’t. At the end of the collision everything is at rest. 

 DancingOnRock 29 Jun 2018
In reply to wintertree:

If your reference frame is the car travelling at 45mph and you consider it at rest then at the end of the collision it is now travelling at 45mph. The other car was travelling at 90mph but is now only traveling at 45mph wrt to the reference frame. 

Now look at it from the car that is traveling at 90mph as the reference frame. The car it hits is travelling at 90mph and after the accident both cars are now apparently travelling at 90mph wrt to the reference frame. 

 Robert Durran 29 Jun 2018
In reply to DancingOnRock:

> They don’t. At the end of the collision everything is at rest

Only after friction with the road and so on has had its effect. We are talking about the actual impact. Unless the cars roll, hit further objects and so on, it is the actual impact which does the damage.

 DancingOnRock 29 Jun 2018
In reply to Robert Durran:

Yes. Now we are getting somewhere. 

Friction with the road, rolling cars, debris strewn over a larger area. There will be more energy involved and larger distruction. 

The collisions are not equivalent in any way. 

 wintertree 29 Jun 2018
In reply to DancingOnRock:

> Friction with the road, 

This is absolutely negligible compared to the forces in the short moment of the crash.

> There will be more energy involved

Theor really isn’t.  Your 06:55 Fri post was wrong and until you take the time to understand that, all I see is a lot of confusing posts.  Although you say there is a different total KE in the two situations, that is a difference born purely of your choice of reference frame and it does not actually differentiate the two situations.  There is no contradiction here because KE is not  invariant across reference frames.  

What does the damage?  Force.  Where does force come from?  Impulse (change in momentum).  The change in momentum is the same for cars at +v/2 and - v/2 colliding and for cars at v and 0 colliding.

> The collisions are not equivalent in any way. 

The collisions absolutely are.  What happens after the collisions is not, but that’s not the bit where people die.

The only important physical difference at the instant of collision between the two is the agreeably different rotational angular momentum and rotational KE in the wheels and power trains between the two situations, but again this is frippery compared to the crash forces.  

Do you understand now why your 06:55 post was wrong?  

Lusk 29 Jun 2018
In reply to wintertree:

Elon's going to have to send another car into space on a collision course with the first one to settle this argument.

 TMM 29 Jun 2018
In reply to wintertree:

youtube.com/watch?v=r8E5dUnLmh4&

Car hits immovable object at 50mph

Car hits immovable object at 100mph

Two cars travelling at 50mph hit each other in a head on impact. Will the damage resemble the 50mph impact or the 100mph impact.

The evidence is very clear.

 wintertree 29 Jun 2018
In reply to TMM:

The case I have been talking in relation to was about two cars (1) moving towards each other with equal speeds and (2) one stationary and one moving towards it with twice the speed of a car from (1).

These two situations are the same despite the incorrect argument used in the 06:55 post.  I’m not commenting on car vs immovable object in these posts.  I’m discussing a misconception DoR has about KE.  The movable/immovable business comes down to momentum and mass ratios etc.  That is the correct way to predict the outcome, not simply stating that one case has more KE than the other without recognising (a) the somewhat arbitrary nature of that KE and (b) where that KE goes based on conservation of momentum.

Post edited at 11:39
 Robert Durran 29 Jun 2018
In reply to wintertree:

> The collisions absolutely are.  What happens after the collisions is not, but that’s not the bit where people die.

To be fair they might if the two cars moving at 45mph after one at 90mph hits the other one stationary then both slide off a cliff.

But that is not, of course, what the discussion is about!

 Robert Durran 29 Jun 2018
In reply to DancingOnRock:

> Yes. Now we are getting somewhere. 

No we're not.......

> The collisions are not equivalent in any way. 

The collisions are equivalent. What happens afterwards is not, but that is not what is being discussed.

 

 Hooo 29 Jun 2018
In reply to DancingOnRock:

> We are not in the vacuum of space and we are not taking about a frictionless elastic collision. 

It doesn't matter. I don't know how we can make this any clearer, but I'll try.

Identical cars travelling at 45mph hit head on. Cars both stop dead. All the KE in both cars is dissipated in the crash, split equally between the cars.

Car travelling at 90mph hits stationary identical car. Loads of KE is dissipated in the impact, but not all of it. Moving car will still be moving, and stationary car will now be moving. That's where your missing KE is. Both cars will eventually come to a stop, but this later dissipation of KE doesn't add to the impact forces, it's a much lower force spread over a much longer time. As far as damage is concerned, this post-impact KE dissipation can be ignored.

 Hooo 29 Jun 2018
In reply to DancingOnRock:

> They don’t. At the end of the collision everything is at rest. 

No it isn't. That's where you're going wrong. They will eventually come to rest, but that can be ignored as far as collision damage concerned.

 wercat 29 Jun 2018
In reply to wintertree:

it might be worth considering that there really is no such thing as a stationary car, might help with people having misconceptions about one vehicle having a uniquely stationary situation in the universe during the the collision

Post edited at 19:40
 Hooo 29 Jun 2018
In reply to wercat:

How about if both cars were on a treadmill?

 Robert Durran 29 Jun 2018
In reply to Hooo:

> How about if both cars were on a treadmill?

They are - the earth.

 

 Robert Durran 29 Jun 2018
In reply to Hooo:

To be fair to DancingOnRock, although it has been made perfectly clear to him what the discussion is really about (and he doesn't seem to get the point), he does also have a point in that a head on crash with both cars doing 45mph is probably preferable to a car at 90mph hitting a stationary car - if both cars then career off at 45mph into a solid wall, the second collision with the wall could be just as bad as the original collision between the two cars.

 Hooo 29 Jun 2018
In reply to Robert Durran:

Hmm. I see your point, but I'm not convinced that a head on would be preferable. Unlike in crash tests, in a real crash the chances of a perfect collision where both cars stop dead are very low. It's more likely that the impact would be off-centre and both cars would spin off and roll and suffer further impacts.

But now we've gone off-topic on our off-topic! What was the OP about again?

 wintertree 29 Jun 2018
In reply to Hooo:

> It's more likely that the impact would be off-centre and both cars would spin off and roll and suffer further impacts.

Not if the cars grabbed each other.  I can’t actually imagine a workable solution to do that mind you. 

Long ago I recall a Tomorrow’s World (I think) in which they looked at a solution to the “overriding” problem in train crashes.  This is where one carriage slides on top of another, crushing the lower one.  The solution was ribbed metal plates on the carriage ends that meshed in a crash preventing vertical separation.  

Another off topic for being in a head on vs compatible stationary crash - if I’m in the head on, I am facing forwards in my seat and wearing my seat belt.  If I’m in a stopped car the chances are it’s because I’m parked, have my belt off and am sorting out drama with the urchins in the back, leaning between the front seats.  I’d rather be in the right place for the airbags and seat belt explosive tensioner to do their things...

Post edited at 22:53
 DancingOnRock 29 Jun 2018
In reply to Robert Durran:

Thank you. You cannot consider the crash over until all vehicles have come to rest. If one car is spinning end over end, it’s still dissipating energy. 

 wintertree 29 Jun 2018
In reply to DancingOnRock:

> Thank you. You cannot consider the crash over until all vehicles have come to rest. If one car is spinning end over end, it’s still dissipating energy. 

However the brief moment of the initial collision is identical in both cases.  Do you now accept that? Looking way back to your fundamental misunderstanding:

> There is twice as much energy in the second collision. Where does all that energy go?

If has been explained by different posters that (1) this difference is an artefact of the choice of reference frame and (2) in the reference frame with an initially stationary car, the extra KE goes in to that car due to conservation of momentum and (3) the impulse and therefore the forces in the two collisions are the same.

Do you now accept these three points?

If so it is then reasonable to move on to consider what happens after the initial crash, but that is so situation specific (stationary by a cliff edge, glancing head on and rolling off a bridge etc...) as to be a pointless discussion. 

Post edited at 23:04
 Robert Durran 29 Jun 2018
In reply to DancingOnRock:

> You cannot consider the crash over until all vehicles have come to rest.

But the collision which is what we are talking about, is over when it's over.

 DancingOnRock 30 Jun 2018
In reply to wintertree:

No. Because the occupants are strapped into the cars. And they stay strapped in until they are ejected, cut out, or climb out. 

This is applied mathematics in the real world. 

In reply to Robert Durran:

I suspect that the fact that cars do not behave like perfect, inelastic balls in simple mechanics problems probably has a significant effect on the outcome.

I suspect significantly more than half of the KE from the fast/stationary crash is absorbed on impact, by deformation of the vehicles. I doubt they both come out at the end travelling neatly at 45 mph. Just as two cars crashing into each other at 45mph don't bounce off each other and come out travelling at 45 mph in the other direction.

Thus, I would expect the fast/stationary crash to have a worse outcome than the head on collision.

 wintertree 30 Jun 2018
In reply to DancingOnRock:

> No. Because the occupants are strapped into the cars. And they stay strapped in until they are ejected, cut out, or climb out. 

> This is applied mathematics in the real world. 

You are changing your argument with the apparent effect of making it (wrongly) seem you were right all along.

Do you now accept you were wrong to state there is different ammounts of KE in the case of cars at +v/2 and -v/2 vs v and 0?

 DancingOnRock 30 Jun 2018
In reply to wintertree:

No. The v term is squared. So there’s 4x the energy in a car travelling twice as fast. And the reference frame doesn’t matter as the final energy has to be 0.

 wintertree 30 Jun 2018
In reply to captain paranoia:

> I suspect significantly more than half of the KE from the fast/stationary crash is absorbed on impact, by deformation of the vehicles.

It physically can’t be as momentum must be conserved in the impact.  The only way the stationary car could accept more KE as deformation without taking on momentum (and thus speed) whilst conserving momentum is to transfer the momentum to something else, which would mean using the brakes and wheels to transfer it to the Earth.  But wheels lock up under 1/10th of crash forces.

For identical mass cars, half the total KE must remain in the cars to obey conservation of momentum.  This is the extra KE dancing on rock wrongly claimed makes a stationary/90 worse than a 45/45.  That KE is physically not available to the impact. If you chose a reference frame where the cars are converging at 45/45 it isn’t even there.

After the collision, the speed of the cars with respect to Earth (again this is where the extra KE is) is different and more bad things *may* happen because of that, but this is totally unrelated to the collision forces which are strictly forbidden by conservation of momentum from accessing this additional KE.

Think of it another way - how do the crumple zones in the cars know if both are moving, or one moving and one stationary?  How do they decide if they should absorb more KE or not?  The only things they know from their stationary view of themselves are (1) the speed the other car hits them with (the same in every case) and (2) the speed of the road - communicated by contact with the road which is a very weak factor in a collision.

Post edited at 09:05
 wintertree 30 Jun 2018
In reply to DancingOnRock:

> No. The v term is squared. So there’s 4x the energy in a car travelling twice as fast. And the reference frame doesn’t matter as the final energy has to be 0.

This reads to me like a strong attempt to cover up  or justify the fundamentally and clearly wrong understanding of KE you showed in your 06:55 post.

Do you now accept that the actual collision is identical in both cases, but that the aftermath is different and has the potential worse in the one car stationary case?

 Robert Durran 30 Jun 2018
In reply to captain paranoia:

> I suspect significantly more than half of the KE from the fast/stationary crash is absorbed on impact, by deformation of the vehicles. I doubt they both come out at the end travelling neatly at 45 mph. Just as two cars crashing into each other at 45mph don't bounce off each other and come out travelling at 45 mph in the other direction.

But, as has been exhaustively explained, the two collisions ARE identical. Clearly it is impossible for more KE to be absorbed than if both 45mph cars stop, so no more can be absorbed in the 90mph/stationary case than if both end up at 45mph. As Wintertree ha explained, this can all be confirmed by considering conservation of momentum.

In reply to wintertree:

> is different and more bad things *may* happen because of that, but this is totally unrelated to the collision forces which are strictly forbidden by conservation of momentum from accessing this additional KE.

Those different and bad things are what I'd be concerned about in a real accident.

All the kinetic energy must be dissipated for the vehicles to come to rest. It's energy that causes damage to car and human bodies.

In a classic mechanics question, two elastic balls of mass m rolling towards each other on a frictionless plane, at velocity v, will collide and bounce off each other, and come out of the collision travelling at -v. Momentum is conserved.

Is that what happens when two cars collide head on?

Why not?

Where has the momentum gone?

Could it be Ft = mv?

What could that F be?

Could it be the force required to crumple the vehicles?

 

Post edited at 12:42
 DancingOnRock 30 Jun 2018
In reply to wintertree:

> This reads to me like a strong attempt to cover up  or justify the fundamentally and clearly wrong understanding of KE you showed in your 06:55 post.

> Do you now accept that the actual collision is identical in both cases, but that the aftermath is different and has the potential worse in the one car stationary case?

No attempt to cover up. That’s what I’ve been saying all along. The aftermath is not separate to the collision. It’s part of it. The debris will be flying off as the cars crumple and come to rest in both cases. 

As has just been explained Ft=mv. That’s the bit that was missing. The equivalent accident to 45/45 is 63/0. 

Do the maths. You now have all the formulas. 

 wintertree 30 Jun 2018
In reply to DancingOnRock:

> As has just been explained Ft=mv.

I have two degrees in physics and have taught physics at undergraduate and postgraduate levels.  Just so you know I might possibly know what I’m talking about.

I know and understand the maths very well.

> That’s what I’ve been saying all along. The aftermath is not separate to the collision. It’s part of it.

It is not.  You were talking about the collision.  Your words and your early posts were very specific,  The collision is identical in 45/45 and 90/0.  You claimed there was more KE in the later and attempted to justify this with sloppy, implicit, mixed use of references frames when KE is not invariant across reference frames.  

I have been clear that the aftermath of the collision maters, and for sure there is more KE **with respect to the earth** to cause more problems **after the collision** in the 90/0 case.  However, this does not change the collision.

The KE availible to damage both cars **during the collision** is identical in the 45/45 case and the 90/0 case.  The impulse exchanged during both is identical.  In the reference frame of any of those four cars that car has zero KE and the other car has that of 90mph.  Their collisions will be identical in terms of exchanges of impulse and damage to crumple zones.

Do you accept this?  

 

Post edited at 13:37
 wintertree 30 Jun 2018
In reply to captain paranoia:

I’m sorry but I’m getting exhausted by the unclear discussions.

Elastic/inelsatic has nothing what so ever so do with the equivalence of 45/45 and 90/0 crashes.   Considering equal mass cars:

(a) An elastic 45/-45 ends with the cars bouncing back at -45/45.

(b) An elsatic 90/0 ends with the cars doing 0/90.

In both cases no crumple zone damage.

(c) A fully inelsatic 45/-45 ends with 0/0.  This is one where as much KE is absorbed as is possible.

(d) A fully inelsatic 90/0 ends with 45/45

In both cases crumble zone damage is identical.

> Where has the momentum gone?

In (c) net momentum is zero before and after.  In (d) it is shared between the two cars.  

The difference between (a) and (c) and between (b) and (d) is an arbitrary choice of reference frame.  KE changes with reference frame but the collision does not.

Momentum is conserved in inelsatic collisions.  I’m sorry but it is.  No crumple zone in the world can allow an object to collide with a movable object, and for that movable object to remain stationary.

Post edited at 13:36
In reply to wintertree:

> I’m sorry but I’m getting exhausted by the unclear discussions.

My apologies. My questions were genuine, not rhetorical.

> In (c) net momentum is zero before and after.

Good point. Thanks.

Post edited at 13:40
 Robert Durran 30 Jun 2018
In reply to DancingOnRock:

> The aftermath is not separate to the collision. 

You know perfectly well that the rest of us are (perfectly sensibly) treating it as separate. Yes, if a car hits say a tree after a stationary/90mph collision, then that could be as bad as the original collision, but if there are no further collisions after the initial one then nothing further that is dangerous will happen.

So we just don't know what might happen after the initial collision, so it makes sense to treat anything further separately since it simply CAN'T be included in the analysis of the original collision. I suspect you now know this perfectly well, but are obstinately refusing to back down and acknowledge it.

>  The equivalent accident to 45/45 is 63/0. 

That is just plain wrong. The initial collisions are fundamentally different, with only half as much energy disipated in the original collision at 63/0.  If you had to choose to be in one of them, then it would need  specific (quite possibly unknowable) circumstances not to choose the 63/0 and take your chances with the aftermath.

 wintertree 30 Jun 2018
In reply to captain paranoia:

> My apologies. My questions were genuine, not rhetorical.

No apologies needed!  I hope I treated them as genuine questions.  

This is a really good topic of discussion for tackling a common area of hazy understanding.  

What is exhausting is doing it piecemeal over several days on a little screen.  

I’d like to think this could all be resolved in minutes with a whiteboard and face to face time.

  

 

 

 DancingOnRock 30 Jun 2018
In reply to Robert Durran:

Nonsense. We are talking about a car crash. Not two items in a physics lab. I’ve asked all along where this extra energy goes. No one can explain it. Other than the two cars magically head off down the motorway at the same speed in some kind of single conglomeration for all eternity. 

 

 wintertree 30 Jun 2018
In reply to DancingOnRock:

> Nonsense. We are talking about a car crash.

As are we.

>Not two items in a physics lab. I’ve asked all along where this extra energy goes.

We have told you, quite clearly.

> No one can explain it.

Several people have.

> Other than the two cars magically head off down the motorway at the same speed in some kind of single conglomeration for all eternity. 

Nobody has claimed that.

I beleive no less than three of us have said that the “extra” energy present is shared between the cars, which we have all recognised may now go on to suffer other unfortunate consequences.  As the cars have the same velocity however, those consequences will not come from further car/car interaction but from car/environment interaction.

However, the collision itself was no more violet, as no more momentum was transferred.  The presence of “extra” energy in 90/0 has no effect on the collision over a 45/45.  It manifests in what happens after the collision.

We have all clearly agreed with you about after the collision - in most but not all imaginable scenarios there is scope for more problems after a 90/0 than a 45/45, but the collision itself will not and can not be worse.

 

Post edited at 13:57
 Robert Durran 30 Jun 2018
In reply to DancingOnRock:

>  I’ve asked all along where this extra energy goes. No one can explain it.

With luck it is harmlessly disipated by the friction between the tyres and the road as the two cars slide or roll to a halt. As we have repeatedly told you it might, if you are unlucky, be disipated as the cars crash into a tree.

The point is that we simply don't know whether the aftermath of the collision will be the first harmless scenario or the second dangerous one (or something in between). That is why, unless the environment around the initial collision is specified, it is ONLY possible to analyse the severity of initial collision, so that is what we are doing. I really don't think that is hard to understand.

 JimSh 30 Jun 2018
In reply to DancingOnRock:

Hoo hit it on the head earlier.

Momentum is conserved during the collision.

Two cars of same mass hitting at 45 mph. Momentum before and after sums to 0

90mph vs stationary . Consider cars stick together on impact. Both move down road after collision.

Momentum before = 90 m units Momentum after = 2m V where V = velocity of stuck together cars.

V =45 mph. ie cars locked together

Equating energies before and after impact.

Energy before impact =1/2 mv squared +1/2 mv squared

45/45 collision total energy  m 45squared  Final KE =0 Change in KE =m 2025units

90/0 collision initial KE =1/2 mX 90squared =m4050units . Final KE =m X 45squared = m2025units.

Change in KE =2025m units.

Momentum is conserved and the same amount of energy dissipated.

Post edited at 14:49
 JimSh 30 Jun 2018
In reply to Robert Durran: and Wintertree

Sorry, I see you have both covered my answer previously.

I was out for a walk and posted on coming back without reading the full thread again.

In reply to Robert Durran:

> With luck it is harmlessly disipated by the friction between the tyres and the road as the two cars slide or roll to a halt.

I'd idly considered the difference in the 45/45 vs 90/0 collisions in the past, and concluded I'd rather have the former, on the simple basis of energies involved.

Whilst you've conviced me that the initial collisions are equivalent, I suspect most '90/0 collisions' end up with 'unlucky' subsequent behaviour. I also suspect that crash protection measures are designed to mitigate the initial, 'linear' collisions, and may not be as good at mitigating the consequences of the 'unlucky' subsequent behaviour. So my preference remains...

Of course, I'd prefer not to crash at all...

 wintertree 30 Jun 2018
In reply to JimSh:

> Sorry, I see you have both covered my answer previously.

No, it’s good!  I always enjoy seeing different peoples different ways of explaining things.  I hope I learn to be a better thinker and explainer that way.  

> I was out for a walk and posted on coming back without reading the full thread again.

I’ve been dropping in to it when cowering in the shade between jobs in the sun...

 wintertree 30 Jun 2018
In reply to captain paranoia:

> So my preference remains...

I really can’t decide.  I’ve given my reasons against being the 0 in a 90/0 up thread.  But the 90 might be quite advantageous - there is a chance the 0 car is “safed” - fuel pump(s) off, fuel solenoid shut (if fitted) etc, no additional energy in its rotating parts (engine, drive train, wheels).  So it’s a more benign object to hit than a car doing 45.  It might also be empty which saves my conscience from some grief!

> Of course, I'd prefer not to crash at all...

One thing none of us can argue about!

 Robert Durran 30 Jun 2018
In reply to wintertree:

>  I always enjoy seeing different peoples different ways of explaining things.  I hope I learn to be a better thinker and explainer that way.  

Indeed. Your contributions here have helped me appreciate more clearly how conservation of momentum means that KE only differs by a constant between different inertial frames of reference.

 DancingOnRock 30 Jun 2018
In reply to wintertree:

Watching the myth busters video, I can’t see either of those cars travelling very far at 50mph joined together like that following a 100/0 crash. They’ll pretty much embed themselves in the road surface with no wheels. I suspect being in the car that is hit would be much preferable as it’s unlikely to accelerate to anywhere near 50. Whereas the 100mph car will go from 100 to 0 in a very short period of time. 

 Timmd 30 Jun 2018
In reply to kathrync:

> I don't think this is only a problem for women -  a quick poll in my office suggests 3 of the 5 men in here wouldn't be confident to change a wheel either. 

> I have changed wheels on smaller cars by myself, but on my van I don't have the physical mass to get the wheel nuts off - I can't shift them even by physically jumping up and down on an extended nut wrench, which is probably not what I want to be doing when the van is on a jack anyway.  Yes, the torques are correct...  

Is there any chance you might be able to extend the extender, perhaps by making something to fit by buying some steel tubing? 

 

 

Post edited at 17:36
1
 Robert Durran 30 Jun 2018
In reply to DancingOnRock:

> Watching the myth busters video, I can’t see either of those cars travelling very far at 50mph joined together like that following a 100/0 crash. They’ll pretty much embed themselves in the road surface with no wheels. I suspect being in the car that is hit would be much preferable as it’s unlikely to accelerate to anywhere near 50. Whereas the 100mph car will go from 100 to 0 in a very short period of time. 


This, I'm afraid, shows a continued misunderstanding of the situation. The frictional forces which will slow the cars down from 50mph after a 100/0 collision will be tiny (and therefore negligible as far as damage/injuries are concerned) compared with the forces acting during the actual collision, and will not have time to have a significant effect during the short time of the actual collision. Both cars will end up at pretty near 50mph after the collision and then slide harmlessly to a stop.

 wintertree 30 Jun 2018
In reply to kathrync:

> I can't shift them even by physically jumping up and down on an extended nut wrench, which is probably not what I want to be doing when the van is on a jack anyway.  Yes, the torques are correct...  

Do you weight around 50 kg?  That’s my estimate for being too light for the higher end of wheel nut torques when using a standard car brace.

What you want is a 750 mm 1/2” breaker bar.  I keep one in my boot.  It’s also useful for all sorts of other jobs where you need a stout steel persuader.   Make sure to callibrate your weight point for standing with all weight on one foot.  

Also, do the intital loosen and tighten of all nuts before jacking the car up, and do this in an order like 1,3,5,2,4 or 1,3,5,2,4,6 around the wheel to balance stress.

Lusk 30 Jun 2018
In reply to DancingOnRock:

> Watching the myth busters video, I can’t see either of those cars travelling very far at 50mph joined together like that following a 100/0 crash. They’ll pretty much embed themselves in the road surface with no wheels. I suspect being in the car that is hit would be much preferable as it’s unlikely to accelerate to anywhere near 50. Whereas the 100mph car will go from 100 to 0 in a very short period of time. 

Here's a quite good example of a car crashing into a stationary car ... youtube.com/watch?v=sm-kxOkLvGg&.

Both vehicles don't remotely look like they're going to embed themselves into the road.

Removed User 30 Jun 2018
In reply to The Ice Doctor:

The kinetic energy in a collision is mainly dissipated in plastic deformation. If a collision didn't produce plastic deformation the cars would remain the same shape after the crash as before it.

You can think of plastic deformation as dragging a sack along the ground. It takes force to move it a certain distance so work is done but you can't get it back. Elastic deformation is like stretching a spring. You do work to stretch it and you can get it back.

If you look up plasticity on the interweb there's a nice MIT lecture on the subject where stress/strain graphs are described as springs and masses.

 deepsoup 30 Jun 2018
In reply to wintertree:

> What you want is a 750 mm 1/2” breaker bar.  I keep one in my boot.  It’s also useful for all sorts of other jobs where you need a stout steel persuader.   Make sure to callibrate your weight point for standing with all weight on one foot.  

Couldn't agree more with using a longer extension, but I'm not so sure about standing on it.  The trouble with jumping up and down is that it isn't stable enough to *really* jump up and down, and standing on it applies only body weight.  If you move it around to the other side of the nut and lift up, with decent technique you can apply a fair bit more force.  I expect most reasonably fit climbers would be able to go into a gym and deadlift significantly more than their bodyweight, and for someone who is a bit of a beast it will be getting on for double.

 wintertree 30 Jun 2018
In reply to deepsoup:

> but I'm not so sure about standing on it. 

There are pros and cons in my mind.

Pro - given a marker on the bar you can apply a calibrated +/- 10% torque when tightening the nuts by standing one footed on the marker, using hands to stabilise yourself on the vehicle.

Con - may not be enough to un-do the nut, although with a 750 mm bar and some jumping I’d be surprised.

> If you move it around to the other side of the nut and lift up, with decent technique you can apply a fair bit more force. 

Very true, although such a situation is prone to a sudden release leading to almost explosive, out of control flailing, the breaker bar smashing the vehicle or person, and shoulder pain.  At least if you’re as gormless as I can be.  

 deepsoup 30 Jun 2018
In reply to wintertree:

Get you with your calibration. 

I have one of those telescopic thingys - about 45cm retracted I guess, and maybe 75cm extended.  The nuts get done up as tight as I can do them with the short lever, in the hope that I'll then be able to undo them again with the long.  (I have a set of winter tyres on spare rims, so change all four wheels twice a year.)

 Mal Grey 30 Jun 2018

 

I am loving this debate, it is becoming a classic of UKC. 

I even understand some of it. 

 

 

In reply to Mal Grey:

I missed the bit where it morphed from 'do cars have spare wheels?' into 'is a 45/45 head-on collision worse than a 90/0 collision?'...

 kathrync 01 Jul 2018
In reply to wintertree:

> > I can't shift them even by physically jumping up and down on an extended nut wrench, which is probably not what I want to be doing when the van is on a jack anyway.  Yes, the torques are correct...  

> Do you weight around 50 kg?  That’s my estimate for being too light for the higher end of wheel nut torques when using a standard car brace.

Yes, that's not far off the mark.

> What you want is a 750 mm 1/2” breaker bar.  I keep one in my boot.  It’s also useful for all sorts of other jobs where you need a stout steel persuader.   Make sure to callibrate your weight point for standing with all weight on one foot.  

> Also, do the intital loosen and tighten of all nuts before jacking the car up, and do this in an order like 1,3,5,2,4 or 1,3,5,2,4,6 around the wheel to balance stress.

Thanks

 Timmd 01 Jul 2018
In reply to Dax H:

> Sorry but they are not men, some kind of metro sexual fop but not a man. 

Ha, the gay man in a family I know seems to be the one who takes the most pride in doing practical things, and wonders at his brothers who are fathers, in liking their bubble baths and face moisturising cream respectively. It's a case of 'Oops wrong stereotype'.

> On the spare wheel subject. I would never rely on sealing gunk. I stopped to help a guy on the road between Durness and Toung late one Saturday night, foreign guy on holiday who had a puncture that was actually a hand sized hole in the side wall. I ended up dropping him and his family in Durness to look for a hotel. 

Do you know (or anybody else know) whether the sealing gunk has a usable lifespan, so that one needs a new tyre after a certain amount of time? 

Post edited at 23:20
 Ridge 02 Jul 2018
In reply to Timmd:

> Do you know (or anybody else know) whether the sealing gunk has a usable lifespan, so that one needs a new tyre after a certain amount of time? 

AFAIK it's a temporary repair to get you to the garage, I wouldn't drive round on a sealed tyre for any distance.


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