UKC

/ Bendiness of metal

Please Register as a New User in order to reply to this topic.
G. Tiger, Esq. - on 12 Jun 2018

I'm no metallurgist so I'm after some advice.

Having built my own bike rack for my car roof bars, it came to the time to put the bike on and it was a bit bendy. 

The front of the bike was on the wheel rest on the end of the tube, about 70 cm overhanging the front roof bar, and it was flexing by about an inch . Probably not ideal for bumpy roads, one pothole might bend it irreversibly. Bodged it with some wood to take the weight, but not an elegant or long term solution.

Other than shortening the overhang of the tube, what can I do to reduce flex? The tube in question is 30 mm diameter aluminium. Would I be better off with steel? Thicker walled tubing?

 

Thanks 

 

Gte

NaCl - on 12 Jun 2018
In reply to G. Tiger, Esq.:

How long ago did you make it? Aluminium hardens as it gets older as crystals develop in its structure (age hardening) but it can take some time unless you artificially speed it up. Wouldn't have a clue how to do that though

spenser - on 12 Jun 2018
In reply to G. Tiger, Esq.:

Put something rigid inside the tube extending beyond the root of the bend/ overhang, this will increase the second moment of area of the tube and prevent it from bending. If a broom handle would fit I would try that for a start, you could weather proof the very ends using Wax plugs to prevent wood from rotting if it appears to work.

Experience in the rail industry (EN14663) would suggest that you should design it for a vertical acceleration of 2g, lateral of 1 and forward 5.

elsewhere on 12 Jun 2018
In reply to G. Tiger, Esq.:

Two tubes side by side - twice as stiff sharing the load.

One tube double wall thickness - twice as stiff 

Flex also scales with square of overhang so 50cm overhang flexes 50% less than 70cm if that is viable opion.

Combining thicker walls or side by side plus reduced overhang could have 4 times less movement at the end.

One tube twice the diameter - eight times as stiff - scales with cube

Two tubes stacked vertically and *fixed together* - ten times as stiff?

 

 

Post edited at 08:00
G. Tiger, Esq. - on 12 Jun 2018
In reply to elsewhere: and others :

Very informative, thanks all.

Made in the last month, although I have no idea about the stock used to make it.

Broom handling (is that a word?) the inside is worth a try.

I can reduce the overhang a little, so that is really worth knowing.

Any mileage in changing to steel rather than aluminium?

ta

Gte

 

elsewhere on 12 Jun 2018
In reply to G. Tiger, Esq.:

steel 3 times stiffer than aluminium

stainless to avoid rust stains from drips onto car

LastBoyScout on 12 Jun 2018
In reply to G. Tiger, Esq.:

Much as I applaud your home-made efforts, I'd point out that you can get purpose made ones from the likes of Thule for a pittance on eBay.

Toerag - on 12 Jun 2018
In reply to G. Tiger, Esq.:

snugly fit a piece of stiff strip metal vertically inside the tube, that'll stiffen it up massively. Be careful if using aluminium in an interference (tight) fit situation - once it grips it's very difficult to unstick.

jkarran - on 12 Jun 2018
In reply to G. Tiger, Esq.:

Increasing tube diameter, thickening tube walls, shortening the lever, lowering the load, stiffening the material, adding triangulation will all reduce bending to varying degrees. Pick the most appropriate fix(es).

Are you sure it's all the tube bending, not what it's attached to twisting (torsion load over a long length or applied to an open channel section), free play in the joint or distortion of a bolted joint? 30mm aluminium tube likely has at least 1.2mm wall and it'll be an alloy not pure soft aluminium unless you bought something quite odd, it should be plenty stiff enough in bending over the lengths involved in a bike rack *if* the loads are carefully fed back into the car.

Aluminium alloys subject to high numbers of load cycles below their elastic limit (like the application you describe) tend to crack quite quickly. Filling the tube with something vibration dampening may help longevity, it needn't be clever, squirty foam would probably do the trick but it won't do much to solve the stiffness problem.

jk

Post edited at 10:13
summo on 12 Jun 2018
In reply to G. Tiger, Esq.:

A short length of armoured steel used in concrete inside the tube. Off cuts just get slung away for scrap. 

However, compared to the cost of wrecking your car or bike, just buy one.  

Post edited at 10:29
Dave Garnett - on 12 Jun 2018
In reply to G. Tiger, Esq.:

Is the final version likely to be Growl Tiger's Last Stand?

Jon Greengrass on 12 Jun 2018
In reply to G. Tiger, Esq.:

I guess you have built a rack for your tandem following the discussion on the other thread

https://www.ukclimbing.com/forums/biking/homemade_bike_rackroof_carrier_for_the_car-683620#x8773162

70cm overhang for a 30mm aluminium alloy tube that is deflected 25mm by the weight of the tandem is not going to end well once you add the dynamic load while driving. I expect the fatigue limit would be met rather rapidly what with the constant oscillating deflection, followed by your precious tandem leaving the roof of your car and the possibility of causing a fatal accident.

Steel is more than twice as stiff as Aluminium alloy but you will get a bigger increase in stiffnes for the same weight by switching to a larger diameter alloy tube.

I used 3" x 2" x 1/4" wall thickness U -channel. you could narrower to 2" x 2" if your tyres would fit.

https://www.metals4u.co.uk/aluminium/c1/channel/c14/76.2mmx50.8mmx6.3mm-(3x2x14)/p829

 

 

elsewhere on 12 Jun 2018
In reply to Jon Greengrass:

> 70cm overhang for a 30mm aluminium alloy tube that is deflected 25mm by the weight of the tandem is not going to end well once you add the dynamic load while driving. I expect the fatigue limit would be met rather rapidly what with the constant oscillating deflection, followed by your precious tandem leaving the roof of your car and the possibility of causing a fatal accident.

> Steel is more than twice as stiff as Aluminium alloy but you will get a bigger increase in stiffnes for the same weight by switching to a larger diameter alloy tube.

Steel has a pretty much infinite fatigue life below a certain limit too.

https://en.wikipedia.org/wiki/Fatigue_limit#/media/File:S-N_curves.PNG

Eric9Points - on 12 Jun 2018
In reply to G. Tiger, Esq.:

I'd just buy one and I've spent much of my life designing stressed structures.

It would avoid the possibility of you feeling like a total dick when you execute an emergency stop or avoidance manoeuvre and the whole lot goes through the back window of the car in front.

Rigid Raider - on 12 Jun 2018
In reply to G. Tiger, Esq.:

Why don't you just put the bike safely inside the car? 

JamButty - on 12 Jun 2018
In reply to G. Tiger, Esq.:

I'm a metallurgist by degree but I think most of the points have been made.

However for my sins I am now a H+S Manager,  so just to give you some different food for thought.  If you have an incident and your bike comes off causing an RTA or worse,  you could be challenged about the suitability and design of your bike rack.  Not a position I'd want to be in.

 

jkarran - on 12 Jun 2018
In reply to G. Tiger, Esq.:

A picture is worth a thousand words.

There's nowt wrong with a good diy design but you do want to get it right, it's subject to some quite substantial forces swinging a big bike swinging about up there on the roof. The temptation could be to rely too much on material bending resistance, not enough on design/triangulation which is probably ok until it starts vibrating.

jk

tom_in_edinburgh - on 12 Jun 2018
In reply to jkarran:

> There's nowt wrong with a good diy design but you do want to get it right,

The thing that would worry me is the lack of resource to test the hell out of it in lots of different situations before trying it on the road where people are at risk.  

 

jkarran - on 13 Jun 2018
In reply to tom_in_edinburgh:

When did we climbers become quite so risk averse? Plenty of people build their own bikes and cars let alone their own bike racks.

jk

LastBoyScout on 13 Jun 2018
In reply to jkarran:
> When did we climbers become quite so risk averse? Plenty of people build their own bikes and cars let alone their own bike racks.

A car would need to pass a number of checks to be allowed on the road and a bike is probably only going to damage you if it falls apart in use.

The consequences of a failed bike rack at 70mph on a motorway is potentially catastophic to quite a lot of other people and I think JamButty is spot on, on this occasion.

tom_in_edinburgh - on 13 Jun 2018
In reply to jkarran:

> When did we climbers become quite so risk averse? Plenty of people build their own bikes and cars let alone their own bike racks.

I had my engineering hat on, not the climbing one

I do think there's a difference between taking personal risks climbing and taking risks with other people on a public road.  The thing about a bike rack is that if it fails when the car is moving at speed the people in the car with the rack aren't the ones who will get hurt by bits of bike and rack flying through the air.

jkarran - on 13 Jun 2018
In reply to LastBoyScout:

> A car would need to pass a number of checks to be allowed on the road and a bike is probably only going to damage you if it falls apart in use.

That isn't actually the case, I've put one through the process and would happily do so again. There is a mandatory inspection of home built vehicles but it's not testing in any meaningful sense of the word. There is no requirement the design or performance be proven theoretically or practically, just that the inspector (not a structural engineer) isn't struck by poor design or material choices, that it's screwed together and that it complies with the few rules on lighting, mirrors, seat belts etc. By bike I meant motorcycle. Test requirements are very similar.

The list of what's inspected is on the fist few pages, even items like 'seat strength' are no more than a look and a wiggle with a hand... https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/696880/individual-vehicle-approval-inspection-manual-passenger-vehicles.pdf

We're rapidly mystifying our built world, convincing ourselves it's all too complicated, too risky to do things ourselves, it isn't but it does need to be done thoughtfully.

jk

G. Tiger, Esq. - on 13 Jun 2018

I’m not surely ifs trying to justify myself is wise, or if I'll open myself up to loads of criticism, but here goes.

 

It's for a tandem, so "off the shelf" options are limited, and certainly not "a pittance on eBay". 

It's essentially a 2 metre tube with wheel rests bolted to each end. This tube is bolted to the roof bars with M8 U-bolts. 

It is built with parts from another bike rack, so I have confidence in the ability of the wheel rest straps to keep the bike secure. Although I appreciate they are more to stop the bike bouncing out of the rests then to give it structural integrity.

Further to that I have uprights on the bars (from my kayaking days) to which I strap the seat tubes. 

Currently I have a rest on the front bar for the front bottom bracket to rest on, taking the weight of the front end of the bike 

With four (or more) straps holding the bike on the whole assembly, and the fact that it is bolted together gives me confidence that it is as secure as any other bike rack out there. Certainly more secure than some of the roof loads I've seen. And more securely attached than many kayaks I've had on my roof (never had one fall off - famous last words). 

I can reduce the overhang easily, although that has an effect on my ability to open the boot, as the whole assembly has to move backwards by about a foot. But using the rack won't be a frequent thing so no big deal (and also partly why I'm reluctant to fork out too much for it).

So my next steps are to swap it out for a stainless steel tube, reduce the overhang, and investigate lengthening the uprights, and perhaps have a cross member between the uprights to give more flexibility with attachment points.

 

Ta

Gte

LastBoyScout on 13 Jun 2018
In reply to G. Tiger, Esq.:

Ah, you never said it was for a tandem, in which case I'll withdraw my remarks about "available" and "pittance"

Your full explanation makes much more sense - I got the impression you were knocking up something rather more Heath Robinson to carry a mountain bike.

My preferred approach to transporting my tandem was with front wheel off and diagonally inside an estate car with the seats down, but that does rather limit what else you can carry at the same time. Neither of our current cars are long enough to even consider doing that.

I have toyed with using 2 of my other Thule bike racks bolted together to create one long carrier (rather than a length of pipe) and probably, like you, using my kayaking uprights and a couple of straps as extra support.

The other idea would be to lie it flat on the roof bars, front wheel out and handlebars secured, which seems a much simpler solution, although you might need to take the pedals off one side to clear the roof. Mine it quite an old steel-framed one, though, so should be fine - if you have something fancy in aluminium/carbon this might not be an option.

My other comments were from someone who HAS had a kayak fall off the roof - on the M4 on a Friday evening, doing about, um, 70mph, m'lud. Fortunately nothing else around and the boat ended up on the hard shoulder, so easy to retrieve. Figured out why it had happened and changed the way I strapped it on to prevent it happening again.

jkarran - on 13 Jun 2018
In reply to G. Tiger, Esq.:

I'd misunderstood/misread your OP, I thought it was one of the uprights was flexing side to side.

It's hard to say without seeing it but I'd probably try to rearrange the layout so the bike's weight is supported as directly as possible by the roof bars front and rear not the ali tube. Whether that's through blocks and straps for the bottom brackets or the wheels depends where they conveniently sit. Supporting the bike's weight on long cantilevered tubes isn't a great idea though I'm sure it helps with loading and unloading.

You could increase the stiffness of the existing tube by adding a few layers of unidirectional fiberglass packing tape lenthwise. Wrap the lot in a spiral of black PVC tape to protect the glue from UV. That said, fix the layout first before putting a sticking plaster on it

jk


Please Register as a New User in order to reply to this topic.