/ Dynamic rope and child leader falls

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flopsicle - on 06 Nov 2017
I'm hoping for some rope geeks to weigh in here! A good friend and experienced climber was telling me that children effectively lead and take lead falls on static rope as it takes 80kgs of weight to stretch a dynamic rope as they were never made for children/uber light climbers.

I have a 9yr old getting keen on leading and I only weigh in at 70kg. This guy knows more than me (unquestionable) but there's something about what he was saying that didn't sound right.

I haven't the foggiest what my kid weighs but I got a bit lardy over a lazy summer and am now working to be 65 as opposed to 70kg and my kid is a random tidge pot! If what he was saying was correct, no amount of hopping up would make it safe for me to belay her - let alone men!

I would love any references for dynamic ropes and weeny climbers but thoughts are also welcome. I've done some looking at rope specs and know the lower the KN impact rating the better - but also realise that these figures will be based on an adults weight.
richlan - on 06 Nov 2017
In reply to flopsicle:
No idea but are you sure he didn’t mean 80kg of force rather than 80kg of weight, that’s a very different thing when you add the dynamics of a fall and other ways the rope acts.
Post edited at 20:29
AJM - on 06 Nov 2017
In reply to flopsicle:
I have no idea how much a 9-year old weighs but I've spent quite some time climbing with a partner who weighs 55kg (I'm 70-75kg) with no issues - if this was a big issue then by 1/3rd under weight you'd start to notice. The tests are done at 80kg but lots of folk weigh less and can fall safely including all those child comp climbers and what have you.

If you're concerned about sharp catches then potentially a skinny stretchy rope, a non-grabby belay plate and practise at stepping in would help.
Post edited at 20:30
Ian W - on 06 Nov 2017
In reply to flopsicle:

Dynamic belaying by its nature removes the shock load from lead falls. From reading this it appears you should learn to use dynamic rather than static belaying. Any climbing wall would be able to help.
ellis - on 06 Nov 2017
In reply to flopsicle:

I'm using a half rope with my sub 30kg lad as I figure it will stretch a bit more.
MFB - on 06 Nov 2017
In reply to flopsicle:

Half ropes tested with 55kg but more prone to chopping
flopsicle - on 06 Nov 2017
In reply to richlan:
I think that was the confusion but he was certainly convinced it was weight based as I discussed my own weight and he felt I would not get a rope to stretch. He is qualified and usually spot on - but just didn't sound right.

Comp kids were discussed and there was mention of study into damage - I have asked for reference.

Hi Ian - the discussion was at a wall comparing the benefits of stepping in vs hopping up, in addition to increasing rope drag on overhangs by the extent you step out. I think where the confusion has arisen is that I don't view belaying as something 'learned already' more something I'd hope to constantly refine, consider and improve. Also considering buying a separate rope for the child (lower kn impact rated such as tendon smart lite).
Post edited at 20:42
flopsicle - on 06 Nov 2017
In reply to ellis:

Do you get flack for it in public? I have thought about this give little tidginess, but we want to get on rather than having to debate rights and wrongs each session.
stp - on 06 Nov 2017
In reply to flopsicle:

The lack of stretch shouldn't matter too much as long as you give a dynamic belay.

There's a few ways you can give dynamic belays to lighter climbers. The easiest is simply to stand out from the wall a bit then when the leader falls simply step/walk forward as the rope comes tight. This is fine as long as they not likely to fall below the level of the first bolt. In that case they could hit the rope between the belayer and bolt one.

Secondly some ropes are more stretchy than others. Using a more stretchy rope could help. If you're outdoors using roller biners also help by distributing the force over the whole length of the rope which allows more stretch and softer falls.

In general with all lead falling it makes sense to practice a bit first, starting off with short falls and building up to longer ones. That way you can see exactly what's happening in a safe way.
ellis - on 06 Nov 2017
In reply to flopsicle:

No, not had to justify it yet.
pec on 06 Nov 2017
In reply to flopsicle:

> A good friend and experienced climber was telling me that children effectively lead and take lead falls on static rope as it takes 80kgs of weight to stretch a dynamic rope as they were never made for children/uber light climbers.

This is complete nonsense. Ropes don't magically start to stretch only when 80kg or more is added to them. I weigh just over 60kg and whenever I've abseiled on any climbing rope it definitely stretches and that's just under a relatively static load before you even think about the extra force from falling.

The stretch of a rope (within its normal operating range) is proportional to the load applied (Hookes Law) so it will stretch under any load.
Whilst it's its true that a lighter climber will stretch the rope less, the reason the rope needs to stretch is to absorb the energy of the fall and since the lighter climber generates less energy in a like for like fall then the rope doesn't need to stretch as much anyway.

Small climber, low energy fall, small rope stretch. Large climber, high energy fall, large rope stretch. It's effectively a self regulating system in which the rope stretches by exactly the amount it needs to absorb the fall's energy.



Fraser on 06 Nov 2017
In reply to flopsicle:

I think you need to be more discerning about who you listen to!
flopsicle - on 06 Nov 2017
In reply to stp:

Hi - we've generally used stepping in but the discussion arose after seeing a vid in slo mo of jump vs non jump where the climber is lighter and it made me think about it again, also the amount of slack at higher bolts which I felt maybe needed increasing (which we agreed on).

I get really interested in things like this, partly out of wanting to continuously refine and understand what I do but also a little out of curiosity. It's something I like to understand really well.

I asked about them running a bit of a masterclass on dynamic belaying, sport vs trad belaying (I did tend to stay a bit traddy indoors!).
flopsicle - on 06 Nov 2017
In reply to pec:
That's pretty much what I thought, however I like to double check.

Fraser - listening and agreeing are different things. I can be wrong about things I know a great deal about so assume others can too, I will generally double check when something doesn't sound right regardless of who said it. I like to listen first though.
Post edited at 21:31
Ian W - on 06 Nov 2017
In reply to flopsicle:

Hi Flops,
you'll always be learning to belay better, alongside the rest of us!
Not sure who i've upset to get the dislikes; nothing was intended to offend - but dynamic belaying is the way forward. i would suggest stepping in rather than hopping up, as hopping up will only provide a small amount of "dynamism". Stepping in and out can regulate the amount of free rope and therefo the length of a fall. if your young'un is entering the ycs in 2018, the bmc are intending to provide instruction / guidance / supervision to lead belayers at each round. As i suggested above, pretty well any climbing wall with route facilities would be able to help. Theres much more to it than can be sorted on a forum discussion!
elsewhere on 06 Nov 2017
In reply to flopsicle:

The rope won't need to stretch as much as it stops a lightweight climber.

That means they stop quicker.

That means greater and harsher deceleration.



flopsicle - on 06 Nov 2017
In reply to Ian W:

Hi - they're already offering instruction. I belayed at last years but they were just checking you not instructing - I can't remember where I saw instruction offered since, I think it was the lead finals in Sheffield but we were just spectators there. I agree re dynamic belaying and that was never in question but which methods and what rope for the particulars of a climber half the weight is worth considering. Also - she is on occasion belayed by her adopted grandad who's skilled but certainly not agile, so being aware feels doubly worth it, especially if I can preserve something that means so much to both.

Honestly, I thought your first response was a bit off, maybe because I knew where the conversation was had in the first place. It was a bit ironic.
Robert Durran - on 06 Nov 2017
In reply to flopsicle:
What matters as far as comfort/possible injury is concerned is the maximum deceleration. Assuming an elastic rope, this is given by the formula:
a = g + sqrt( g^2 + 2gkF/M)
where g is acceleration due to gravity, k is the stiffness of the rope (tension/fractional stretch), F is the fall factor and M is the mass of the climber.

Clearly this is greater the lighter the climber.

For example, for a rope which stretches 10% with a 100kg climber hanging statically on it, and a fall factor of 0.5 (quite high - runner at 6m with climber 2m above it ) this gives a max decelaration of about 3g for a heavy 100kg adult and about 7g for a very light 25kg child. So more than double the deceleration. I'm not sure whether this would be a problem. This assuming absolutely no dynamic belay.
Post edited at 22:36
Ian W - on 06 Nov 2017
In reply to flopsicle:

Well keep at it, and i hope to see you at te final in Ratho next April, although probably not using the name flopsicle!
flopsicle - on 06 Nov 2017
In reply to Ian W:

Hehehe... doubt it, she just scraped qualifying last yr and is in the older age group this yr. Still just a about the least competitive kid in the series - she goes for the craic!
Ian W - on 06 Nov 2017
In reply to flopsicle:

O ye of little faith!
Glad she enjoys it (bit of detective work, found out real name) - more important than pure competitiveness at that age; if shes enjoying it, she'll be competitive enough, and imho, climbing outdoors is a massive help to young climbers in comps.
flopsicle - on 06 Nov 2017
In reply to Ian W:

I agree completely and we've been getting out more. I wouldn't swap her for the world but she does make me laugh - she's always the one starting shenanigans! The comps are lovely for time with other kid climbers.... playing with chalk. She didn't even ask where she'd come at Ratho till someone asked her that evening.
obi-wan nick b - on 07 Nov 2017
In reply to flopsicle: The UIAA test weight for single ropes is 80kgs I don’t have the rope length and fall length info that’s used i the test but it results in a test fall factor of 1.77 (quite harsh). The rope must take at least 5 test falls of this factor in order pass the UIAA test and call itself a climbing rope. This may be where your friends figure of 80kgs comes from rather than a minimum weight to make it safe

Zebdi - on 07 Nov 2017
In reply to elsewhere:
> That means they stop quicker.


Only if you let them. Skinny single rope + Fig8 belay device = dynamic belay. If there's still too much friction, you can use setup shown here: https://ae01.alicdn.com/kf/UT8Fe5IXbRcXXagOFbXC/201953439/UT8Fe5IXbRcXXagOFbXC.jpg

Fig 14, Super 8.

I know it's not recommended, but it works well.
jkarran - on 07 Nov 2017
In reply to flopsicle:

A lighter climber will experience higher deceleration when caught by a climbing rope than a heavier one all other things being equal. Real world climbing ropes used in the real world aren't ideally elastic which I suspect mitigates this effect to a degree. In my experience belay technique has more impact on perceived deceleration rate (comfort) than detail differences in the rope used so that's something to consider and work on. I'd say if your kid isn't finding the falls uncomfortable then that's all the information you really need.

Saying a dynamic rope behaves like a static one for a child is an overstatement.
jk
cameronmurdoch on 07 Nov 2017
In reply to flopsicle:

I work in London as a coach and regularly lead belay young children. I weight about 75kg, and when belaying very light kids it can be tricky to give a dynamic catch, however it IS possible, it just takes practice. Learning to do it has made me better at belaying in general.

As far as I know there hasn't been a proper study done on the effects of leader falls in children. There was something talked about at the BMC injury symposium a few years ago, but I don't know if anything has come of it.
oldie - on 07 Nov 2017
In reply to pec:
> Small climber, low energy fall, small rope stretch. Large climber, high energy fall, large rope stretch. It's effectively a self regulating system in which the rope stretches by exactly the amount it needs to absorb the fall's energy. <

However it seems to be generally accepted that light people get less protection when using shock absorbing via ferrata lanyards, and in fact some manufacturers now produce lanyards adjustable for different body weights.
Will not the same effect occur in a lead climbing fall with a light/child climber?

Proviso: I know little about mechanics myself and don’t use a lanyard.
I suppose the very high fall factors possible on via ferrata might mean the situations are not that comparable.
Post edited at 12:12
Jamie Wakeham - on 07 Nov 2017
In reply to Robert Durran:

Your calculation is obviously right, but isn't the 'comfort' of a catch more to do with the maximum force exerted, not the maximum acceleration? Using your numbers, the child would feel around half the max force that the adult does, despite having a greater acceleration.

Of course an adult body can withstand this greater force more readily than a child. But I'm fairly sure that, in the real world, all belays are sufficiently dynamic that there's no significant issue. if there were, we'd be hearing about children getting hurt.
Robert Durran - on 07 Nov 2017
In reply to Jamie Wakeham:

> Your calculation is obviously right, but isn't the 'comfort' of a catch more to do with the maximum force exerted, not the maximum acceleration? Using your numbers, the child would feel around half the max force that the adult does, despite having a greater acceleration.

Fair point. But can bigger people comfortably put up with bigger forces? If I stood on a mouse, it would die, but if I stood on an elephant it might not even notice. I think the whole thing is probably quite complicated.......

And don't fast jet pilots worry about g's (acceleration) rather than forces?
deepsoup - on 07 Nov 2017
In reply to oldie:
> I suppose the very high fall factors possible on via ferrata might mean the situations are not that comparable.

There's that, but also the different nature of the shock absorption.

A dynamic rope behaves kinda-sorta elastically - as the load comes on it begins to stretch, and how much it stretches relative to it's length is approximately proportionate to the load that comes on it. So from the time that the rope pulls tight the 'impact' force increases from zero up to some maximum when the faller has stopped and the rope has stretched to it's furthest extent.

A 'screamer' type via ferrata shock absorber (or an industrial fall-arrest lanyard, come to that) is basically static, with stitching that rips at a certain tension to limit the maximum force. I don't know about via ferrata, but the upper limit for an industrial 'screamer' to meet the EN standard is 6kN and in practice they generally go at about 4kN. That means that as it pulls tight the 'impact' force will rise more or less instantly and if/when it reaches 4kN the stitching begins to tear limiting the force at 4kN until either the faller has stopped or it runs out of stitching to tear. A light faller experiences the same 'impact' force as a heavy one, and therefore decelerates more rapidly.
deepsoup - on 07 Nov 2017
In reply to Robert Durran:
> Fair point. But can bigger people comfortably put up with bigger forces? If I stood on a mouse, it would die, but if I stood on an elephant it might not even notice.

Also, can lighter people comfortably put up with higher accelerations?

If you drop the mouse off a 2m ledge it'll hit the ground, bounce and run away. If you drop an elephant off a 2m ledge you have a big mess of dead (or severely injured and very, very cross) elephant to clean up.

(Actually, according to professor google, a mouse can survive a fall from any height - it's light enough and strong enough to survive hitting the ground at its terminal velocity.)

This bit of reading might interest you if you have trouble sleeping at night. ;-)
http://www.hse.gov.uk/research/hsl_pdf/2003/hsl03-09.pdf
Robert Durran - on 07 Nov 2017
In reply to deepsoup:

> Also, can lighter people comfortably put up with higher accelerations?

> If you drop the mouse off a 2m ledge it'll hit the ground, bounce and run away. If you drop an elephant off a 2m ledge you have a big mess of dead (or severely injured and very, very cross) elephant to clean up.

Is that not a pressure thing? Make an animal 10 times as big and it has 1000 times the mass but only 100 times the surface area, so when it lands on the ground it feels 10 times the pressure. So we might need to consider the area of the harness slowing the climber down.
jkarran - on 07 Nov 2017
In reply to deepsoup:
Dropped mice bounce where a dropped horse would splash because of the very different weight/drag ratio resulting in very different terminal velocities. Weight goes up with the cube of scale, drag with the square of it for similar shape/composition creatures. As I understand it something fluffy like a cat is the sort of size/shape/weight to provide a 50/50 chance of surviving a huge drop in good condition, any smaller and it'll likely survive, any bigger and it'll likely break. Interesting but not much use to the OP
jk
Post edited at 14:51
deepsoup - on 07 Nov 2017
In reply to jkarran:
> Dropped mice bounce where a dropped horse would splash because of the very different weight/drag ratio resulting in very different terminal velocities. Weight goes up with the cube of scale, drag with the square of it for similar shape/composition creatures.

Very different terminal velocities, to be sure. But that's only relevant once terminal velocity is actually reached.
(I've no idea what the terminal velocity of a mouse it, but I think it's quite unlikely to be reached in only a 2m fall.)

I'd guess the strength of bone and connective tissue also increases with cross-sectional area, so goes up with the square of the size of the animal as it's mass increases by the cube. So I think you'd expect the smaller/lighter animal to better cope with impacts well short of terminal velocity, and 'g forces' generally.

Edit to add:
> Interesting but not much use to the OP
I think that goes for the thread generally tbh. It's pretty much all speculation.
Post edited at 15:15
deepsoup - on 07 Nov 2017
In reply to Robert Durran:
> So we might need to consider the area of the harness slowing the climber down.

As above, perhaps more to the point would be the cross-sectional area of bones and ligaments.
duchessofmalfi - on 07 Nov 2017
Having climber a lot with small children (including those that are leading and weight as little as 20Kg) and never had a problem. You'll get no where near the gear failure forces and as regards the softness of the catch and the damage to the climber I doubt you'll be exposing any child to the sort of falls where this matters much.

For <40Kg children I'm perfectly happy to climb with them on a single 1/2 rope but I only do this to save them (or me) the trouble of dragging a heavier rope up (to) a long pitch. (I'm pretty sure 1/2 ropes are fall test individually with 55Kg to similar standard single ropes are tested to at 80Kg).

Indoors I don't really think it matters for safety and the routes tend to be so short that the weight of the rope is largely irrelevant. I'm pretty sure it matters for wall rules and insurance and that a single rate rope would need to be used.

To qualify this - that is all indoor or outdoor "sport" leading with no serious run outs. By the time any serious trad leading is going on the (child) climber should have their own well informed opinion or shouldn't be leading trad.

Robert Durran - on 07 Nov 2017
In reply to deepsoup:

> I think that goes for the thread generally tbh. It's pretty much all speculation.

Yes, I think what is needed is some drop testing of small children to destruction.

Jamie Wakeham - on 07 Nov 2017
In reply to Robert Durran:

It's the only reasonable way forward.
climber david - on 07 Nov 2017
In reply to pec:

This is exactly right. If you want to prove it, tie one end of your rope to a tree or such like and then pull the other end. You will feel the rope stretch as you pull it, even before you start needing to pull too hard. The harder you pull the more the rope will stretch but even a relatively light pull will still cause some stretch. If you want to compare to something get a static rope and try the same experiment with it. You will get almost no stretch with it, especially with lighter pulls.
deepsoup - on 07 Nov 2017
In reply to climber david:
> You will get almost no stretch with it, especially with lighter pulls.

A bit more than that. Static ropes (the kind sold to climbers/cavers etc.) are required to be a little bit dynamic by the standard they're tested to, you'll get something like about a quarter of the stretch of a single climbing rope.
deepsoup - on 07 Nov 2017
In reply to Robert Durran:
> Yes, I think what is needed is some drop testing of small children to destruction.



Further up the thread you wrote: "Assuming an elastic rope, this is given by the formula: ... "

Perhaps you shouldn't. You're a mathematician aren't you? While you're waiting for diminutive volunteers for empirical testing, here's something to get your teeth into. Enjoy:
http://kristinandjerry.name/cmru/rescue_info/Technical%20Rescue%20Research/Viscoelastic%20Theory%20O...
pec on 07 Nov 2017
In reply to oldie:

> However it seems to be generally accepted that light people get less protection when using shock absorbing via ferrata lanyards, and in fact some manufacturers now produce lanyards adjustable for different body weights.

> Will not the same effect occur in a lead climbing fall with a light/child climber?

As others have pointed out, lighter climbers will decelarate more rapidly and so experience a higher g force, though being of a very short duration the importance of the g force is debatable.
Robert Durran's example above gives a g force of 7 but this would appear to be well within human tolerance for the fraction of a second it is experienced.
This suggests flopping down into a chair generates over 10g
http://www.iaapa.org/safety-and-advocacy/safety/amusement-ride-safety/g-forces

Fighter pilots wear pressurised suits to cope with g's of 8 or 9 but those forces act for a long time and would cause them to black out otherwise.

For very short durations, huge g forces can be survived
from this
http://www.medicaldaily.com/breaking-point-whats-strongest-g-force-humans-can-tolerate-369246

"It’s difficult to calculate the exact level of G-force that would kill a human, because the duration of exposure is such an important factor. There are isolated incidents of humans surviving abnormally high G-forces, most notably the Air Force officer John Stapp, who demonstrated a human can withstand 46.2 G’s. The experiment only went on a few seconds, but for an instant, his body had weighed over 7,700 pounds"

I don't know how valid comparisons of via ferrata lanyards and climbing ropes are, they function in very different ways. A climbing rope approximates to an elastic substance hence my reference to Hookes Law (though probably not exactly because of the differential stretch of core and sheath) whilst a VF lanyard absorbs energy by a series of non elastic stitching failures.

I'm sure there's a PhD thesis in this thread somewhere but I think we can be confident that small kids falling off at climbing walls aren't going to break their backs because their rope behaves like a steel cable
purplemonkeyelephant - on 07 Nov 2017
In reply to flopsicle:

Sounds like I need to put some weight on to get my rope working properly!
climber david - on 07 Nov 2017
In reply to deepsoup:

I did say almost ;) . In my experience pulling on a static rope doesnt give you that much stretch, and certainly much less than you would get with a dynamic rope which is the purpose of my post
FactorXXX - on 07 Nov 2017
In reply to Robert Durran:

Yes, I think what is needed is some drop testing of small children to destruction.

Best get testing before the anti-smacking laws stop such valid and needed research.
flopsicle - on 07 Nov 2017
In reply to flopsicle:
I've been at work and only just got to read the many new responses!

First of all, given the mess my kid has just left in the living room, I would like to volunteer her to be tested to destruction!!

More seriously, I think research would make sense. I have a good friend who is by trade a physicist but acknowledged much is in the land of speculation when I bugged her for advice.

I have no concern that with careful belaying there should be no major harm from standard dynamic ropes stopping a child climbers lead fall. For some comp kids it's multiple times a week, falls practice pushing limits, falls that are unexpected and from less than ideal start points (eg, hands slip during high heel hook) - all across a decade when the body is growing and is fundamentally different to an adult's. More than all of that though, they're kids. An adult chooses their own rope and have opportunity to make informed choice. If I choose something for someone else then it's my job to consider their differences.

In the end I decided to get a second rope. My current Eldelrid Python has a impact rating of 8.9kn. I've bought a 30mtr Tendon Ambition 9.8 with a 7.1kn impact rating (20% less) - it was less than a day's pay for 30m of good rope, no loss if no benefit. Also trying to find research meant I found this: http://bjsm.bmj.com/content/bjsports/41/12/852.full.pdf definitely worth the read!

My kid's only really been after leading indoors cos her team mates do, I can't see her wanting to spend days on end red pointing, it's her hobby not her life. She looks like she prefers outdoors trad and no way she's leading that for a few years!
Post edited at 21:11
krikoman - on 07 Nov 2017
In reply to flopsicle:

You can build some absorption into the rope by tying the knot a little looser, you could also tie a few into the rope itself above the tying in point. Under fall conditions the knots will tighten and absorb some of the force in the system.

Obs. don't tie them too loose But you're cleverer than that anyway.
stp - on 08 Nov 2017
In reply to flopsicle:

> I get really interested in things like this

Have you seen the Day in Nature Youtube channel? They've done loads of videos on falls, some comparing fall types and which are safer. Others are about overcoming the fear of falling. Well worth checking out.

https://www.youtube.com/watch?v=UCnbbOxqwPK2lfkEP3FzIf2Q
Xharlie on 09 Nov 2017
In reply to flopsicle:
Why does a dynamic rope stretch? The clue is in the answer.

The objective of rope stretch is to reduce the effective force on the falling body of the climber (and their harness) when arrested by the rope. A falling 30 kg climber and an 80 kg climber will accelerate at the same rate (air resistance being negligible, here) but the lighter body will have less momentum even if it is descending at the same velocity when the rope catches it. The lighter falling body will stretch the rope less but this is because there is less momentum, not because the rope exerts a stronger force for the lighter climber.

I would imagine that the peak force exerted by the rope on the climber is actually lower for a lighter climber even if there is less observed total stretch. Think of an elastic band: the force increases as you stretch it, assuming it doesn't snap.

EDIT: That said, a more bouncy rope could be "nicer" for a kid even if it has nothing to do with safety. I think a lot of adults don't like extremely stretchy ropes because they're "annoying" but since a light person will stretch the rope less, it might be less annoying. Their trade-off of annoyance vs. comfort might be different.

P.S. can we all agree to use newtons as the unit for force, because "80 kg of force" is meaningless. 80kg is a mass, not a force! I assume that "80 kg of force" would be circa 784 Newtons, i.e. the force that an 80 kg mass would exert against a surface on which it was resting at sea-level gravity of -9.81 metres per second per second but this is just a guess. As I said, it's meaningless.
Post edited at 14:49
deepsoup - on 09 Nov 2017
In reply to Xharlie:
> I would imagine that the peak force exerted by the rope on the climber is actually lower for a lighter climber even if there is less observed total stretch. Think of an elastic band: the force increases as you stretch it, assuming it doesn't snap.

Yes, quite right. Lighter climber, lower force.

But having accelerated to the same speed as a heavier faller before the rope pulls tight, more rapid deceleration as they then come to rest over a shorter distance.

So, subjectively, will the lower force feel like a higher force from the climber's PoV? We don't know.
(And there's very little chance of a definitive answer emerging from this thread before it either runs out of steam or goes all plane-on-a-treadmilly.)

> P.S. can we all agree to use newtons as the unit for force, because "80 kg of force" is meaningless.

'fraid not, no. There's no need to be pedantic about the difference between force and weight, "80kg of force" is meaningful enough, you said it yourself, it's the weight of an 80kg mass on or around the surface of the Earth.

Which is nearly enough constant give or take a bit of height or latitude that it makes no difference to any sums likely to be done here. May as well just round g up to 10m/s/s while you're at it. Any answer coming out of reasonably simple maths will have an "ish" attached way bigger than a couple of percent.
tom_in_edinburgh - on 09 Nov 2017
In reply to deepsoup:
When my daughter was younger I was shown by her coach how to get a more dynamic belay for a small/light person by using your arm to do most of the catch. The forces when a small kid falls are massively lower than in an adult lead fall. The idea was to hold the live end of the rope out to the side a bit when you think the fall is going to happen, keeping it relatively tight to the belay device. Then your hand gets pulled upwards and inwards when the rope comes tight, but because you didn't leave slack between your hand and the belay device it doesn't pull through your hand and you don't end up with a rope burn. You resist the force of the fall with your arm and the result is a bit less than a meter of deceleration for the climber before the belay device is involved - at which point you can also step in towards the wall if necessary.

Before I get flamed obviously this only makes sense for smaller kids.
Post edited at 17:15
krikoman - on 09 Nov 2017
In reply to deepsoup:

> 'fraid not, no. There's no need to be pedantic about the difference between force and weight, "80kg of force" is meaningful enough, you said it yourself, it's the weight of an 80kg mass on or around the surface of the Earth.

What about when we're climbing on the moon?
Robert Durran - on 09 Nov 2017
In reply to krikoman:

> What about when we're climbing on the moon?

We never are.
deepsoup - on 09 Nov 2017
In reply to krikoman:
> What about when we're climbing on the moon?

That's the only way I'm ever going to get anywhere on a campus board.
john arran - on 09 Nov 2017
In reply to deepsoup:

> That's the only way I'm ever going to get anywhere on a campus board.

You should try installing one in a lift, pressing the 'down' button and timing your 1-5-9 attempts very carefully ;-)
deepsoup - on 09 Nov 2017
In reply to john arran:
Sounds complicated. I think I might see if I can get one of those trampoline places to install one.
krikoman - on 09 Nov 2017
In reply to Robert Durran:

> We never are.

Speak for yourself
Scott K - on 10 Nov 2017
In reply to krikoman:

Please don't tie knots above the tying in point for kids. They will end up catching on something or clipping in the wrong part of the rope (one of the strands you have created). Also, for competition climbing, the person should be used to seeing the same thing when they climb in a comp as they do when climbing normally, so they are relaxed.

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