CTT National Council - Helmets on the Agenda

[pjclinch]

Given that sports cycling seems to generate rather more accidents than utility cycling (despite the signs the idea is to go as fast as possible over the course, and some riders will get closer to the line than others) it's not too much of a stretch to suggest sports riders may be in a benefit group.

And your evidence for that is? Its not much of a stretch to suggest that cyclists generally may benefit from a bit of soft foam between their head and anything it might hit but the evidence says otherwise. So using "common sense" to justify a helmet in any cycling sub-group is not a sound idea. Any intervention should be evidence based otherwise you don't know whether you will cause more or less harm.

You seem to have missed the bit where I went on to say an expected benefit would be in minor injuries and thus not something requiring an intervention.

The oft-quoted 12 mph is the speed you'll get from the vertical gravity-powered component, but it's not the speed that's the problem so much as the deceleration from that speed to rest. In the vertical plane this will almost always be very well constrained by the ground stopping the falling cyclist completely. If our notional TT-ist is doing 40 mph along the way, unless they hit a fundamental constraint like a solid wall they're not actually going to decelerate from 40 to 0 in effectively damn-all space, but they will still decelerate from 12 to 0 in effectively damn all space. So in a lot of cases saying a helmet is useless over 12 mph is misunderstanding the issue. There are further complications in "grabbiness" of the shell sliding on tarmac as opposed to scalp, and twisting motions etc., but it terms of the simple impact absorption the excess speed over 12 mph in the horizontal is not the issue it's widely assumed.

That is an oft quoted claim but a misunderstanding of the physics. You can't just resolve it into the vertical component of velocity and then square it because impact is proportional to the square of the velocity. You need to calculate the velocity vector, square it and then resolve it into the vertical direction not resolve the velocity into the vertical direction and then square it. Its a bit like the counter-intuitive fact that a 90 degree side wind will slow you down (because aerodynamic drag like impact is proportional to the square of your velocity)

Taking your figures of 12mph vertical and 40mph horizontal the velocity squared is (144 + 1600). So the vertical component of the impact is 12/40ths of 1744 - 3.6 times the 144 you are assuming - the equivalent of 23mph not 12mph. You can finesse the calculations with the coefficient of friction of the tarmac helmet interface but even just taking a 4.5kg head mass and a 100G deceleration it still comes out at a vertical force of half a tonne pushing the helmet onto the tarmac so it will be a fairly high friction interface to say nothing of the rotational forces that will create.

It's not the KE when you hit, but how fast you lose it. And unless you stop dead (as you do in the vertical) then you haven't lost all the KE. As I freely admitted it's not going to be that simple, but you're fundamentally over-egging the pudding in thinking that a falling cyclists will stop dead the moment they hit the tarmac.

Pete.

[pjclinch]

Here's a pretty famous TT crash from earlier this year. Note the way the rider maintains some of his KE for some time after the initial impact (look how far he ends up from the bike), though he never manages to dig a hole for himself in the road.

[youtube]jI8iOqgIy5I[/youtube]

[TonyR]

You seem to have missed the bit where I went on to say an expected benefit would be in minor injuries and thus not something requiring an intervention.

Not missed at all. There are lots of expected benefits of helmets that are not there in practice so where is your evidence that your expectations are any more real and/or there are not far more significant disbenefits? Otherwise it's nothing more than idle speculation with the risk that e.g. wearers could be much worse off through risk compensation.

It's not the KE when you hit, but how fast you lose it. And unless you stop dead (as you do in the vertical) then you haven't lost all the KE. As I freely admitted it's not going to be that simple, but you're fundamentally over-egging the pudding in thinking that a falling cyclists will stop dead the moment they hit the tarmac.

I think you should write to the helmet standards bodies then to let them know they are wrong in specifying their tests in terms of impact KE. And it's nothing to do with stopping dead or not. It's to do with the impact energy a helmet is designed to mitigate and the impact energies you might expect in a racing fall. If it were only to do with the vertical height they've fallen from motorcyclists could get away with wearing cycle helmets.

[pjclinch]

Impact KE against an unyielding object though. That's how they do the tests.

How come Sr. Quintana's helmet didn't do quite what you said it would when he came off in the Vuelta TT? Perhaps it's not as straightforward as you make out?

[andy65]

There's little evidence that overall they offer any benefit over a cotton cap, is there? This may be because they encourage the sort of risk- taking behaviour that leads to some people writing several of them off.

Why would a time trialler be going that much faster than if they were rushing to catch their train? This isn't about bunch racing on closed roads. This is on the open roads, with big signs telling you not to take chances like staying in an aero tuck too much (at least on the course near me).

[andy65]

If anyone races for the train like I used to push myself in a time trial, then they must be very late. I used to be physically sick at the end of tens.

[TonyR]

Impact KE against an unyielding object though. That's how they do the tests.

How come Sr. Quintana's helmet didn't do quite what you said it would when he came off in the Vuelta TT? Perhaps it's not as straightforward as you make out?

Because it never hit the ground and instead he rotated over to land on his back not his helmet?

And when I last tried it the road was a pretty unyielding object too.

[pjclinch]

Impact KE against an unyielding object though. That's how they do the tests.

How come Sr. Quintana's helmet didn't do quite what you said it would when he came off in the Vuelta TT? Perhaps it's not as straightforward as you make out?

Because it never hit the ground and instead he rotated over to land on his back not his helmet?

And when I last tried it the road was a pretty unyielding object too.

Difficult to tell whether it hit or not. Say he was slightly less far around his roll and the top of the lid had definitely struck the tarmac, it would have stopped dead? Or, I would suggest, have taken the vertical component force and the rider would have continued rolling because there was nothing much scrubbing off his horizontal KE. That comes slightly later, and his horizontal KE gets scrubbed off along with his skinsuit and probably some of his birthday skin suit too.

The road is indeed unyielding if you try and go down in to it, but "merely" a very uncomfortable one if you try and slide along it. It doesn't stop you dead though.

As for motorcycle helmets being that much chunkier, that's because in an accident you want as much as you can get, but of course there are limits as to what you can lug around before you get there. For a m/cyclist there are fewer issues of overheating and impeding your own progress, so they take more armour. And going that much faster, they're more likely to need it coming up against walls etc. that stop you more in the horizontal than a road does. Downhill MTB and freecross BMX use chunkier helmets, because being mainly gravity powered they can, without impeding forward progress.

None of this makes a case for a compulsory TT helmet though. It may make a useful difference between being able to get on and ride off after a spill thanks to being less dazed, but that's not something a safety intervention is needed for. I'd have thought the main point for a typical TT, hill climbs excepted, was they improve your aero: again, not something you need to require.

Pete.

[TonyR]

Difficult to tell whether it hit or not. Say he was slightly less far around his roll and the top of the lid had definitely struck the tarmac, it would have stopped dead? Or, I would suggest, have taken the vertical component force and the rider would have continued rolling because there was nothing much scrubbing off his horizontal KE.

First what happened would be very difficult to observe because the critical impact period - the time it takes to decelerate from 12mph to zero at 100G over 25mm is 5ms.

Second, lets do a thought experiment. Assume that he does land vertically on the top of his head and instead of relying on friction there was a 20cm high kerb that stopped his helmet dead. Do you think he would have stopped dead with his body magically in a vertical position or do you think all the KE of his body - which does not feature at all in the helmet specification, design or testing - would mean that his body rotated around the kerb contact and continued rolling and sliding down the road? If, as I do, you think the latter then it just indicates that there is more KE involved in the evolutionary dynamics of a crash than the KE of the head and helmet and that body KE effects will most likely mask what is going on with the head and helmet.

[SilverBadge]

The evidence of little benefit comes from whole-population studies looking at serious injuries (minor injuries, being minor, tend to go unreported). A problem with a whole population study is it gives you a very big average, and in a sample like that you'll possibly have groups either side of your mean. Given that sports cycling seems to generate rather more accidents than utility cycling (despite the signs the idea is to go as fast as possible over the course, and some riders will get closer to the line than others) it's not too much of a stretch to suggest sports riders may be in a benefit group.

Though having said that, the sort of benefit they can really expect is not life-saving, but being able to get back on and finish the race. I don't really see that that needs a rulebook entry.

The big question to ask is why wouldn't whole population studies not reflect the benefits that are claimed for helmets? If you take a total number of ksi, slight injuries etc, the digital nature of accident classification will ignore some minor reductions (unchanged classification) and overemphasise others (bottom of one classification to the top end of the one below). The flawed nature of case control studies has been apparent when injuries that a helmet cannot in any way be expected to affect are likewise reduced - it demonstrates that the reference population is "not the same".
This is clearly demonstrated in
J Trauma. 1991 Nov;31(11):1510-6.
A prospective analysis of injury severity among helmeted and nonhelmeted bicyclists involved in collisions with motor vehicles.
Spaite DW, Murphy M, Criss EA, Valenzuela TD, Meislin HW.
For cyclist injuries not featuring a major head injury, non-helmeted riders had a far higher probability of being severely injured than those with helmets. The population of accidents with no major head injury should be the same or more severe for helmeted cyclists than non-helmeted, yet the other injuries demonstrate the converse.

The oft-quoted 12 mph is the speed you'll get from the vertical gravity-powered component, but it's not the speed that's the problem so much as the deceleration from that speed to rest. In the vertical plane this will almost always be very well constrained by the ground stopping the falling cyclist completely. If our notional TT-ist is doing 40 mph along the way, unless they hit a fundamental constraint like a solid wall they're not actually going to decelerate from 40 to 0 in effectively damn-all space, but they will still decelerate from 12 to 0 in effectively damn all space. So in a lot of cases saying a helmet is useless over 12 mph is misunderstanding the issue. There are further complications in "grabbiness" of the shell sliding on tarmac as opposed to scalp, and twisting motions etc., but it terms of the simple impact absorption the excess speed over 12 mph in the horizontal is not the issue it's widely assumed.

However, against our 12 mph constrained vertical fall the helmet is still really looking to mitigate minor injuries (bloody (literally!) painful ones you'd rather avoid, yes, but then that's the same as road-rash hips, legs and arms), and one wonders why that needs a ruling for PPE.

Casualty stats for TTers are broadly on a par with that for "normal" cyclists on roads high higher speed limits, which is where almost all TTs occur.

The existing CTT helmet rule (compulsory for U18s, duty of care yada yada yada) also states "Cycling Time Trials makes no warranties or representations regarding the adequacy of any standard or the fitness for the purpose of any brand of helmet . . . "

Stop Press: Motions for compulsory helmets and compulsory rear lights both thrown out. CTT regulations require 67% for (abstentions ignored), there was also a motion to reduce this to 60% which would have been voted on (75% majority required . . . . ) before these rule changes.

[pjclinch]

Difficult to tell whether it hit or not. Say he was slightly less far around his roll and the top of the lid had definitely struck the tarmac, it would have stopped dead? Or, I would suggest, have taken the vertical component force and the rider would have continued rolling because there was nothing much scrubbing off his horizontal KE.

First what happened would be very difficult to observe because the critical impact period - the time it takes to decelerate from 12mph to zero at 100G over 25mm is 5ms.

Second, lets do a thought experiment. Assume that he does land vertically on the top of his head and instead of relying on friction there was a 20cm high kerb that stopped his helmet dead. Do you think he would have stopped dead with his body magically in a vertical position or do you think all the KE of his body - which does not feature at all in the helmet specification, design or testing - would mean that his body rotated around the kerb contact and continued rolling and sliding down the road? If, as I do, you think the latter then it just indicates that there is more KE involved in the evolutionary dynamics of a crash than the KE of the head and helmet and that body KE effects will most likely mask what is going on with the head and helmet.

I think his head would very possibly be a damn site worse off in that situation than if there wasn't a 20 cm vertical kerb there. Are you suggesting otherwise?

Looking at the crash shows us that we can't rely on simplistic assumptions as to what will happen. But we can see that there will be situations where the absolute impact speed being > 12 mph will not automatically render a helmet specced to no higher than that completely useless. Which is rather my point, and saying greater than 12 mph a helmet can't be of any use ignores those situations.

Pete.

[beardy]

You seem to have missed the bit where I went on to say an expected benefit would be in minor injuries and thus not something requiring an intervention.

Not missed at all. There are lots of expected benefits of helmets that are not there in practice so where is your evidence that your expectations are any more real and/or there are not far more significant disbenefits? Otherwise it's nothing more than idle speculation with the risk that e.g. wearers could be much worse off through risk compensation.

It's not the KE when you hit, but how fast you lose it. And unless you stop dead (as you do in the vertical) then you haven't lost all the KE. As I freely admitted it's not going to be that simple, but you're fundamentally over-egging the pudding in thinking that a falling cyclists will stop dead the moment they hit the tarmac.

I think you should write to the helmet standards bodies then to let them know they are wrong in specifying their tests in terms of impact KE. And it's nothing to do with stopping dead or not. It's to do with the impact energy a helmet is designed to mitigate and the impact energies you might expect in a racing fall. If it were only to do with the vertical height they've fallen from motorcyclists could get away with wearing cycle helmets.

When you crash a motorcycle, it only really becomes a problem if you hit a solid object like a lamp post or a fence (aka as stopping dead). Otherwise you can normally crash at high speed without even hitting your helmet on the deck and if it does hit the deck it normally just slides along and gets a few scratches because the fall off the bike to the ground is pretty minor and not that much changed by your forward momentum.
As somebody who has crashed his motorbike more times than he can count but never hit anything solid in the direction that I was travelling at speed, I would have been fine on each occasion with a cycling helmet or nothing. I dont watch races but if anybody does they will have seen what I mean.

[pjclinch]

The big question to ask is why wouldn't whole population studies not reflect the benefits that are claimed for helmets?

It rather depends what claims are being made: different people are making different claims. Anything lower than reports to police or hospital admission isn't really going to be in the population data set, because it won't be reported and that takes most minor injuries out of your study.

I will claim a reasonable expectation of mitigation of minor injuries (e.g., bumps and grazes) in the event of an accident where the helmet is hit and the head would have been without it. This is based primarily on my caving experience, where I hit my head a fair bit but it doesn't hurt much, against times when I've hit it against hard things without one (typically in the home). This is not really much of a claim, especially as it ignores the chances of being in an accident anyway, and the lack of obvious reason to wear a helmet on my bike is why I don't wear one (technical MTB excepted, where I expect to fall off and comfort is a non-issue). I think most people that do are wasting their time, but I think it highly dubious to suggest they've never been of any use to anybody just because the average use in a population study is zero plus or minus error bars.

Stop Press: Motions for compulsory helmets and compulsory rear lights both thrown out. CTT regulations require 67% for (abstentions ignored), there was also a motion to reduce this to 60% which would have been voted on (75% majority required . . . . ) before these rule changes.

That's a good result, at least!

Pete.

[SilverBadge]

The big question to ask is why wouldn't whole population studies not reflect the benefits that are claimed for helmets?

It rather depends what claims are being made: different people are making different claims. Anything lower than reports to police or hospital admission isn't really going to be in the population data set, because it won't be reported and that takes most minor injuries out of your study.

I will claim a reasonable expectation of mitigation of minor injuries (e.g., bumps and grazes) in the event of an accident where the helmet is hit and the head would have been without it. This is based primarily on my caving experience, where I hit my head a fair bit but it doesn't hurt much, against times when I've hit it against hard things without one (typically in the home). This is not really much of a claim, especially as it ignores the chances of being in an accident anyway, and the lack of obvious reason to wear a helmet on my bike is why I don't wear one (technical MTB excepted, where I expect to fall off and comfort is a non-issue). I think most people that do are wasting their time, but I think it highly dubious to suggest they've never been of any use to anybody just because the average use in a population study is zero plus or minus error bars.

But it's precisely the fatalities and serious injuries that are highlighted by DfT, BHIT, Brake etc as the reason everyone should always wear a helmet except when not cycling. DfT blurb talks about ksi then states helmets are effective across a broad range of injuries. That's about as honest as pointing out that wearing trousers reduces leg injuries compared to shorts and then talking about nothing less severe than falling down a ravine. If helmets aren't putting a measureable dent into the number and severity of hospital admissions then their effectiveness is very limited indeed. And in theory there are fatal impacts that can be reduced to no more than concussion with CE1078.

[TonyR]

I think his head would very possibly be a damn site worse off in that situation than if there wasn't a 20 cm vertical kerb there. Are you suggesting otherwise?

You seem to be deliberately avoiding the point. And that is that the sliding you claim is evidence of no horizontal deceleration of the head is no such thing, Its just a consequence of the body's KE.

Looking at the crash shows us that we can't rely on simplistic assumptions as to what will happen. But we can see that there will be situations where the absolute impact speed being > 12 mph will not automatically render a helmet specced to no higher than that completely useless. Which is rather my point, and saying greater than 12 mph a helmet can't be of any use ignores those situations.

All speculation as helmets are not specified or tested for higher speeds and how they behave and what the consequences are is unknown. But rather irrelevant to the point that in assessing the impact when falling off the bike the horizontal velocity is a significant factor.