TonyR wrote:pjclinch wrote: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.
TonyR wrote:pjclinch wrote: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.