drossall wrote: 2 Apr 2023, 12:27am
Jon in Sweden wrote: 1 Apr 2023, 7:09amThe reason I make the distinction is that when on training rides, I'm on higher speed limit roads going at a much higher speed. When I'm in the village, my speed is low and the speed limits are 30 or 40kph. Consequently the risk is far lower for me.
I still find that logic difficult. Helmets are designed and tested for the forces associated with falling off sideways from a standing start. By the time your head hits the ground, it's probably doing about 10mph. Since most of the "interesting" things in a crash relate more to the square of speed than the speed itself, a rough approximation, and making the assumption of a direct impact, says that 40kph=25mph is exceeding the design parameters of the helmet by 6.25 times.
I'd find it much easier to understand people arguing for helmets at low speed and questioning their value at high, just as I'd find it really weird if soldiers were to decline bullet-proof vests in gun battles and then insist on them for tank fire.
The speed thing...
It's not how fast you're going when you hit whatever you hit, but how rapidly you
stop. Mostly what people hit at speed is the ground, and they end to keep moving in the direction they're going horizontally so the KE is scrubbed off slowly and doesn't break things. Take a look at this...
https://www.youtube.com/watch?v=kZZTapcPRSk and think you fast he's going, and with nothing but some leather and padding he's fine, because the (horizontal) energy he had on coming off was scrubbed off slowly.
The ground, however, doesn't let you dig a hole unless it's particularly soft, so you stop
very suddenly, at ~ 12 mph, and that's why helmets are rated at 12 mph.
The going over the rating by going fast would certainly be an issue if you hit e.g. a wall, tree, lamp-post etc. head on, but other than that horizontal speed mainly removes skin. That's not very nice but isn't likely to kill you.
Pete.
Often seen riding a bike around Dundee...
