tuco wrote:The 25km/h was most likely set by politicians or a committee and not people who know what they're talking about.
It's to protect you from the fall. You're falling on to pavement not riding into a brick wall.
MJF read the current Australian Cyclist magazine, there IS definite evidence that helmets prevent serious head injury.
The literal, physical, attribute of a helmet is that it reduces the G force that your head is exposed to in the event of striking a solid object - such as the ground, or a vehicle windscreen.
If you are hit by a car, you are likely to be scooped onto the bonnet and windscreen. Some of the energy will be absorbed by the bonnet collapsing (for modern design cars), some by the windscreen braking, and the driver will be braking at the same time. The helmet helps you if your head hits the windscreen, or if you fall forward onto the road after the collision. The effective *impact* speed for your head will hopefully be less than 20kp/h, but even faster than that it's better than nothing.
e.g, struck from behind by a car travelling 50kp/h, you are travelling 28kph, the car will hit your bike & legs first, you fall onto the bonnet, hit the windscreen at an effective 16-18kp/h.
e.g, riding at 55kph through a bend, hit loose gravel on the road, tyres slide, you fall about 1.2m out of the lean directly onto the road. Yes it's going to hurt, and you're going to lose a fair bit of skin - but you may be saved from severe head injury.
tuco wrote: She reckons she only let go of the brakes for a second or two so I've told her to be more careful, keep the brakes on...
Pulse the brakes, don't drag them. I.e, break for a second, let go, slow down again, repeat. If you hold them on for a long descent, you can overheat your rims and lose your brakes/burst your tyres.