There’s a few reasons why we extend our legs to shift our weight back when we’re braking hard but one of the big reasons is down to tyre load sensitivity.
When we brake, about 75-80% of our braking is done by the front brake leaving 20-25% of the braking being done by the back brake. As the brakes are attached to the wheels and the tyres on those wheels are the things in contact with the ground, these ratios also apply to our tyres.
In order to stop us, our tyres have to ‘grip’ the ground and, in physics, this grip is described as friction between the tyre and the ground. The main substance providing friction in our tyres is rubber and the property of rubber that is important in this case is it’s viscoelasticity. If you take a rubber band and stretch it, it will go back to it’s original size and shape as you take the stretch off. However, if you stretch the rubber band and keep pulling, eventually it stops feeling springy and starts to feel stiff. In other words, it’s viscoelasticity has decreased.
As we brake (or turn for that matter) we’re relying on the viscoelasticity of the rubber in our tyres to provide the friction with the ground that allows us to stop (or turn). As the load on your tyres increases however e.g. when us going faster results in harder braking, the viscoelasticity of the tyre rubber decreases and this results in less grip. Need proof?
Take a ball and a rubber band
Wrap the rubber band around the ball and then slide it across a smooth surface.
What you’ll feel is a certain amount of friction between the surface and the rubber band.
Now stretch the rubber band around the ball twice and again slide it across the surface.
What you’ll feel is a lot less friction than the first time as the extra stretch (or load) on the rubber band has resulted in it’s viscoelasticity reducing. What you have is a lot less grip.
If we want to combat this effect, what we need to do is get both tyres doing a similar amount of work rather than one doing 80% and the other doing 20%. This effect is one of the main reasons car makers spend so much time talking about 50:50 weight distribution. The advantage we have on a bike compared to driving a car is that we can shift our weight backwards to balance the load between the two tyres and thus make our braking much more efficient than if we just keep our weight still.
Next time you’re watching a pro rider braking hard, see how far they shift their hips back over the back wheel to even out the tyre load.
If you’re going to take the time to practice, practice like the elite.