Originally Posted by
Yellowbeard
Because the rider's center of gravity is higher and further forward. The force of braking acts at the front wheel contact patch, parallel to the ground, creating a moment around the center of gravity that tends to tip the bike forward. Under normal braking that moment is countered by the shifting of weight from the rear wheel to the front, which generates an opposing moment.
Yes, the rider center of gravity is shifted forward but that has no effect on the rate of deceleration.
If the rider does nothing, the deceleration will remain the same regardless of the height of the rear wheel. Moving the CG forward increase the risk of going over the handlebars because it is easier to lift the rear wheel but it doesn't cause the deceleration to decrease.
Think about what happens when you start down a hill. Depending on how steep the hill is, a bicycle can easily have a vertical differential of 2" to 6". Does this mean that the bike's ability to slow down is less than on level ground? Of course not. If what Wilfred Laurier says is true, a bike headed down an incline would reach a point where it couldn't be stopped.
Originally Posted by
Yellowbeard
When the rear wheel lifts off the ground (assuming the rider stays in the same position relative to the bike) the moment from braking force gets stronger because the C of G is higher (as long as braking force stays the same) and the opposing moment gets weaker because the C of G is horizontally closer to the front wheel contact patch (assuming the rider doesn't spontaneously take on or jettison some form of ballast).
The CG of a bicycle rider is located around the mid-torso of the rider and moves forward but not up when the rear wheel lifts. But that still doesn't cause a decrease in deceleration. Moving the CG forward increases the normal force...i.e. gravity...on the contact patch. With the CG behind the contact patch, the rear wheel can still be pulled down to the ground if the brakes are released which means that there is still a part of the normal force pulling the rear wheel down. If the CG is directly over the contact patch, the normal force is at it greatest on the contact patch. That's why (some) cyclists can balance in a nose wheelie.
But none of this has any effect on the rate of deceleration.
Originally Posted by
Yellowbeard
If braking force doesn't change, the only way a rider isn't going over the handlebars at this point is if they are going so slowly that they don't have enough kinetic energy to lift their center of gravity past the front wheel contact patch (or if the front wheel skids for some reason, but I'm not arguing about that).
You and Wilfred Laurier assume that the bicycle is going to naturally stop when it gets to the point of a nose wheelie. It won't. The rider has to feather the brakes and delicately adjust the CG to avoid falling forward. If the braking force doesn't change and
if the rider is over the deceleration needed to put them over the handlebars, there is nothing that is going to stop them from falling.
Originally Posted by
Yellowbeard
You seem to be assuming that when someone brakes hard enough to lift their rear wheel they release the brake, and arguing against people who are assuming that braking remains constant throughout.
No. You and Wilfred Laurier are assuming that someone who lifts their rear wheel is releasing their brakes as they approach the pitch over point. That's the only way you can balance in a nose wheelie. If you maintain constant braking and are over the maximum possible deceleration, the rider is going to pivot around the CG over the front wheel and do an endo.
To be clear, a
reasonable rider will start to release the front brake when the rear wheel starts to skid. Part of the reason to use
both brakes is that the rear wheel skidding is an indicator that you are headed towards going over the bars. Mountain bike riders know that when the rear wheel skids, you get off the
front brake to stop the rear wheel lift. This is because they tend to ride on inclines where the CG is further forward and they have less deceleration to work with before they go over the bars.