Originally Posted by
Wilfred Laurier
You interpret it as already having the wheel as far from the ground as possible. I think at that point you are pretty far into your journey over the bars. I interpret it as applying so much front brake that your rear wheel has begun to lift, and barring any change, will continue to lift until you have gone around the handlebar. The diagram you posted showed a bike sitting on level ground, but rotation around the front wheel, any amount, is a part of `going over the handlebars. We just generally release the brake when we get to that point.
A rider isn't going to go over the handlebars until such time as the center of mass of the system is in front of the tire patch. If you find your center of mass at the point where the center of mass is at point 3 in the drawing, you have reached the maximum possible deceleration. Anything forward of that is when the rider has gone over the handlebars but not until you have reached that point. I suggest you
try to use the front brake to get your wheel further off the ground than simply losing contact with the ground. It's not that easy to get the center of gravity to Point 3. Yes, if you don't change anything you can get to that point but it's not easy to do.
The diagram is of a bicycle standing on the ground but it shows the different locations of centers of mass. You could easily represent the bicycle's center of mass as a simple point.
Originally Posted by
Wilfred Laurier
First, a nose wheelie can be done in motion - you don`t have to have stopped. Secondly, zero velocity does not equal maximum deceleration. This is another indication of your lack of understanding of mechanical systems. I can`t teach you that here, you need to take grad 11 physics again.
A nose wheelie doesn't
have to be done in motion and the way that you get into that position is to apply enough brake to lift the center of mass to point 3, then back off the pressure before you go over the bars. Secondly, you can't get to a velocity of zero without applying some sort of force to decelerate the system. In the case that Wilson is discussing, you may not even
reach zero velocity before the rider is thrown over the bars. It is only the maximum value of deceleration that you can
attain before you thrown over the bars.
This is also not a high school physics problem.
Originally Posted by
Wilfred Laurier
This statement is exactly how I know you (or the author) are wrong - the maximum theoretical deceleration when over the handlebars is zero.
What Wilson has said isn't that this is the maximum possible to stop the bike, i.e. reach zero velocity. He is saying that this is the maximum possible deceleration before the rider goes over the bars. There's a large difference. You can go past the point of maximum deceleration and be on your way over the bars and the bicycle isn't necessarily anywhere near zero velocity. If you are in a nose wheelie, you have reached the maximum possible deceleration, used it, and have reached zero velocity. But if the bike is still moving forward and you aren't balanced in a nose wheelie, you can reach maximum possible deceleration and still have enough speed to carry the rider forward of the contact patch. Then gravity takes over and it's probably time to call your dentist.
Originally Posted by
Wilfred Laurier
When the centre of mass is behind the front wheel, gravity wants to pull the rear wheel back to the ground, but deceleration makes the rear wheel want to go forward and up. These two forces working against eachother are what makes braking possible when the centre of mass is behind the front wheel.
So far so good. No argument from me
Originally Posted by
Wilfred Laurier
If the centre of mass is directly over the front wheel, gravity is not pulling the rear wheel back to the ground, but it pulling it directly toward the pivot point which does not affect the movement one way or the other, and so the forward rotational force caused by braking has nothing to counter it, so no deceleration is possible. The theoretical maximum deceleration is zero. The practical deceleration is slightly higher than zero as the rider can scoot his weight back a bit before braking, but this eliminates the weight-over-the-front-wheel scenario.
Yes, you are approaching a limit. When you are still approaching that limit, you are approaching maximum possible deceleration. Once you have crossed that limit, you no longer can attain maximum possible deceleration because the rider is now falling.
The rider can either move his center of mass rearward of let off on the brake if they are
behind the contact patch because they haven't reached the maximum possible deceleration yet. But once they have crossed that limit, there is no going back. That's what Wilson said in the quote above and why he talks about "risk".
I've also noticed that you've painted yourself into a corner. Your statement above says to me that you don't believe that the maximum possible deceleration occurs when the rear wheel just leaves the ground nor that "going over the handlebars" happens at that point either.