Keep an eye on Cyccommute's posts - he presents some very... interesting opinions on how a bike behaves during braking. I generally don't bring it up except to debate him. It is one of my oldest and most treasured relationships.

Turns are tricky. If you enter too fast, you're likely to go wide. If you grab front brake and the bike "stands up", you're likely to go wide. If you try to add more speed, you're likely to go wide. What CAN you do?

The typical answer, short of "don't get into this situation in the first place," is to apply just enough brake to slow the bike and cause it to "fall" deeper into the turn.

Je connais bien.

Be careful of the word "never". It leads to a confidence level that is usually unwarranted. A rear brake won't make a difference if you don't know how to use it. Otherwise, you're just plain wrong. (You can argue the point till the cows come home but I've got the upper hand, I'm stupid stubborn.)

Okay never yet. But a rear brake didn't magically stop you it slows you down just like my fixed rear cog

Bonne. Merci.

And I have explained to you your error in thinking that the "pitch over" point is when the rear wheel just leaves the ground. A "pitch over" can also be called an endo, a header (from the days of the ordinary), a face plant or a few other colorful things. They all describe crashes in which the rider leaves the bike by going over the handlebars. To do that your rear wheel has to be quite a bit further in the air then the instant your rear wheel has zero force on the ground. That's a skid if the rear brakes have locked the rear wheel and isn't nearly as disastrous as a faceplant.

Go find and read Bicycling Science by David Gordon Wilson.

You are right for the rear wheel. But you can't skid a front wheel on a single bicycle. The bike and rider will rotate around the front hub before the front wheel will slide. We don't have enough mass and our center of gravity is too high to reach the limit tire-to-road adhesion.

The other bit of the puzzle that people miss is that it takes some time to lift the rear wheel off the ground if you are trying to reach maximum braking. During the period the contribution of the rear tire is not zero. Not using the rear brake is throwing away about 20% of your deceleration up to the point where the rear wheel leaves the ground.

Even ideal weight distribution just changes how hard you can brake before the rear wheel leaves the ground and therefore how fast you can stop.

From Jan Heine's bicycle quarterly article on braking

http://bikeforums.net/attachment.php...hmentid=454448

I have two of my bike's set up with front brakes only. I have been running front brakes only on both these bikes for over 10 years with no issues.

Would it be acceptable to refer to that moment when the front tire stops turning a skid? I don't know if you're right about not being able to skid the front wheel. I'm feel certain that I've skidded in the past... or maybe I just $hit my pants.

The "it hasn't happened therefore I'm safe" argument. Don't take it to the bank.

Imagine a rear brake and your fixie? Huh HUH? That'd be something!

I wont pester you anymore, I'm jelly.

Sheldon did list some exceptions to front braking in the article. The front brakes only is for dry ground.

• Slippery surfaces. On good, dry pavement, unless leaning in a turn, it is impossible to skid the front wheel by braking. On slippery surfaces, however, it is possible. A front wheel skid almost always leads to a fall, so if there is a high risk of skidding, you're better off controlling your speed with the rear brake.

So you actually proved Sheldon Brown correct.

And for the Tandem riders. They are an exception to a lot of road biking rules.

And Sheldon's articles are aimed at road bikes, so I'm sure that MTB's have a lot of exceptions to his rules when on the trails.

GH

Phil_gretz: ?same brake / front? used, when holding a cell-phone???
I would think that most people eat with the same hand that they hold a phone.

Please quote the section from the book that defines the term 'pitch-over point'.

I won't quote the whole page but he goes through the calculations for determining the deceleration on a specific bike and rider. At the end of the discussion he say:

Bicycling Science, 3rd edition, David Gordon Wilson, 2004, p245. Here's a (very) crude reproduction of his figure

http://i144.photobucket.com/albums/r...psad49fda8.jpg

As you can see "point 3" is over the front wheel. "Moments of torque around..." implies rotation about that point, not simply lifting the rear wheel. Simply lifting the rear wheel isn't "going over the handlebars". His language is very clear.

No. Stopping a wheel from rotating is different from a skid. A skid is sliding friction. You can slide a wheel out during a turn but that is different from a straight line skid of the front wheel.

Again, you have provided your interpretation of the text (or a figure in the text), and you obviously misunderstand the text.

When the rear wheel is lifted, the rider has begun going over the handlebars. If nothing changes (rider keeps the brakes on with the same force) then he will continue rotating around the front wheel. The diagram you created does not show the centre of mass over the front wheel, just the point about whicht he rider and bike are rotating, and that point is the same regardless of how far the rear wheel is lifted.

THe problem with your understanding is that deceleration when the centre of mass is over the front wheel is impossible. The maximum theoretical deceleration when balanced n a nose wheelie is zero. In order to stop from rolling forward in a nose wheelie the rider must shift his weight and rotate slightly back. Your failure to understand this indicates a major and disturbing misunderstanding of physical systems in motion.

That's not what I asked. But have a nice day.

I provided you with a quote. There is no "interpretation" on my part. That is what Wilson wrote. It's pretty hard to take a statement like "...before he risks going over the handlebars..." and interpret it any other way.

I doubt that you would find anyone else who agrees with your interpretation that lifting the rear wheel is equivalent to "going over the handlebars". Lifting the rear wheel on just about any bike is fairly easy...every time you slide the rear wheel you have lifted the rear wheel off the ground. I don't think anyone who has actually experienced "going over the handlebars" would equate sliding the rear wheel with doing an endo. Having done lots of both, I can tell you that they aren't the same.

Actually braking hard enough to get the bike into a situation where the rider is "risk[ing] going over the handlebars" is an altogether different situation. To put the rider over the bars (and carry the rear wheel with the rider) requires lifting the bike and rider into the air. It's almost something you have to want do on level ground or you have to stop the wheel immediately.

Granted, you are correct that "if nothing changes" the center of mass of the system will continue rotating around point 3. But most people aren't that ham handed and will back off on the brakes as the rear wheel lifts off the ground. But lifting the rider and bicycle's mass to the point where the whole system rotates around point 3 isn't that easy. If it were, we could all do nose wheelies all the time. Getting a bike to the point where you are doing a nose wheelie is very difficult.

If you are balanced in a nose wheelie, the bicycle has stopped, then you have reached the point of maximum possible deceleration. If you go past that point, you transition from stopping to falling.

But that is not what Wilson is saying. He isn't talking about "balance" but pitch over (aka face plant aka endo aka header aka dental appointment aka head trauma). That isn't implying that the rider stops just before pitch over. His calculations show a theoretical maximum deceleration but not a real practical one. He sets the load on the rear wheel to zero which will happen when the wheel lifts off the ground but you are missing the moments of torque (i.e. rotation) about point 3

You have your own failings. The first, and foremost, is not understanding what "going over the handlebars" means. Your interpretation of going over the handlebars is hardly "risky".

That's exactly what you asked. If you were asking something else, it is unclear what you are asking.

IDK. Let's put it out there. I've added bold for emphasis. I asked, "Would it be acceptable to refer to that moment when the front tire stops turning a skid?

You responded, "No. Stopping a wheel from rotating is different from a skid."

The "moment a wheel stops" and "stopping a wheel" are two different things. Sorry if I confused you.

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.


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.


This statement is exactly how I know you (or the author) are wrong - the maximum theoretical deceleration when over the handlebars is zero. 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. 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.

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.


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.


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.

So far so good. No argument from me

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.