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Sliding Rear Tire - Brake Not Locked
Highside NOT Likely

By: James R. Davis



It has been said here (exhaustively) that I consider there never to be a time when aggressively using the rear brake makes sense. I have used as my argument the high likelihood that should you do so you will probably lock the rear brake and, if you then release that brake (and sometimes without releasing it), potentially cause a highside.

That your rear wheel, if spinning, provides gyroscopic stability for about 80% of your motorcycle has also been repeated several times. I do not mean to suggest that gyroscopic precession provides roll stability. It can cause yaw movement of the front wheel, but not the rear wheel. Since the rear wheel is constrained by its attachment to the bike's frame, whether spinning or not, it does not provide yaw movement. What it does do if locked, however, is make the restoring force generated by trail become ineffective in maintaining path of travel stability. Thus, if you lock your rear wheel you have made your bike essentially unstable. (Gyroscopics provide no lean angle stability. Indeed, only a change of travel direction and speed (if in a turn) effect lean angle.)

But what I have failed to discuss is how to handle a rear-wheel slide if you have not stopped its spin. For example, assume you are in a curve and you encounter some sand that causes your rear tire to break away. What do you do about that? Is all lost? Must you end up riding the ground instead of your motorcycle as a result?

First, I want to make a few comments that might seem out of place in this discussion.

  • Over-controlling your motorcycle is dangerous.


  • Over-reacting is similarly dangerous.


  • Your motorcycle, more times than not, attempts to correct instability by itself requiring NO INPUT WHATEVER from the rider.


  • Centrifugal force does NOT push you away from the center of a curve.

Virtually all riders know that if a bike begins to slide in a turn you should turn your front wheel in the direction of the slide. What too few riders seem to know is that *YOU* don't have to do anything and the bike will, of its own accord, turn the front wheel in the direction of a slide. Your only real job is to not inhibit that self-correcting effort by the bike.

Should you try to 'steer into the slide' and either over- or under-shoot the amount of turn required to offset the slide you place the bike into an even less stable configuration. In other words, over-controlling is dangerous.

Similarly, over-reacting to a bit of instability almost invariably makes things worse. When you ride over rain grooves and your front-end becomes squirrelly, if you put a death-grip on your handlebars you merely cause the instability of the front-end to be broadcast through your arms into the rest of the motorcycle. If your rear-end squirts briefly to the side (slides) while in a curve, corrective action on your part can turn it into a disaster just as easily as it might 'cure' the problem.

While the rear wheel continues to spin there is essentially no danger that your bike is going to fall down - gyroscopic forces are tremendously strong. Further, unless your slide is the result of hitting an oil slick or ice, you have not LOST traction, just diminished it. You are still able to accelerate (or VERY MODESTLY decelerate) while in a slide.

And though it certainly feels like centrifugal force is attempting to push you away from the center of a curve, in fact what it does is attempt to make you go in a straight line tangential to that curve.

Thus, as your slide progresses there is less and less centrifugal force at play. That means that more and more traction is becoming available to the tire. In other words, if you do NOTHING (other than allow your front-end to steer itself in the direction of the slide), the odds are overwhelming that the slide will end of its own accord.

There are three things that you could do:

  • Slow down - WRONG, WRONG, WRONG - this causes weight transfer to reduce what traction you have left and the bike will almost certainly end up on its side.


  • Nothing but allow the front-end to steer itself into the slide - works most of the time and requires no skill whatever.


  • Modestly accelerate - increases rear-wheel traction and shortens the slide - but requires a gentle touch (skill).

Honest! The best course of action for almost anybody is to let it slide.

Look at any motorcycle race film and you will observe that 100% of the turns are negotiated with the rear wheel sliding! Nothing magic about that, now that you know what's going on. Right?

Aren't you glad, now, that your motorcycle generally has over-steer rather than under-steer built in? There is essentially no 'fix' if your front-end slides out from under you while in a curve. But if the rear wheel continues to spin, it can slide a little and you can continue your ride, a little more 'puckered' but none the worse for it.

Copyright © 1992 - 2018 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
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