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Motorcycle Safety/Dynamics

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Your REAR wheel does it more often than not

By: James R. Davis

"Steer" - To direct the course of.

When your motorcycle is stable in any course, whether in a straight line or in a curve, it is your rear wheel that is primarily responsible for maintaining that course and stability. Indeed, it's the job of your front wheel to DESTABILIZE the bike in order to change course. That is, your front wheel changes course, your rear wheel maintains it.

How is that possible? Well, I suppose it is easiest to think of in terms of influence. The rear wheel attempts to keep the bike moving in the same direction it is pointed toward because it is directly connected via its axle/swing-arm to the frame of the motorcycle - it does not have motion freedom analogous to a steering head. The front-end provides steering input used to change the path of travel.

[Rather a lot of attention has been given to this article in recent days because certain readers have maintained that if rear-wheel gyroscopic forces are so great it *should* be virtually impossible to 'flick' a fast moving bike into a lean as you enter a turn. Well, what the gyroscopic forces generated by a spinning rear wheel does is to try to maintain the direction of travel of the bike - and, just as with the front wheel, when a change in direction happens the wheel responds with 'precession' and leans at a 90 degree vector to that change. When you use counter-steering on the front of the bike to go, for example, to the right, you press the right grip forward. That causes the direction of travel of the bike to momentarily change to the LEFT which, in turn, is felt in both the front and rear wheels and the result is that both of them lean towards the RIGHT. The harder/faster the counter-steer effort is, the greater/faster that lean will occur. And THAT is why you can 'flick' the bike over onto a significant lean. (i.e., you are using gyroscopic precession.) Actually, that is only a small part of it - it's centrifugal force that accounts for the vast majority of counter-steering functionality.]

When a motorcycle is stable it will maintain its current course until an outside influence or steering input to the front-end results in destabilizing it and a new course is sought that will once again result in a stable motorcycle.

Proof that the rear wheel is directing the course of your motorcycle is easy to come by. Watch any motorcycle that is performing a 'wheelie'. Whether it is going in a straight line or it is in a curve, the motorcycle will continue that course even while the front wheel is off the ground.

The significance of this otherwise esoteric bit of insight should be to cause you to rethink about locking your brakes. For example, it should now no longer be a surprise that if (while going straight) you lock your rear brake and cause a skid that the motorcycle does not simply drag the rear tire along in a straight line - the majority of the motorcycle is deprived of the stabilizing effect of a spinning rear tire and it will try to fall over to one side or the other. On the other hand, if you lock your front brake (while going straight) and cause the front tire to begin to skid, there is every reason to believe that (so long as the rear wheel continues to spin with some speed and you leave the front wheel pointing straight ahead) the bike will continue to stand tall and track straight while you correct the problem (by releasing the front brake lever.)

Indeed, so long as there is meaningful speed and you are moving in a straight line, locking the front brake (for a brief time) is less dangerous than locking the rear brake. Obviously you do not want to lock either brake, ever, but it will happen. Further, we all know that we should not aggressively use either brake while the bike is leaned over in a curve. But now you should know that it is NEVER reasonable to aggressively use the rear brake, and why.



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