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Does NOT provide stability

By: James R. Davis

Like other folklore, a popular misconception is making its round within the motorcycle community that is simply wrong - and dangerous to you should you believe it - that speed provides a motorcycle's stability. The impression conveyed by this is that gyroscopics (precession) keeps the bike upright.

Indeed, even the MSF teaches this inaccuracy to its students during the first day of range work. So how, do you suppose, can they explain the following fact?

  • If you lock your rear brake while traveling at 100 MPH your bike will fall over.

Speed is NOT what provides stability (or ease of balance) to a motorcycle. Nor is it the strength of the gyroscopic force generated by your spinning rear wheel. Instead, it is the stabilizing force (known as the restoring moment) provided by the trail of the bike's front-end.

Stability, at least to me, is lack of wobble - it is, in other words, the ability to maintain a path of travel. Many, I know, are of the opinion that stability is somehow related to a bike's roll (lean angle.) Gyroscopics can affect only the steering head angle of a motorcycle's front wheel, not roll (lean) angle. A bike's lean angle is determined by a change of direction of travel or, in the case of a bike that is not traveling in a straight line, its speed.

Since that spinning rear wheel is constrained from changing the direction it is pointing to relative to the rest of the bike, it's rotation cannot effect direction of travel, thus it plays no part in affecting the bike's stability.

But since the front wheel can point in a direction other than path of travel (because a tire can provide a slip angle), any gyroscopic effect that tries to change that steering angle affects stability.

Should you lock your rear brake, thus stopping the rotation of the rear wheel, your bike will fall over in a matter of seconds - far less time than it takes to stop when traveling at any meaningful speed!!! Any steering anomaly (such as provided by roadway surface changes) begins this process of yawing the rear-end and is what causes your rear-end to slide out from under you. Once it starts to fall to one side or the other there is nothing the direction you are looking at can do to cause the bike to change gravity and try to fall UP rather than DOWN.

Gyroscopics play no role in maintaining your bike's upright position - balance does that at low speeds, and the restoring force from trail does it at higher speeds as a result of trying to keep your bike moving in a straight line. This is indeed fortunate, because if gyroscopics did maintain a bike's vertical orientation at speed, you could never change directions- certainly not 'flick' the bike into a turn.

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

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