Why does it wobble?

By Bill Ballou

It's also my understanding and experience that, it happens mainly on deceleration but it can also happen due to road surface irregularities, driving conditions and other mechanical considerations. Although it's obviously important that steering head bearings, leading link bushings, tire pressure and alignment be adjusted properly, the real reason for front-end shake, has to do with basic physics. With respect to sidecars, the need for a steering damper is really governed primarily by trail and the mass of the sidecar and the location of that mass. The're an easy way to visualize the dynamics behind front-end shake. The next time you're pushing a grocery cart through the store, stand still directly behind the cart and twist the cart slightly to the left. You'll see the front wheels twist counter clockwise. With a sidecar outfit this would be the first period in the oscillation of the wobble. What happens with a sidecar when you decelerate, the mass of the sidecar tries to push the front of the motorcycle to the left. Like the front wheels of the shopping cart, the contact patch of the front tire of a motorcycle trails the actual steering axis of the motorcycle, so when the front of the motorcycle/sidecar is pushed to the left under deceleration, it tries to rotate the front-end counter clockwise. But because there is also forward motion the next period in the wobble begins when the self centering effect of trail takes over and tries to straighten out the front-end. It whips the wheel back in a clockwise direction and depending on the amount of mass and rate of forward motion, the wheel goes past center in the opposite direction and the whole cycle starts over again much like the pendulum in a clock. The longer the trail is, the more leverage there is to twist the front-end under these conditions and the lager the oscillation, the less trail, the less leverage their is to cause this reaction and thus smaller oscillation. Bottom line is, the less trail there is, the less mechanical advantage there is to effect front-end wobble. If it were possible to drive a sidecar outfit with zero trail it's very likely it wouldn't shake. A steering damper simply slows down and controls the rapid acceleration of mass necessary to induce a wobble. If tightening the steering head bearings solves the problem, I would argue the bearing are tighter than necessary, and in effect acting as a damper. The need for a steering damper in nearly all cases is a reflection of the amount of trail a specific sidecar setup has and the use of one should not be viewed as a cover up or in any other negative way. In my opinion, a damper should be view as a necessary safety device. After all the damper most of us use is original equipment on a Volkswagen. Many solo bike come from the factory with steering dampers as well. For similar and other reasons.

A much better explanation of this topic can be found in the book Motorcycle Handling and Chassis Design by Tony Foal. Someone I consider to be one of the most knowledgeable people on the subject. His web site is http://www.tonyfoale.com/

Bill

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