All mechanical things have natural vibrations, it's just a characteristic of the physical system that's made from masses, springs, and dampers. Motorcycles of course are subject to this effect too with the extreme effect being a tank slapper. Two considerations that control a front end shimmy are the tightness of the steering head bearing and steering dampers.
Since most bikes don't have steering dampers it's extremely important that the head bearing is properly adjusted. Even so, the possibility is always there, it's just waiting for the wrong surface conditions, a series of bumps, in a turn, and so on. Since I returned to biking I've read of a few incidents that made me aware of this issue. Not that I want to say that any bike with a little vibration will eventually have a tank slapper, but that a rider should be aware of any vibrations getting uncharacteristically worse.
__________________ As much as I've tried to get lost, everywhere I went, there I was. |