Motorcycle Safety & Driveability: vulcan shimmy., wheel alignment, shimmy

Question
I own a 1998 1500 vulcan classic.I always check tire pressure, oil ect. before riding. At highway speed ,75mph, bike started to shimmy in a slight crosswind.Slowed to 60 shook bars lightly ,this set up a shimmy that lasted 10 seconds or more.Although uncomfortable it was not uncontrollable.I have had a few high speed shimmys before(hit jackrabbit at 80 ect.)on other bikes. Don't seem to have loose spokes,Where do I start.Bike is garaged and maintened.  

Answer
Robert,

Thanks for the question.

The most common cause of a shimmy is mis-alignment of the wheels, and/or problems with the tires. Usually, incorrect wheel alignment is the culprit. Have you had new tires installed lately? When the wheels were put back on the bike, the rear may not have lined up correctly (since the front turns and is non-adjustable, it is usually not the cause).

If you have new tires on the bike, are they the factory-specified size? A lot of cruiser riders opt to go a size larger for the rear tire for a "beefier" look. However, running tires larger than specified can cause the shimmy you  describe, especially if an outside force (the crosswind) aggravates the situation. Also, jsut getting new tires with a different profile may cause a small shimmy, but that's unusual.

Another thing to check is steering bearing tightness. If you have play in the head bearings, the bike can "roll" around the bearings and shimmy. You'd know if they were loose if when you try to stop, the forks go "clunk" or you feel the bars change position slightly. Again, unusual but not unheard of.

Last of all, does your bike have a windshield? If so, they are a common cause of shimmy, especially at higher speeds (like 75mph). A crosswind could set up a harmonic across the shield that transmits through bike, causing the slight shimmy. You then aggravate it by fighting back against it through the bars. The bigger the windshield, the more likely it is to "vibrate" at a frequency that would cause the shimmy.

More than likely, it's the wheels or steering bearings, maybe both. Have a Kawasaki dealer check wheel alignment and be sure to tell them the problem you are having. They may have seen it before with your bike, your tires, or with other equipment you have on the bike. But from your description, it sounds like the problems is with your alignment or tires. Defintely get it figured out and fixed.

Ride smart,

Bill Roberson