The inner cv joints on the two axle half shafts will cause a shimmy in the steering wheel when under acceleration and usually at speeds up to 35 mph, not when just cruising at a steady, higher speed. Struts and shocks have a different feel from a bouncing tire. Warped brake rotors are real common. New rotors can warp too but they are usually fine after one machining. You will feel very little shimmy from them while cruising. They are mainly felt when braking and the brake pedal MIGHT pulsate up and down depending on how the rotors are warped. A misalignment will cause unusual tire wear but not a shimmy.
For some things to look at, start with a broken belt in a tire. Real bad ones will show up on the tire balancer, however, they have protective hoods that must be lowered for the tire to spin so the mechanic might not have seen the tire tread wobble unless he knew to look for it. It is also possible to have a broken belt that can not be easily seen because the tread wears flat. You have to watch the grooves in the tread for signs of irregularity. Rotate the tires front-to-back. If the feel of the shimmy changes, suspect a tire belt. A good tire and alignment shop can find that. Most shops also have a "road force" balancer that puts about 700 pounds of force on the tire to check for irregularities. Inspect the engine mounts. If the rubber is deteriorated in one of them or if the metal insert in the middle is broken it will allow that end of the engine to sag. That can severely change the angle of the half shaft and cause the shaft to hit the housing of the inner cv joint as it rotates. The clue is how fast the shimmy occurs. If you feel it about once per tire revolution, suspect a tire. If the problem is related to a half shaft, the shaking will occur three times per tire revolution.
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Wednesday, March 9th, 2011 AT 4:59 PM
