Shaking at 55 - 60 mph, but then goes away once you slow down to about 20 mph?
Hmm - this could be a lot of different things.
First - check tire pressure all around. While down there, check the inside of the rims (on the side not out to you normally - the inside of the rims where mud can hide)
Second - walk back about 20 feet away front and back. Look at the tires, do they look like they are straight up and down? Or slightly angled inward or outward? They should be straight up and down - if not, then you probably bent something.
First gut - mud in the rims. VERY common problem and can be a major pain cause it'll make you think the vehicles is in serious disrepair. If you did have mud there, I would also take the brakes in to be checked. If mud or dirt gets in between the brake pad and the rotor / drum, it can lead to premature brake wear, which could then become a later problem. Many places will do a brake inspection for free, and when they do, it generally gets that dirt out of there.
If none of these, then it could be a slightly worn brake component such as a caliper or wheel cylinder sticking. (Even a caliper slider sticking) this would cause one wheel to grab more than the others, and thus cause some vibrations where depending on how fast you went would become more severe. It's possible but gut still thinks mud / tire pressure as those are more common. The brake thing would also be found during the brake inspection recommended earlier / cleaning :D
Sunday, June 8th, 2008 AT 5:13 PM