I have a '79 911SC that I bought a few months ago. I had it lowered last week, and it has a bad shimmy around the speed limit. I'm hoping that it only needs tires and wheel balancing. The tires have a lot of tread, but are not new. I bought this car with 29k miles on it. It's basically immaculate for 27 years old, but there's no telling what parts may be just old.....
Any ideas?
Thanks,
Joe

 

Well, its possible that the tires just need balancing, but my overall thought is to get the car looked at quickly. Its possible that lowering it exposed a weakness in a joint or bushing that needs attention. Its not good to delay service on suspension parts that need attention.
First, I'd ask the shop that did the lowering what they think.

Next, I'd do some thinking: did they remove the tires to do the suspension lowering? if so, then it is possible wheel balance is all it is. I assume so. Remember, tires that were balanced on the car also balance out any descrepency in brake rotor imbalance, etc. Its rare, but you get situations where the rotors or something on the car is slightly imbalanced, so you mount the wheel -- also imbalanced maybe -- and its just luck whether the imbalances add up to a real large imbalance on one side and the technician has to seriously work at adding weights, or whether they cancel out (one on one side, one on the other). Regardless, moving a wheel and the putting it back on would of course make it random chance whether the wheel's and rotor's "imbalances" added up like they originally did when they were balanced. This happened a lot more back in the days of drum brakes - drums that had been turned often would be really out of balance: that is why it was so common in the 60s to see tire shops balance your tires "on the car" whereas today most shops balance them off the car .

Anyway, this could be the cause, although I suspect it isnt. First, I would not drive the car at high speeds until I had this fixed. I would start by get the tires balanced, very well, if possible with an "on the car machine" if you can find a shop like that but one way or the other. I'd make a point to ask the technicians if they had to add any balances - if they did then the tires were a problem (maybe not the problem, but a problem). Then test drive it over a wide speed range (not tellling you to break the law and speed, but I would) to see if a shimmy is there at any speed now). If balance was not the problem then I suspect a bad bushing that the lowering has exacerbated.