new tires/steering wheel shimmy

This has me baffled. I just had new tires put on front wheels of '73T. The new tires(Dunlops) are different than the previous tires. At about 62 mph the steering wheel begins to shimmy and it increases as the speed increase to about 70. Above 70 the shimmy is inconsistant, sometimes much, sometimes less. I took the car back to the wheel shop and they redid the balance, I watched, and according to the machine, the tires seem to be balanced up right. The shimmy pattern repeated itself after this second balancing! What's going on? By the way, in the last 2 1/2 years there's new Koni's and ball joints. Obviously, there was no shimmy with the previous tires in place.

FWIW, I had an unexpected shimmy this spring and after some time of denial i took it into my shop for them to look at, and I had reverse mounted the left and right fronts so they where rotating "backwards" according to the "rotation arrow" on the tire. my shop fliped'em around and no more shimmy. have you checked for proper rotation direction?

Had the exact same thing on my 89 Targa. A alignment shop cleared up most of it by doing a balance-on-the-rack (doing a second (fine-tuning) balancing with the front wheels mounted on the car and the car on a rack with high speed rollers simulating the road).

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DW

89 Targa

Until they retired, all local mechanics used to take their wheel and alignment jobs to Bernie Brothers. Some folks just took NEW cars STRAIGHT to the Bernie Brothers. We're all sad they retired. They NEVER had ANY electronic equipment. I watched as my tires were balanced while still on the car. When done, they were PERFECT. The only tools used were a floor jack and pliers. Why can't this be done with the fancy computer machines?

Vibrations around 60 mph suggest (strongly) a tire balance problem, regardless what the computer says. Bearing play can be looked at, etc. but I suspect a tire is still not balanced. Perhaps find someone with one of those old bubble balancers who knows how to use it.

Shimmy is another thing, and suggests looseness in the steering/suspension parts.
FWIW.

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'83 SC

How bout alignment?

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Stephen Schafer 1973.5 911T targa

Even though the tires are new, perhaps they have a flat spot, or have some sort of seperation. What is the date code on the tires?

Jack the tires up a little off the ground, spin them, see if they are symmetrical as the tire goes around...

I had a perfectly nice pair of P7s (almost new tread) where the front had a seperation problem, found out they were really two years old.

Good luck.
Nick

There are cases of bad tires that have defects that prevent them from ever being right. If continual balancing does not make them right, talk to the store you got them from about an exchange.

I worked at an NTB for a year, and saw this type of problem more than you would think. Most likely one (or both) of the tires has an internal belt shift, or seperation. Ask the shop to dismount the tires and rotate them 180 degrees on the wheels, then check the balance. If the shimmy/vibration is still there on a test drive, be firm with them and have the tires replaced-some are defective right from the molds.

Thanks for all the ideas!! I was getting ready to take the wheels off to have them balanced yet again and, in the process, I spun one of the wheels. I noticed that tire seemed to be out of round. So i set up a little jig and one of the tires is perhaps 1/4 inch or slightly more out of roundon one side! The other tire is lumpy but not not as pronounced on one side. Should tires be very round and this is the problem or is some of this normal?

1/4" out of round is waaaaaaaay too much-sure sounds like belt seperation to me-take 'em back for exchange. When you get new tires mounted to your rims, ask to see them when they're spun on the balancer-you'll be able to see right away if they're round or not right away-remember-just because they can be balanced (according to the machine) doesn't mean they're round.