wrong ..

in fact vibration at specfic speeds is allmost always wheel balance. an earlier poster has the right idea you need to have the car " wheel force balanced" this takes into account the entire rotating mass , wheel hub , CV and shafts .. i good tech will also be able to tell you if you have one corner showing issues.

 

upon sighting the G-force tread, my first thought was "unpleasant ride."

 

Well OK, it is possible that if you balance a 944 just right, the problem will go away for a while, but it will soon be back as the tires wear down. Either way, this is just the most sensitive car for this type of thing. My 928 is dead steady. I don't think I would bother to get the car "wheel force balanced," I've never heard of a car that required that. And besides, if the problem is intermittent (some pavement surfaces do not induce it), what does that tell me? It cannot be *entirely* due to the spinning of the wheels, it is partly an interaction with the roadway.

 

New tires and a perfect balance will almost certainly make your problem go away. Look over your front suspension good. Make sure everthing is tight. If everything feels tight then get the balance and I am sure it will smooth it out. I have a Chrysler Sebring with 85K miles on it that is doing the same thing. I have not fixed that one yet with a balance because the rims are shot and seep air. 2002 Chrysler junk.

 

As per a 25 yr veteran front end/suspension tech I deal with every day and tech reports from a vehicle manufacturer: First..inspect for any worn front end components, replace if nec. second..perform a road force variance balance, correct mounting issues if nec.

A wheel/tire can come up as needing nothing on a balancer and vibrate like crazy on a car. When put on a road force balancer, it will show all issues with the rim, tire, mounting issues. We routinely correct discount tire's mounting/balancing issues. Customers are always amazed when they come directly from them after multiple attempts and we fix in an hour.

Balancer calibration and a good machine are key.

 

Anyone have a link showing a 'modern' wheel force balancer? I'm using an old Alemite with a strobe but it does the job...

 

Check the play/distance between the stop ring and rebound washer on the strut hats. I don't know about the later style strut mounts but the ones with the stop rings squish/collapse over time/engine weight and exacerbate vibrations if they are worn.

 

I couldn't agree more. There are no design flaws in the suspension. The critical parts are the "pick up" points. The control arm bushings/strut mounts need to be checked. Bushings may look OK and not be. Replacement is suggested if they are over a few years old. Second, the pivot points - Ball joints/tie rods...should be verifiable good. Then good tires on good, properly fitting, rims.
Yes, balance comes and goes with speed - most cars see it around 65mph. It IS basic physics - called constructive interference. it's why the Tacoma Narrows bridge collapsed. Also, prolonged driving in this mode will further destroy all the above mentioned parts like ball joints and tie rods and bushings.
A good tech with experience in these cars and proper equipment are key.

 

I was still having a small vibration start @ 80-90 Mph after my recent rebalance because the sticker lead weight flew off my front tire and I had a crimp-style one installed. After checking everything, I noticed the strut rebound washers were resting away from the stop rings so I put in new strut mounts with polyuerethane and all vibes are gone. Steering is precise and tight too....

 

Where did you get the bushings?

 

Off my lathe. The factory rubber in the mounts felt like foam almost. My CD player skips a little more often, but this is the best front ride out of all the 944's I've owned.

 

Well this is sure interesting. I did not know that these vibrational interactions were normal on cars. What sort of shop would specialize in this type of balancing? My 944 has been doing the shimmy for 11 years, off and on, during which time I have read about it over and over as a peculiarity of these cars. It was better with new tires, but this depended on the tires. It has had a new steering rack, struts, and alignment, with no effect from that. Obviously the tire shops I've used have not done this type of balancing.

 

I concur that blaming the problem on a design flaw is wrong. The more communicative the steering, the more likely you are to notice slight vibrations. I've noticed that the average dynamic wheel balancers don't spin the wheel very fast. It seems to me what may be considered adequate balance at 50 mph will not necessarily be good at 90 mph. I'm sure that's an oversimplification but when I balance my fronts (on the car), I spin them up to at least 100 mph. When I get it smooth there it's usually perfect at slower speeds.

 

If you have custom arms, most aftermarket control arms ef the caster up as the B-joint center is not in the correct position as factory, severely negative.