greg928GTS

As many of you know, I recently installed new 18" wheels/tires. They were new take-offs, and came from the factory with factory air and balancing. When I first ran them from my mechanics shop back home, one or more of them were badly out of balance. I took the car immediately to a tire shop that has the Hunter Road Force machine and had them balanced. It made a great improvement.

I've made a couple of trips with the car and put about 800 miles on it since that balancing, and recently started noticing a vibration that comes and goes rythmically, every couple of seconds, while driving along between 70 mph and 110mph.

It's the type of vibration that is like going over a rumble strip, only not as loud, but that speed. It makes looking in the rear view mirror fuzzy, then smooth, fuzzy, smooth, every two seconds. I just had the tires balanced again, and it made no improvement. It doesn't really get any worse as you go faster, but the rythym is gone over 110mph or so. I can't really say that I noticed the rythym changing speed as I increased speed. Hmmmm, I"ll have to pay more attention to that next time. Just a general vibration from 110mph and up.

Had it up to 145mph today. The car tracks fine, handles fine at that speed. put 60 miles on it today, with lots of over 100mph runs.

Any suggestions for what to check next?

S4-on-your-back-door

motor mounts?

jon928se

Check the bump stops for the rear hatch - the rear wing sheds vortexes at around that sort of speed and can cuase the hatch to bounce if the stops aren't adjusted. Took me a few wheel balances and then driving in a strong wind (moved the speed at which vibration occured up, going downwind and down, going upwind) to figure that one out.

worf928

Symptoms sound similar to those I had when a rear wheel bearing was on its way out.

I was able to isolate the sound by removing all the stuff from the hatch area - carpet, spare, tools etc. and then driving around while CarChick crawled around in the hatch area listening...

greg928GTS

That's interesting. I had both rear wheel bearings replaced when I had the rear wheel studs replaced. Do you suppose one of them was bad, or installed incorrectly?

This is exactly the type of thing that I would think would make the vibration that I'm feeling.

I'll check the rear hatch too, but I think it's very tight.

hupp

Rear axles and cv's ok?

Mike Frye

Greg,

I have a similar thing with mine, although I haven't gone 'through' the range to see if it stops at some higher speed (in Joisey we have traffic EVERYWHERE). I start to get a da-dum-dum, da-dum-dum at about 85 mph. I feel it in the whole car and haven't isolated it to anywhere in particular.

Could it be something in the suspension or alignment of the car that has a harmonic vibration at that speed? Anyone?

ROG100

I will go for MM as well.
Have you had them changed? If not they are way overdue.

chaadster

Greg,

I would suggest putting on the old set (or another set) of wheels to start narrowing things down. If it doesn't repeat the vibration with the new set, you know you have a wheel or tire issue; if it does, you've at least eliminated the possibility of tire/wheel trouble.

Mike Frye

Chaad,

Good call.

Rog,
Are the MMs a possible cause of this? If so then I might have to take a high speed run tonight. I did the MMs and pan gasket over the weekend just because they were due, didn't think this could be related.

Wouldn't that cause a vibration at the same RPMs in every gear though, and not just at a certain speed?

ROG100

On my GTS it started at higher speeds and I could feel the vibration through the pedal.
Once they settled totally I could feel a vibration between 1800rpm and 4000rpm.
MM would also be an easy check - does your engine rock when you blip the throttle?
Chaad's suggestion will iliminate the wheels as a possible cause.

SharkSkin

New wheel bearings? Did they heat the knuckle and freeze the bearing or did they hammer it? If they hammered, they could have damaged the races. As Chaad says, eliminate the tires as a potential cause. It's possible that you have two wheels vibrating at different resonant frequencies that you don't really notice, and that's setting up a harmonic vibration that comes and goes.

Bill Ball

I have had the same subtle, rhythmic vibration, although between 85-100MPH, with a period of 5-6 seconds, twice after new tires were installed. In both cases, I had the tires rebalanced at a different shop and they found one rear wheel off 1 ounce in both cases. That resolved it. The rhythmicity was intersting and I don't have a good explanation for it. Another time one of my rear wheels was found to be off an ounce, the symptoms were more classic - a steady vibration at 100+ MPH.

AO

I'm going to go with tires/wheels. When you had them road force balanced, did you happen to note the lateral and radial runouts?

I'm with Chaad on swapping to a different set of tires/wheels and seeing if the problem persists.

One other thought... I remember a rennlister who came by my house once and we were going to check his front wheel bearings. When I went to take his wheels off, I noted that he had severly over torqued the lug nuts. Luckily there was no damage, but that's a good way to introduce some wobble into the wheel. Make sure the wheel lugs are at 95 ftlbs.

Sharkbait

I had this exact problem. It was the steering rack. Fixed and the problem went away.