worf928

John, if you feel more vibration through the seat and less vibration through the wheel then it's more likely vibration coming from the rear. Rear wheel bearing(s), CV joint(s), rear tire balance. If it is a rear wheel bearing you may hear a rhythmic thrumming noise at low speeds (5-20 mph) that effectively disappears as you increase speed due to higher frequency, tire and wind noise. However, that kind of noise could also be tire noise.

The first thing I would do is have the tires rebalanced. (The GTS exhibited the same behavior you have described and rebalancing the tires cured the problem while on the '91 it was a rear wheel bearing causing the problem (and noise.)) If, after balancing, the vibration is still present then proceed to the more expensive and time-consuming diagnostics/repairs.

EDIT: To be absolutely sure of the tire balance you can consider finding a shop with a Hunter Road Force balancer. The Hunter web site can help you locate a shop in your area so equipped.

deliriousga

I'll have them balanced tomorrow. I have a lifetime balancing deal with the local Kauffman Tire. I chose them because they use the road force balancing equipment so hopefully that will take care of it.

Thanks

Mark

The Hunter road force bal is great. The price usually also includes any tire shaving necessary if it is an out of round problem!

deliriousga

Got them balanced today to no avail. Not a bit of difference. I asked the guy to try and shake the wheels to check the bearings and he said everything is very tight.

Torque Tube question regarding this vibration. When I was first looking at the 928 my Boxster mechanic said the two things to watch for are the timing belt miles and torque tube. He said you would feel "wobbling" if the torque tube were wearing out. After I check out the CV joints and bearings for myself, is the torque tube something I should look at? If so, what do I look for?

Thanks everybody!

AO

You could have a bent rim... When you had them balanced did they do it on the hunter road-forace? I believe that has a set that has the mech check the lateral run-out (wobble) and trueness of the circumference (roundness). It doesn't take much.

I really doubt it's the torque tube so let me ask a few more questions.

How long did the car sit? It only takes 30 days for tire to begin to flat spot and usually when it's minnor it will work its way out, but flat spotting can become permanent!

How old are your tires? What brand? You may have a bad tire from sitting. It's amazing how new tires can make an old car feel new again.

Do you feel the vibration in the steering wheel? How about the brake pedal? The vibration could also be from a warped rotor.

Let us know. We can probably give you some other things to look at before you start doing the expensive stuff.

Mark

John - Wally is only about an hour from you. I would consider taking the car to him to check out!

WallyP

John,

I would have said tire problem or balance is most likely.

One possible approach - Be careful!!
- Chock the front wheels very carefully.
- Jack the rear of the car SECURELY. I would put non-wobbly jackstands at the jack points, and leave a floor jack under the center of the rear crossmember, with just a little pressure on it.
- Get a reliable helper who will do what you say with no hesitation. Have them crank the engine, put the car in first (or drive), and let the engine idle, while you make sure that both rear wheels are turning. If only one wheel turns, it turns at twice indicated speed. This is tough on the spider gears, and can explode a tire if you spin it at 240 mph!
- Check visually for out-of-round conditions, side-to-side wobble, etc. on the tires. Also check the rear axle shafts for smooth operation, with no visual wobble. Be careful!
- If all seems well at idle speed, run the wheels to the speed where you normally get the vibration, and repeat the inspections. Be careful!

deliriousga

Andrew - They used the road force and said there are no rim problems. The car sat for about 3 1/2 months while I was working on it. First on ramps in front while I did the TB/WP, etc. and the worked on the basement, then for about 3 weeks with the front wheels off the ground and the rear on ramps while I did the clutch re-build. I think the tires are 3 years+ old. The PO only had it for 3 years and put a little over 3k miles on it in that time so it sat in a storage place alot. The tires were put on just before he bought it and they are Dunlop Z-rated, don't know the model right now. They have tons of tread. When they wear out I'm putting the Kuhmo Ecsta 712 on front and Kuhmo ASX on rear like we have on the Boxster. I like that setup, but I'd rather put the bucks into other maintenance items to "catch up" from the PO before tires.

Wally - I'll try to do that this weekend. I was already planning on changing the CV boots I ordered awhile back so I'll do your tests first, then take everything apart for inspection and re-packing. It is looking more and more like the tires so I'll keep her under the 90 range until I figure it out. If it's the tires, I guess I'll just keep her low until they die so I can keep my other maintenance going and get everything else into perfect shape.

I know the PO never drove it very far. He said he never went more than 100 miles when he took it out. That may have fouled the tires up too.

Thanks for all of the great ideas!

mark kibort

its not only balancing as i said, its roundness. you can spin them to see if they are true.
torque tube can be eliminated by putting in the clutch and waiting for the tube to stop. (also put it in neutral )

mk

AO

Well if it's not the wheels, then my guess is the tires. Now the question becomes which one(s).

Is the vibration coming through the steering wheel, or just in general.

Oh... and how sever are we talking here? Does it feel like the car is shaking apart or is it more like it makes your coffee dance in the glass?

Here's another idea... do you know any fellow sharks in the area? Why not swap wheels and go for a drive? It would eliminate a whole lot of guess work.

worf928

IIRC the Hunter Road Force balancer is not going to let a tire/wheel combo pass wherein one or the other is not-round or true. Assuming that a road-force balance was both performed and charged-for (and not only the latter) then it is less likely a wheel problem at this point. Nonetheless a swap would allow you to be absolutely positively sure.

goliver

You might have a seperated tire. Happened to me once after I had to really get on the brakes hard. Use your spare on different corners to see if you can isolate the offending wheel.

Regards,

Mark

ANY of the suggested tire/wheel-related problems would have shown up with the Road Force balance - PROVIDED it is the REAL Road Force. Done only with the Hunter GSP9700.

Info here -> http://hunter.com/pub/product/balancer/index.htm

deliriousga

Can the spare tire go up to 90mph? The swapping sounds like a good idea. I'll look at the Boxster wheels/tires and see if the sizing works with the 928, then maybe I can do a swap for a test run. The balancing was done with the GSP9700, I've seen it and it's all they use at this garage.

Mark

Boxsters are 205 on the fronts - bu should be fine for testing purposes...