petercherry

1990 928S4 - I hve building vibration felt mainly throught he floor at higher speeds (over 140kph) that is exacerbated under braking. Upper control arms are new, disks new, bearings and TCA's new, steering rack and mountings OK, engine mountings and torque tube Ok, anybody got some ideas out there. My local Porsche workshop has tried abaout everything so far that we can thing of. Wheels and tyres new and balanced of course.

Sharkbody

Have you had a road force analysis done?

AO

Yep... Road force balance is the only way to go if you're going to travel at those speeds. Ask your tire shop if they can do this for you, if not, go to a place that can. Go to http://www.gsp9700.com/ and do a search in your area. When they do the balance make sure they use the face plate on the front of the wheel. I was told by the manager of my local tire shop that it's the only way to get a reliable balance - but most technicians don't bother because it's a PITA.

The great hing about this system is that it will measure the runout (Radial & Lateral) of the tire and the wheel separately and tell the tire shop how to match the tire to the wheel for best results. It's a superior balance in every way as long as the technicians use it properly. Remember... face plate!

Nicole

I once had a Renault Fuego that could have been called a vibrator. The aluminum rims were not exactly round, so whenever I bought new tires I had to have the assembled wheel grinded to become round. After grinding it, it was smooth for at least some time. Sometimes it got uneven again after a while...

BIMMERMIKE

' Remember... face plate!'
Please explain or show what you mean by 'face plate'
Thanks

AO

Read this. It will explain everything. Basicaly you should opt for what's called back cone mounting, but because it's such a PITA for the tech they won't do it.

http://www.gsp9700.com/pub/technical/4442T/4442T.cfm

The one difference that I observed is that the scratch guard (sorry I called it "face plate" previously) that my tire shop used had "fingers" that went into the lug holes. Other than that, it's the same.

Daryl

I had an annoying vibration as well. Tried the first brand of tires and had a vibration that couldn’t be balanced out. So we tried a top of the line Michelin and still no change. I took the car to a shop that had a Road Force balancer. They told me that one of the tires was faulty. Tried a new tire and same result. Turned out that the hole in the center of the rim was slightly out of round. We needed to find a balancer that used the bolt holes in the rim to attach to the balancer. Bottom line! Make sure your rims are good and remember the wheel balancer is only as good as the tech. using it. Extremely annoying. Good luck.

petercherry

Hi
We've done all the wheel/tyre work, definitely not a problem there. Definitely speed related though as opposed to RPM. I wonder if it's not in the chassis somewhere, am going to pull all the front running gear off and check everything especially damper mountings. Any other ideas? Otherwise car goes great, been around Kyalami GP circuit (cool) and 4th place in a skid-pan speed trial at Zwartkops raceway recenty againf mainly 911 opposition (GT3's, Carrera Rs's and the like) - proof that an old (1990) S4 can take them on!

Drmark

Road Force, Road Force and Road Force - do nothing until you have had this done. Finally sorted a vibration in my GTS that hadn't responded to new tyres, bearing checks, alignments, new brake discs etc etc. The NSR was egg shaped but normla wheel balancers hadn't detected it.
The problem arises if the Road Force comes back as fine!!

Sab

NSR?

Garth S

Was there any asymmetric wear pattern in your old tires? If so, any feathering of rib and block edges beginning to show in the new rubber? - actually, it can only be determined by touch/touch in the early phases.
Obviously, I'm exploring possible alignment issues: once a wear pattern is well established, it can cause high speed vibrations that no amount of corrective balancing, by any technique, can correct. If caught early, it is generally correctable.
The other sources of chassis vibrations ( not rpm related) are torque tube and rear half shafts. Do you feel the vibration in the chassis ( where you sit ), or through the steering wheel ? .... or 'everywhere'?

AO

Check wheel bearings, suspension bushings, ball joints, and steering linkage (inner and outer ball joints). Rear shafts are also a good place to look.

Could also be a slightly separated belt in the tire that doesn't show itself until you reach those speeds. No balancing would ever detect that.

Daryl

Are the calipers allowing the pads to release completely? I had a car that ran smooth cold but had a vibration after a few miles. The problem wheel was hotter to the touch than the others. Also use the search option. I remember Eric form Denmark did a lot of work on vibrations.

AO

Thanks Daryl!
A sticky caliper could cause this as well as an out of balance rotor. An out of balance rotor is rare but possible. How old are your rotors? Maybe time for a replacement?

petercherry

Hi and thanks for all your help. We've been through a complete front end rebuild and everything checks out OK, including disks, wheels and tyres. Tried some off other cars too. Just reassembling today for another test. Skimmed the rear disks to adjust a tiny run-out so that may help. Nest step is to check torque tube again and rear half-shafts - thanks for the tip. Having dynamic balancing done today as well. Interestingly, had this car (with 195,000 km on the clock and original engine) on the dyno at 211 bhp at the wheels - and the altitude here is nearly 6000 feet! Standard spec for the car is 320 bhp at the flywheel, does this figure seem right? Maybe the dyno was in kilowatts?