Resident Sicko

1998 993 C2S, 44k miles, clean carfax and history. Vibration felt in seat starting around 80pmh. Not bad, but enought to get on my nerves. I have rebalanced and re-checked balance several times on multi piece rims, even deflating and turning the tire on the rims to zero out 1mm of "high spot". Finally put a different set of monoblock rims/tires on the car and noticed the same vibration. As speed increases vibration lessens or so it seems to the point where driving up to 120 is smooth, flat and stable...now I know it is the car and need some advice...

Bearings?
Rear shocks are Bilstien HD (less than 5k miles)
Any other bushings or rubber that could be old and needing replacement?
Bent hub?
Out of balance half shafts?
???

Mike J

So it sounds like its not the wheels, so something else is out to cause a harmonic that eventually dissapears.

How about the rotors - can you measure their run-out?

Does the steering wheel vibrate, or its coming through the car and not the wheel?

If you touch the brakes lightly while at speed, does the vibration continue?

can you feel it through the shifter?

Do you think its front or back?

Cheers,

Mike

nile13

Warped rotors were a common cause of vibration on 80s Japanese cars after the gorillas at the tire changing shops would use the air guns to put on lug nuts. Some time in the mid-90s the gorillas started to use torque extensions and the problem largely subsided.

The vibration is caused by some rotating mass getting harmonic resonance. It's either wheels, tires, rotors or bearings. I assume bearings would cause a discernible noise by the time they would get so loose as to cause vibration. So I'm with Mike here - check rotors.

Steve Weiner-Rennsport Systems

Hi,

When did this begin? recently? suddenly?

Original brake rotors or OEM replacements?

Front struts original?

I would be measuring radial and axial brake rotor run-out with a dial indicator if the rotors are not factory Porsche. I've also seen balance issues with some of the OEM replacements, too.

Possible rubber bushing issue here, but the above items must be eliminated first. If the rotors check out, close inspection of the front A-arm bushings as well as rear toe, camber & kinematic link bushings, too. If they are all solid, time to check out the wheel bearings.

Resident Sicko

Rotors may be a good direction to look. They were replaced by PO 8k miles ago with genuine Porsche parts.

No vibration in steering, just seat. (rear)

No chage with brake use but may recall "throttle off" being more noticeable but could be my imagination.

No feeling of vibration in shifter. I have stiffer than OE motor mounts as well.

I am sure it is in the back. Steering is fine.

Symptoms were practically overnight. It has been an ongoing issue as I drive the car on the weekends, sometimes every other weekend, so I have take steps to avoid "parking flat spot" on the tires by parking on foam.

Shocks, front and rear are fairly new and look fine. No leak, no damage. (Bilstien HD) They have been on the car for 2 years.

I will look into the rotors.

Thank you for your help.

Resident Sicko

Turned rotors and noticed a slight difference (less harsh). Ordered new rotors today and will report back if not totally fixed.

BesideTheBox

If turning the rotors resulted in less symptom, and did not eliminate it, then I would say this is an indication that you need to check the lateral runout of your rear hubs with the rotors removed.

I replaced front rotors due to shake through steering wheel. All was great for about 2K miles, no symptom whatsoever. Then the shake slowly returned. Dug into it, bought a clamping dial indicator from Harbor Freight. Now I know my RF hub is bent and needs to be replaced.

I don't see a max lateral runout for rear hubs specified in the workshop manual. The max runout for a front hub is .04 mm, and .09 mm for the rotor installed on the hub.

Good luck with this.

chaoscreature

Did you try a road force balance yet? I had a very odd and fairly violent shimmy in my Cayman that would get significantly worse under braking over 70mph. I was about to condemn my rotors when I took the car in to RFB... turns out I have flat spots in the tires from lack of use. They moved the tires on the rims to balance this out as best as possible but recommended replacements... shimmy is 95% gone now but at least I didn't start to swap parts.

ble2011

+1 on the road force balance. Your tires could be "spun" balanced perfectly, but there could be an issue with the tire that will only show up under load. The example I was given - there could be a heavy section in the tire and still be within their tolerances for quality control. The spin balanced was done and corrected the weight difference. Then a "thump" shows up when your driving.

ToSi

Does the shimmy follow vehicle speed or engine speed, i.e. same symptoms regardless of gear?

if engine speed, -> https://rennlist.com/forums/993-foru...uspension.html