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Front hub lateral runout

 
Old 12-09-2012, 02:40 AM
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BesideTheBox
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Question Front hub lateral runout

I've had my '97 C4S about a year now. Had 69K miles on it in November, 2011 when I got it. There was a bit of front end shake felt through the steering wheel when braking above 40mph. The PPI indicated that the front rotors were warped, so I knew I would be replacing them.

Maintenance receipts received with the vehicle indicated that the rotors had been turned at 66K miles in March, 2011.

The shake was there when I got the car, so I assumed that the job was not done properly. And since the rotors had already been turned once, I felt that new rotors and pads would be the solution to the problem.

So in June, 2012 at 71.5K miles, I installed new front rotors and pads. Problem solved, or so I thought. Slowly and surely, the shake returned. Today the car has 75K on the odometer and it has been shaking for months, so the rotors warped inside of 2K miles.

I bought an inexpensive Pittsburgh clamping dial indicator from Harbor Freight. It arrived this week and I finally set off on measuring the lateral runout of my front rotors.

First up is the left front rotor. It measured within .001" (one thousandth of an inch). Hmmm. Maybe it's not the rotors after all.

Moved onto the right front rotor. It measured within .007" (seven thousandths of an inch). This is good. I am getting somewhere. Explains the shake. Porsche specs the maximum lateral runout for the rotor installed on the hub assembly at .09mm (.00354" or 3.5 thousandths).

I then removed the big red caliper and the rotor to measure the right front hub runout. The hub runout measured at .0045" outside of the studs. Porsche specs the maximum lateral runout for the hub at .04mm (.00157" or 1.5 thousandths). Due to the angle that the dial indicator has to be at in order to clear the studs, I think this number is conservative. Ideally, the dial indicator would be perfectly perpendicular to the hub for this measurement, but in order to clear the studs, the dial indicator had to be angled about 20 to 25 degrees from perpendicular.

The 993 workshop manual says to make 2 sets of measurements on the hub. Since the outside measure was so far out, I did not bother with a measurement inside the studs.

So, workshop manual says "Replace wheel hub". Ouch. Any other remedy? How about shims between the hub and rotor? Anyone have a good spare right front hub for a '97 C4S laying around?

If I need to replace the hub, will this require an alignment? Or can this job be done without disturbing the alignment? If it will require an alignment, then I may take one step down the slippery slope and freshen up my suspension while I'm at it as it all appears to be original.
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Old 12-09-2012, 05:40 AM
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Steve Weiner-Rennsport Systems
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Hi,

If the drive flange or hub is warped (likely from over tightening lug nuts with an airgun) you will need to replace the wheel bearing and re-align the front end since the camber adjustment is lost when the hub is removed.

You will need access to a hydraulic press to replace the drive flange & wheel bearing.

At 75K miles, I'd also check out the front control arm bushings and the rubber inner tie-rod couplers very closely.
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Old 12-10-2012, 06:25 PM
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BesideTheBox
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Steve,
Thank you for your reply.

Carfax when I bought the car said RF accident damage when the car was less than a year old. One of the receipts with the car suggest that new rotors were put on the car some time before rotors were turned. Problem is that there was no date or mileage on the parts receipt and I haven't been able to correlate it with any of the service receipts.

How can I verify that the steering nuckle and the spindle are straight?

Or would I have other symptoms if one or both weren't straight?
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