964: broken rear hub
 

I was turning 1:31s at Willow Springs a few weeks ago when I hear a clunk and felt a wobble after turn in at turn 3. I then went to brake for turn 4 and the pedal went all the way to the floor! Luckily that is the slowest part of the track and one pump was enough to get my brakes back. I limped off the track and back to the pits and up on jack stands it went.

After a bit of searching, I found that the left rear wheel was wobbling; enough so that the brake pistons were being pushed back in the calipers, which is why I had to pump the brakes once. The lug nuts and axle nuts were all tight, so my amateur diagnosis was a failed wheel bearing. This was a larger repair that I was prepared for at the track, so on the trailer it went and back home.

The shop took the rear end apart today and I was quite surprised to find out that it was not the bearing that failed, but rather the hub itself. The car does have 107k miles, the last 5k of which have been almost all track miles. I also run 18" wheels with sticky tires, which goes against the factory's advice.

The plan is to replace the hubs and bearings on both sides, so hopefully it won't happen again anytime soon. I'm just glad it didn't fail in turn 8 headed into 9 at 125mph, or there would have been an off track excursion for sure!


http://forums.pelicanparts.com/uploa...1210910455.jpg

Wow. Lucky guy.

Any issues with axles and CVs? I've had lots of problems with the rear axles and CV joints lately. Probably due to the crap grease Porsche uses that appears to liquify after a few hours of hard use on the track. I've just had my rebuilt with better lube that I hope will help. My main symptom was grease leaking out of the hub and out onto my wheels (inner face). I think with my large slicks, I generate a bit more heat than most would. (I run 13" slicks, equivalent to a 335 or larger tire size)

How tight was the axle nut? The axle nut is supposed to put pressure on the hub and bearing spit inner races and CV axle splined end. I can't imagine how the hub journal will break off if everything was tight.

I have not had any problems with the axles/CV themselves, beyond replacing split boots. I have had the exact same problem with liquefying grease creating cool patterns on my wheels, but mine was the outer face. :) Our solution was the same as yours: repack with high quality grease; no problem since.

At the time this happened, the nuts were tight. I forget the exact number, but the torque spec is some huge number requiring a 3 foot torque wrench to actually achieve. Not only were they tight, but they were double nutted as a precaution against coming loose on the track.

That being said, they have been loose in the past; this was one thing we found when we were trying to figure out why the axle grease was getting all over the front of my wheels, as Tom W and I were just discussing. So there is a chance that they were fatigued earlier in their life (over one year ago). This is part of the reasoning for replacing both sides now.

Broken Hub
 

I experienced a broken left rear hub on my RS America at Mid Ohio in May. Here is a link to the write-up and photos: http://www.dorkiphus.net/porsche/showthread.php?t=17582.

In addition, thanks for the pointers to the NEO grease. I had both axles cleaned and repacked - good thing, the outer CV on the brand new RS axle was DRY!

Eric