[Eric S]

Thanks for the comments, guys. For all I know the hub just failed from fatigue. The last thing I would want to do is wrongfully accuse a shop or a person, so that is why I couched my description the way I did. However, the damaged studs cannot be explained, and it is at least plausible that whatever force was used to damage the studs also contributed to the failure.

Dave O - If I were to guess, it looks like someone used some sort of hammering force in an attempt to get the hub removal process started. This doesn't make any sense, as the trailing arms were off the car and all they needed to do was place it in a press...

Jase - Thanks for the link; his failure looks very familiar and his description was eerily similar. Bob M. had also forwarded a couple of links on this, repacking of CV's and the grease to use, and bearing replacement; I appreciated the info as it prevented me from installing a brand new axle with a dry CV.

Kurt - Haven't been able to reach John B. and the band of tin kickers yet; I would like to have them look at both the broken hub and the other one (magnaflux/x-ray for cracking). I'm interested in whether the experts can shed any definitive light on it, other than "it broke"

Andrew - I've seen the photos of another broken hub on the Pelican link above, and I know they can break. But like you I don't think that this is a "common" failure. Heck, no one in the US even stocks a single hub, not even the Porsche warehouses in Atlanta or Ontario, and if it was a common failure they would have some in the country (and no, I don't believe the failure is so common that they can't keep them in stock). I don't know if the bearing was properly installed, but it came out relatively easily and since the hub was broken, it could be removed as a whole unit and is available for inspection. I don't have an adequate torque wrench for the axle nuts, so they were tightened using the following process: (1) tightened with an air impact gun, (2) tightened further with a breaker bar and pipe, and (3) properly torqued to 340 ft-lb by Synergy Racing before I drove it. Also, I use the second nut as specified on the RS, torqued to 147 ft-lb.

If it turns out that this is nothing more than a fatigue-related failure, then there are a bunch of other guys who better start thinking about hub replacements...

Eric

[Manny Alban]

we upgraded my hubs to those used by the C4 which evidently is the same as the 964 Cup.

[Eric S]

Manny - I remember that you had a lot of front hub/bearing issues and went to the C4 hubs to resolve it. Did you do anything on the rear? In my case, it was the LR that broke; when you think about all of the force that corner takes, a preventive replacement wouldn't be a bad idea.

Eric

[Manny Alban]

I had a left rear hub/bearing failure many years ago coming out of T9 at Summit. Left the car stranded in Pit In because it sheared off the axle. If I recall, the only thing holding the wheel was the brake caliper. Not too pretty. The hub was determined to be out of round, I believe. At that point, the car had at least 30K miles of track duty.

I'd have to ask the shop, but I thought we upgraded all four corners of the car. Since then, I've only had to replace one wheel bearing (left rear). That bearing got five years of use.

[Eric S]

Manny,

My guess is that it was just the fronts. I just checked the PET and the part numbers for the rear hubs is the same for all narrow body cars (C2/C4/RS). There are different part numbers for the front hubs for each (C2/C4/RS), and the wheel carrier is the same for C2 & C4, but differnt for the RS.

Eric

[MLIN]

Eric, sorry to hear the problem. Hope you find a good solution.

[slbates]

Based on what I see in the photos, it looks like classic mechanical failure due to stress concentration. Failure precipitated by stress concentrations are usually conjoint with two other mechanical happenings – impact and fatigue. The location of the failure is located right at the change of section. Note the very small radius at the change of section. This small radius does help reduce the stress riser at this location, but is too small to be very effective. Any corrosion or scratching in this area, no matter how small, can lead to failure. The smooth surface you see in your photo at the 1 o’clock position was the likely the starting point for the cause of the failure. I don’t know much about the type of hub on your car, but I hope the “machined” step I see in the photo was from rubbing post failure, not sloppy machining. Also, the notches at the edge of the radius are very undesirable. Those hubs have seen a lot of stress under the constant acceleration, deceleration (in gear), braking and impact loading. Banging on the hub bolts likely did not cause the failure (probably didn’t help things either. You probably had a crack and didn’t know it. 20k of track mile is one heck of a bunch of fatigue cycles!

There are several things you can do to reduce the probability of future failures:
*Shot peen your new hubs.
*Make sure no machine steps appear at section changes or edge of radius.
*Use a penetrating dye test to check for cracks that you cannot see when you have the hubs off for any reason.