[Eric S]

For those of you who weren’t at Mid Ohio, I suffered a DNF and had to be towed in (fortunately after the session so no others lost track time). Here is what I found.

To put things in context, there are four factors to be noted.

1. Between the third and fourth sessions on Saturday I rotated tires from side to side and installed new Pagid RS14 rear brake pads. On inspection, everything was normal; i.e., properly torqued, no unusual movement, etc.

2. I had been dialing out understeer between each session, and had it pretty close so I made one final front sway bar adjustment while rotating the tires.

3. My car has about 20k track miles on it (50k miles total); I use 18” wheels which are not “approved” by Porsche, and I use 275 Hoosiers that provide relatively high grip.

4. I had a shop, un-named but not my regular shop, install monoballs and rear wheel bearings in the rear trailing arms prior to this season. Oh, hindsight; you cruel bitch...

I spent the first couple of laps of the fateful session mating the rear pads and started picking up the pace. I detected more oversteer than expected and trail braking was loose so I was thinking “Gee; maybe I went too far and need to dial back a tad…”, and I also detected a bit of a slight thump that made me think that I had a CV beginning to go bad. Then I noticed a really strange symptom: turning into Turn 13 (left-hander at the top of Thunder Valley) I let off after trail braking and felt the sensation of the brake being applied more. Strange. Well, I get to Keyhole and on exit I shift from 2nd to 3rd and get nothing but a loud thumping whir and total loss of drive.

I get it back to the garage and find that the left rear wheel is not only wobbly, but it can turn independently of the axle! It turns out the hub sheared off right at the bearing, and the axle subsequently shredded all of the splines in the hub! In addition to no drive, this explains the trail braking sensitivity and the sensation that the car was being slowed after I let off the brakes – in a turn the LR wheel was changing camber and canted so that the rotor was being forced against the pads thereby “applying” the brake!

In searching for the root cause, all of the potential load-inducing factors above were considered, and it may have just been a mechanical failure due to fatigue. However, I noticed that several, but not all, of the heads of the 10.9 hardened wheel studs were deformed – apparently by use of a hammer or impact chisel. The un-named shop swears they did not do it, but they cannot explain why the studs were undamaged when I took them to the shop but they were damaged when I got them back. When inspecting the RR hub, it showed the same damage to the heads of the wheel studs. Every professional Porsche mechanic I’ve consulted with, including the PCA National Tech Rep, believes that if the hub was struck it would not only be patently inappropriate technique, but could likely induce a stress riser and could have been a contributing factor in the hub failure.

As a result of seeing the stud damage on both sides, I was obligated to replace both rear hubs, both rear bearings (they had a whole 7 days on them), and the LR axle. By the way, I had been hearing that some of my CV failures mirrored the experience of others and that the factory grease was the cause so I had the new axle cleaned and repacked – good thing, too as the new CV was nearly DRY from the factory!!!

I am not accusing anyone because I don’t have a smoking gun (only a smoking axle – boy, was it hot!), but I also do not have satisfactory explanation of the source of damage to the wheel studs. I cannot identify a single root cause of the failure, but in the event that the shop’s technique was a contributing factor I feel an obligation to inform the 5 or 6 friends who have had their cars worked on there of my situation. I'll contact each one personally.
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