Mid Ohio Mechanical Failure
 

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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Holy Crap! Thank goodness you made it in with only a lighter wallet.

Peter

Any way they pounded on the back of the hub (at the wheel studs) in order to remove it when they replaced the wheel bearings?

Sweet Jesus!!!

Those studs were hammered on by someone. :mad:


FYI SirTools BMW / Porsche specific Master bearing removal / install kit runs about $450 ... prolly < the cost of one of the hubs let alone labor ...

*Oh, shipping an axle nearly dry ... WTF?????


** Another 964 with hub failure pics.

Bloody hell Eric. That sucks - great post mortem write up, though. I'm off to make sure my new CVs have grease in them...

Making the determination that this was a direct result of a procedure failure would require more than looking and thinking. Did you contact the tin kickers?
I did some digging after looking at the parts and found that this is not an unheard of failure on a highly stressed point. Your hub failed in the same manner that other hubs that were not beaten on have failed.
2. You have 20,000 high stress miles with some to most using oversized track compound tires and rims that Porsche has in print recommend not be used.
3. There are no reasons to beat on the back of the hub other than short cut service methods. On the other hand look at what the part is built to do. It 100% holds a rear wheel that holds the car up and transmits all motor and braking torque energy as well as all cornering G loads and wheel impact shock. Hammer hits are not the same to this part as they might be to a knee cap. It failed at the weakest point as well. My guess is that they might have been struck while a puller was also being used. Some hammer hits can induce a shock wave that makes interference fit parts release under tension that would otherwise not dislodge them. Note the angles of the ball-peen impact dimples. Some swing angles are directly edge to center some are oblique. This pattern might come from the hub being struck while the puller in in place and preventing the hub from being spun.

I don't know what happened to your hub at all but here are people that can inspect, test and determine the failure mechanism.

I don't understand why they hit the hubs, at least why they hit them there? I've done tons of wheel bearings on 911s, I'm not sure how you could get a hammer there, or why you would think it's a good idea... As Jase said there's a tool for it, but even if you didn't have the Porsche tool, or a puller, it doesn't take a genius to get some random bearing press tools and maybe a torch and do it safely. Even then, I'm not sure on a 964 that's a problem... unless you end up scoring or pulling the bearing races apart.

Still, it happens. When I had my shop, my first customer was someone I knew from the Pelican Parts BBS, I didn't even want to have to call him and tell him I was charging him for almost literally everything the previous shop had done on his car a month earlier. Thankfully he only needed 1 new, new hub though.

Note if you ever use a torch to heat the aluminum around the bearing so it'll come out easier... don't catch it as it pops out, regardless of your shops "nothing hits the floor policy" Not that I'm speaking from experience or anything :roll:

I'm going to have to disagree here... The reason being you have not only a 964, but an RS America. Also, to put one thing to rest, a 275 is an "approved" tire size with Porsche, you can even run them on Porsche rims. The blue rims in my avatar are pretty rare factory RS wheels that allow running 235/275 on a narrow body car and Porsche raced on them all the time. I don't think your choice of tire size had anything to do with this failure, people run them on 915 and G50 cars all the time, in many wheel combinations... if you were running a 3-piece fuchs I'd call you a fuching idiot, but.... Also, your wheel studs look fine, no bending no shearing... if you were using a really messed up offset wheel, I'd see those failing and the wheel seperating I don't see how those forces would get transferred to the hub and cause it to shear instead.

Here's the other thing, if you look at hub, bearing, and axle failures on 911s, it's kind of like coming to a cliff with 964s. Those are beefy bearings, axles, and hubs. They didn't suffer nearly the same problems as SCs or Carreras. What you're describing is a nearly unheard of problem on RS Americas. Front and Rear wheel bearings were stouter than on older cars, the axle bolts are even bigger and don't back themselves out nearly as much. The fact that the hub is what sheared is honestly shocking. My gut tells me the bearing was possibly improperly installed, but I'd be looking hard at that bearing... I'm not sure what else could cause that without tearing up the axle, transmission, or cv joints first. Especially since you mentioned the heat...

Or the axle nut... did the shop that did this know the torque specs on it? It's a PITA, I've had the "sniper rifle" torque wrench on it, and a jack handle, and have had to stand on the end to get to the required torque settings before.

O RLY? 275's on a narrow body 915 car?

I've seen it done. Need fender lips rolled, the right offset and camber, oil lines out of the way, the right tire manufacturer, and maybe some judicious fender pulling. I could get a 265 on one side of my car easy, with 10" rims. The other side would require some massaging. It wouldn't be hard to get 275's to fit with 9" rims...

Yep, in fact I've seen custom 11" rims squeezed under an early car. One of the fastest early cars I've ever seen, runs 245s up front and 275s in the rear under stock fenders... it's amazing what you can do when you try... and have.... $80k to blow on an already race prepped racecar.