Well, here is a pic of the damage with the wheel on. I'll take some more pics of the damaged parts later.

You can see that the stub axle is clearly sheered off. You will also notice that the hub is broken in two. Kind of scary. Don't know the sequence of events but I think the hub broke last since that's what caused the car to come to a complete stop. I can't image the car moving with a broken hub.

George

 

Well, I finally got my car back. Pretty much everything in that area was replaced: hub, carrier, bearing and driveshaft. Here is a picture of the broken hub. I can’t believe the bearing didn’t break.

George

 

Well, after two more years, the other side broke this weekend. Just like this one. The stub sheared off (with the nut) and the hub broke at the same place. I wonder if it's just my car since no one else is having these problems........ I had a passenger in the car and really wasn't much attention to the car. The only thing I noticed was that the brakes felt a little funny. Oh well, at least there was no other damage since it happened at the slowest part of the track.

G.

 

Thats really strange George. I used to reman axles for a living and we'd see this type thing every once and a while. Have also seen the axle shaft break as it goes into the joint. Seen the threaded end break on the back side of the nut. But usually the cage and other internals break first since they are the weaker links to the whole assembly. The only answer I would suggest is that you eith have the wrong joints on those shafts (I know hard to believe the same thing happening more than once) in that the length of the spined area is wrong, short or otherwise not right. Or its not bottoming out correctly and actually flexing under stress until it lets go and breaks. Love to know what you end up finding out......if ever

 

Chris, I am stumped. I wonder if I'm just bouncing off the curbs too much. This one did last for over two years. I think the conclusion on the other one was that it wasn't torqued down properly when installed. I have both marked with fingernail polish and nut did not back out at all. It was in the original position from over two years ago. Maybe I should periodically start checking their torque. I'll let you know if anything strange comes up. I'm wondering if I just got a bad batch.

G.

 

I would hazard a guess that it was the hub that failed first - and that failure applied a radial stress to the axle stub causing it to fail.
The stub axle is placed in tension with 330 ft.lbs of torque, mostly to establish the correct preload on the double caged bearing, as well as keep the components attached to the car; however, vertical loads are borne by the hub, bearing and hub carrier.

 

George, are you launching off the curbs or just rolling over them? as clean as that first pic is, maybe the wheel is slowing down to much if the tire leaves the pavement for to long. The sharp contrast in speed when it comes down may be putting to much stress on the stub. Although like I said, usually the internal parts go first............strange

Do you have LSD? Maybe its not working, allowing that slowdown of the tire?

 

wow.. just be thankful you didn't end up in a catastrophic accident from it... you have that to be thankful for anyway..

 

When I have seen this damage in the past, it is generally a bad wheel bearing at the root cause.When the bearing goes bad, it allows radial play an does not then have full force anymore applied to the nut. The nut can then back off, and / or the vibration from the radial movement can cause it to break pretty fast.

 

Chris, I do both, roll of the curbs and launch off the curbs. Once I set the car, I try not to make corrections. I don't mind launching off the curbs as they are typically on the side of the car that isn't under load. Maybe I should calm down a little. BTW, what I thought was strange was that once the stub broke, I could not move. In gear, the engine just revved up freely but nothing went to the wheels. I have an LSD and I would have thought that it would have made the other wheel turn but it didn't. Maybe my LSD is worn out but it's less than a year old.

Bob, if you look at the picture above from two years ago, the bearing doesn’t look like it failed. I’m not sure about the one this time, I’ll find out in a couple of days. The strange thing is that I talked about replacing the rear bearings with my mechanic. He was going to do them in the next week or two.

Garth, I just have a hard time believing that something as thick as the hub broke first. It’s really thick and made of steel. But then again, I’m not the nicest person to the car, so maybe a couple of hard bumps at the right angle caused small cracks which eventually led to complete failure.

G.

 

So what was the answer as to whether your shop used the correct torque wrench? This sounds like undertightening. Overtightening could do it too, but we studied this problem after I had this failure last fall at a club race, luckily at low speed, and concluded it could take in excess of 700 foot pounds to cause it. Undertightening is just as bad, and much easier to do. A good strong shop impact gun "feels" like enough torque, but usually it's not. I discovered this by having the same failure you did. Answer, always torque them with a proper torque wrench, no excuses.

This issue affects centerlock and non-centerlock alike. With centerlock the issue is primarily the torque applied when installing the center-screw. The lockpot won't hold it if the shaft snaps due to undertightening.

 

I think you are on the money for the first one that happened over two years ago. It let go within two weeks of the axel replacement. This one has lasted for two years before this happened. I think it was something different and not the installation.

G.

 

Did they use a torque wrench or not the first time? How about the second time? Same shop or different? Did they replace the axle nuts like the factory shop manual says to do, or did they reuse them on either side? I think the torque is more important than replacing the nut, but you never know. These big nuts have a "pinch" which becomes less effective with each use. Also it is necessary to put a light coat of antiseize (or Optimoly HT) on the threads and nut flange to get an accurate torque. Without that, your guy could be off by around 30%, even with a torque wrench. Raw friction is not the goal here.

2 years from reinstallation to failure does not in itself rule out the undertightening theory. If it was tightened more than the first one, but still below spec, and/or if the nut was reused, or the threads installed dry, that could still explain this. I have seen a lot of guys use the curbs without this problem. You might have other problems from using curbs, like with the wheel bearings or axle main shaft or CV joints, or inside the transmission, but not likely the stub axle thread. I've also seen cars driven on track with worn out wheel bearings for awhile before the owner realized it, without a big failure.

 

Like many have said, proper torque is very important. The components rely on this especially the wheel bearings! Double row, angular contact bearings such as the 993 wheel bearing are picky for proper preload.

One thing we found on our race cars (which are virtually identical to the 993 system) that, we feel, contributed to these types of failures is that the nuts bearing surface needs to be square to the axis of the nut. If not, bending loads get applied to the drive flange when tightened. Despite our driveflange nuts being military spec, we found that machining them square to their axis helped.

 

Oh wow!