tbccrane993

May need to replace axle shafts on 2000 Cup...any advice on street axles versus Cup axles? First season with Cup and learning my way through the (expensive) maze.
Thanks for any input!

Steve113

Blacks get rid of them every year or 2 years depending on how much u drive the car

fstockcarrera

^^^This

Street Blacks, don't bother changing the grease as the weak link is the axel stub not the CV. Toss them every 35+ hours

good hands

Is there a definitive part number for these ? I have done some thread searches and the part number seems to have evolved or changed.

SDGT3

Blacks with Krytox grease and I got 100 hours with no sign of damage. Changed them as a precaution.

fstockcarrera

My experience is that even with the cv grease change the failure I had was the stub axel @ 40 hours not the cv joint. 100hrs is a very long time for an axel. IMO

SDGT3

Agree that 110 hrs is long. I have my car checked after every event and we never saw evidence of wear. At 100hrs determined I was pushing my luck and changed them Still appeared to be usable but no confidence.

fstockcarrera

You honestly you can check all you want but the failures are at the center bolt , stub axel junction. You would have to remove the center bolt and the axel to even have a chance to evaluate it's condition. My last failure came with out warning at about 40hrs on the stub axel at the center bolt flange.

chrcook

Agree with Bob, here's what failure looks like:

Bring your check book....

The P/N for the blacks I use is 996.332.024.16. Purchased them from sonnenporscheoemparts.com. Around $835 at the time.

Bill Farrell

I agree completely with Bob ! It's a painful failure point.

Would still like to get opinions on why these fail at 40 hrs: Hypothesis #1) the 360 Ftlb Torque Spec for the Central Bolt is too high - putting excessive Tensile stress on the Stub Axle. #2) The Stub Axle thread stretches - and therefore, you get a bending stress as the wheel rotates or #3) The Stub Axle is simply a weak design.

If it's #1 - we can reduce the Torque - like they recommended for the 997s. However, Professor Austin has been informed that this is not the solution.

If it's #2 - the Fix would be to Retorque the Central Bolt after the first outing - and recheck the Torque on a regular basis (clearly cold conditions).

There needs to be a better way to prevent these catastrophic failures. 40 hours makes no Engineering sense for the design of this part.

fstockcarrera

Cause #2 Is my guess, the center bolt is a 30mm nut so the treads on the stub axel are carrying the 1.7 G of X the effective rear distribution weight of the cup at the least. I'm no engineer obviously.

Juha G

I agree with you guys; the axle stub is the weak link. I think it's a combination of things but the main cause is fatique from torqueing the center nut over and over again.

I had the stub fail on me in the last race. These were blue axles.
For what I can gather I'd be better off with black axles that are replaced every 40hours instead of hoping for more hours with blue axles.
I can tell you it's not fun at all to loose a rear wheel doing 120mph...that's what happened to me.










It's been cracked for some time already...

amso3

Hi Bill

"Professor Austin" here. To reiterate, you should not reduce the torque value specified for your car. I discussed this with the PMNA engineers when we had this discussion some months ago. Forget that the newer cars have lower torque values, the parts are designed differently.

I believe that most failures occur near the transition from the stub to the CV housing. This is a point of stress concentration due to the design/manufacture. If there are microscopic machining grooves, the stress can rise significantly and over time fail. I think machining is why some fail sooner than others. Then only way to really examine them after use is to liquid penetrant test the stub axle edn to look for cracks that are not visible to the naked eye. Once a minute crack has started, they will fail. If you can't do that, toss them after 40 hours.

Juha G

As an engineer, I can agree with both of your statements. My stubb broke exactly where the machined thread starts. It is very logical.

chrcook

Being a mechanical engineering with a background in materials, I was very curious as well. My failure point looked exactly like Juha's, with the shear happening at where there stub axel ended over the axel flange (i.e.: end of the thread on the main axle assembly). Upon inspection of my crack propagation, I concluded that I had a minor crack for quite some time, slight under the treads, and then it failed. Off the top of my head, the stub axle torque is around 360, slight less than the wheel nut torque of 340 (although I ran an even 350 for year). My theory is that the on / off of the wheels work hardens the entire stub axel / axle flange assembly leading to a failure at the weakest point (smallest cross section which is at the begging of the threads on the flange). If that's true, the "culprit" is the number of cycles of torching he wheels. Also, as the flange "stretches", Bob's #2 situation occurs with "flex".

As an unintended experiment, I put on new axles (including front flange axels) and new stubs, bolts and washer with very light grease. One of my front wheels would not torque, but kept turning. Despite never reaching 350 ft-lbs, the clamping load ramped up and I stretched my new axle flange about 1/8", creating a paper weight. Hence, the tensile load on the axel assy can vary despite the same wheel nut torque. My new approach is to keep everything clean and dry, torque to 300, then take to 340.

Bottom line for me, is I'm 40 hours and out or every other year, which every comes first.

Other option is go to 5 stud lugs and take this failure point complete out of play.....