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Stub Axle "Weak Point"?


paulyg

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Hello all,

 

A recent WTB brought up the possibility that the rear stub axles might represent a "weak point." A search turned up several threads, including one with a sequence of photos purporting to depict a rear hub assembly liberating with the outside part of the stub axle, accompanied by a C.D. Eisel diagram showing common failure points. Billet stub axles are available from a few vendors and supposedly offer insurance to trailing arm rebuilders against such failures, especially racers.

 

I'm scratching my head a little bit over here. The stub axles seem to be plenty beefy for their application. Even motor swapped cars rarely push more than 300HP at the crank. The cars weigh 2600lbs max with a couple fat guys in there. How can it be that stub axles are failing, sending rear wheels past on the highway?

 

My guess is that it all has to do with inadequate torque/preload on the hub castle nut. There are a few threads out there calling out scoring/galling/corrosion of the splines as the root cause, but honestly these things are so overbuilt that the extra fatigue knockdown wouldn't cause a failure over the lifetime of the car (I think?). It's far more likely, in my opinion, that any combination of corrosion, gunk, galling, or fouling of the hub splines or castle nut threads is resulting in the required preload not being reached on the bearing spacer during reassembly. 780341754_2002REARAXLE.thumb.png.7a0cc9498dd5e32b4944e7f75212d3b6.png

 

The spacer is designed to contribute some stiffness in bending to the axle shaft. This only works if the appropriate compression is achieved on the inner races + spacer through the 300lb*ft torque on the nut is actually converted to clamping force on the bearing spacer, and not clamping force on a bunch of tiny hangups on the hub splines. I've indicated that this lack of preload would cause "gaps," but really it's just an area of inadequate compression that would only result in a gap in extreme cases. 

 

In any case, however, the upshot is that if inadequate preload is obtained on the stack, either because the hub nut is not torqued or because there is stiction anywhere in the stack (due to the aforementioned corrision, galling, or gunk), the fatigue test that is the rear axle becomes a lot more interesting. 

 

Please set me straight if I'm wrong but I think that it should be emphasized here that the goal for re-assembling the rear axle is that the torque on the nut compresses the bearing spacer appropriately (and by including the bearing shim, that the inner and outer bearing races don't stray too far from each other). Again, I highly doubt that minute defects in the axle shaft would result in a catastrophic failure unless they caused the described preload issues.

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The stubs are full of stress raisers, and eventually, they will fatigue with repeated curb hops,

clutch dumps and wheel to wheel contact.

 

It DOES take a lot.  They hold up pretty well to 10" slicks.  Personally, I have never broken one.

The broken ones I've seen would have given lots of warning- if they had been crack checked.

 

The E21 stub is formed very differently, and has far fewer raisers.  It's not perfect, but it's a lot better.

 

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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After years of SOLO II in a prepared class (on slicks) I decided it was probably time to pull the axles and look at the bearings.  The bearings were fine the axles had about 10-12 degrees of TWIST in them.  These were the early axles and BMW did get the metallurgy better starting in 1972.  I do however remove, inspect and crack test then on a much more regular basis.

 

We check our Gp2 CSL axles often.    12" wide slicks and a 4.45:1 final drive for the short courses doesn't help. 

 

Axle.jpg

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1970 1602 (purchased 12/1974)

1974 2002 Turbo

1988 M5

1986 Euro 325iC

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My former 74 2002 had the stub axle break back in the early 90's while going 70 MPH on I-15 in the Mojave Desert near Barstow. I had no indication or noise that it was going to break. The back drivers side wheel/tire and brake drum assembly separated from the 02 and the car went down on the rear trailing arm in a shower of sparks. That was an "oh sh*t" moment.  The wheel crossed the median and opposite lanes and went out in the desert. I'm lucky the wheel didn't hit someone in the opposite lanes.  Looking back, that 02 had a lot of rust issues. I brought that 02 back from Germany while I was stationed there. I have no doubt that the rust was a contributing factor to weakening the stub axle.  After I got the 02 fixed, a week later the 02 was totaled by a red light runner. Insurance gave me far more money than the car was worth.  

 

 

 

74 tii (many mods)
91 318i M42

07 4Runner

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One more failure mode data point. The previous owner of my car had the torque side axle, hub, and bearing fail. The wheel was not lost, but the splines stripped off the hub and partially off the stub axle. The PO had claimed it had a lot of bearing noise before "The rear wheel started wobbling, and the car wouldn't move". 

The FA: The outer bearing had come apart allowing the axle to gyrate. The hub splines stripped, the axle splines partially stripped, and the wheel nut clamping surface got buzzed down so the whole assembly was free spinning on the stub axle. An interesting fact is the stub axle was the most durable part, follwed by the hub, and the nut. The nut is definitely a softer material.

What is unknown did the nut or the splines on the hub fail first when the bearing came apart?. If it was the nut the pressure train you described would fail putting all the torque and impacts on the splines only. The nut did not loosen during the occurance. It was complete with cotter pin and factory witness mark intact. 

The cause of this failure in this case was the bearing coming apart. 

 

Regards

 

Dono

 

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On 1/6/2023 at 11:07 AM, Preyupy said:

After years of SOLO II in a prepared class (on slicks) I decided it was probably time to pull the axles and look at the bearings.  The bearings were fine the axles had about 10-12 degrees of TWIST in them.  These were the early axles and BMW did get the metallurgy better starting in 1972.  I do however remove, inspect and crack test then on a much more regular basis.

 

We check our Gp2 CSL axles often.    12" wide slicks and a 4.45:1 final drive for the short courses doesn't help. 

 

Axle.jpg

Nice twist!

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The hub's quite a bit softer than the stub-  usually the splines wear on the hub quite badly before the stub's damaged.

 

One of the main failure modes is the inner race of the outer bearing

sinks into the soft surface of the hub,

releasing the compression of the nut.  Then the splines on the stub start working on the hub.

 

The nut's harder than the hub, but softer than the axle, and can gall itself to the face of the hub. 

Then the drill's the only way...  along with the chisel.

 

t

 

Edited by TobyB

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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After about 15 years of Autocrossing, I did eventually snap the stub axle. With about 140hp to the ground and 275/35R15 tires, i just kept looking for the weakest link in the drivetrain. I broke wheel studs, transmission, half axle bolts, differential, and finally the stub axle. I think that was the last thing to go. Funny thing is that i never broke my giubo. 

 

image.jpeg

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