PRSCH3

Have been looking online and haven't seen too many similar cases.

For reference, I apparently drove for some time on this with a worsening sound coming from the rear end. A droning noise that was worst at 100kmh (60mph) but wasn't evident at lower speeds. Worse under acceleration but present on coasting, noise went away a little when varying speed. Worse when curving right.

I was on a drive into the country and pulled over to check on the noise, which I knew shouldn't have been wheel bearings or trans noise given that the car had a major service 5 weeks ago and these had been inspected I believe (The opposite side had had the stub axle replaced). I checked the wheel nuts on the off chance that they hadn't been fully tightened but they were ok.

Anyway, when I pulled over, I noticed a brake smell and the right rear wheel hub was hot - even the wheel itself was hot. When the wheel was removed, the castellated (castle) nut fell off onto the ground. With a sheared off piece of the stub axle and the cotter pin still intact.

When I think back to it, the car's rear end had jumped a little on a rapid deceleration into a hairpin turn a few weeks ago, however I had put it down to the car having the wrong offset wheels on the back (and possibly needing spacers added) and not being used to driving my NA after coming back from a turbo car. The brake sensor light was also showing, but I ignored this as it had previously been stuck on without reason and had only been reset at the recent service. So I have learned a little and was probably a little lucky not to lose a wheel on the highway.

Anyone else have this happen? Causes?

Pics attached...

f1rocks

I doubt that the wheel bearing caused the issue but you'll definitely want to replace the wheel bearing!!

What was the castle nut torqued to? If you over torque a nut it can cause a very similar metallurgical failure. I know of at least two racers that have sheared stub axles in the last couple years.

PRSCH3

The castle nut was still on the axle thread, it had snapped off cleanly. Wouldn't be surprised if previous mechanic (Porsche specialist) had overtorqued it, they were incredibly incompetent. New mechanic a lot better. But should have done this one myself, didn't realise how much it was going to cost, things a hell of a lot more expensive in Australia than the U.S.

Replaced the stub axle, bearings, hub flange, and brake pads which had worn diagonal. Caliper kept the wheel on at least!

william_b_noble

that break looks like a tension break, not shear, the nut was overtightened. I lost a rear wheel on a 356 due to something similar - it made for a spectacular ride down a mountain pass with a beautiful shower of sparks behind me.

f1rocks

Yeah it does explain both. I lost a castle nut with 3 laps to go during the Sebring enduro. The stub axle moved left to right and the bearings overheated fast. Rotor/caliper kept the wheel on the car but the bearings and stub axle were toasted.

turbodave

My two cents says you are all correct. Since the torque on the castle nut is 332 ft/lbs max. it is usually applied using a torque multiplier. If the mech. torqued it to the (late model) aluminum trailing arm spec. of 368 ft/lbs and then some to get the cotter pin to align with the hole in the stub axle, it would put a significant preload on the wheelbearings resulting in failure over a relatively short period of time. The wheel hop that you experienced may have contributed to the ultimate failure of the axle but I doubt that your wheel selection had anything to do with it. Luckily you weren't hurt and the car will live to see the road again!

william_b_noble

note also, in addition to Tubodave's comments, that the heat would come from the brake pads being the thing holding the wheel in place laterally, and so you would be running with the brakes partly on all the time - that's a lot of heat.