Bill Ball

Sure.

The only problem is that my way of doing things usually involves some things that are not legal. There are aleady are several good write-ups, such as Tony Harkin's, that are done correctly.

Bill Ball

The photos with my camera came out looking just as nasty as Matt's:

Tony

Unless that oxidized quickly, rust is present on the sides of the "splits/fractures". It would be really cool to get that shaft out and send it to someone who knows metalurgy/strain etc etc

I am on SUCH borrowed time in more ways than one!

Bill Ball

Yeah, Tony, the way this came apart is bizarre. Too bad we ground the break off some by running the car, but there's still a lot of awfully strange looking stuff there.

We used to have Dave? the metallurgist here or on the email list. I remember him getting quite sick and then getting away from 928s. Maybe Louie or Wally have some appropiate engineering-based remarks.

Tony, if it going to go, so be it. This is so freaky, I'm not stopping the fun and I doubt you will.

Tom. M

If you attendended sharktoberfest..you should have looked over in the corner of the lift bay....many examples of torque tubes with exactly that kind of cheesestring break.

What I find odd is that doesn't the TT usually break at the rear?...I don't think I have seen one in the front?...also..looks like a constant diameter shaft...? Is that right?...don't see many like that break..as usually it's the later larger diameter shafts ..where they neck down to the splined section...

Goodl luck in the repair..

later,
Tom
89GT

dr bob

Many moons ago, I had some similar 'driveshaft' problems disguised as half-shaft failures in one of the little race cars. Only a couple hundred HP, but a locked spool and sticky tires. Going around slow corners caused the car to "crab" as the driveshafts would load and unload, the sticky race tires would finally slip and release the stored-up torque. In my case, the CV joints seemed to be the weak link, so we went to beefier 935 jonts. At that point, the half-shafts became the weak point. Solution for that problem came from Summers Bros in Anaheim, not far from 928 Intl, coincidentally. The boys there were in the biz of making axles for drag cars, where the shock loading was much greater than anything my little Yamaha-powered sports racer could dish out. But their calcs showed that the shafts I was already using would never break. Turns out the real loading wasn't as much a function of horsepower as it was one of traction and the turning radius of the car. So we did some measurements with a movie camera in the parking lot after a few sprirted laps around the block. (thrilled the neighbors having a herd of 2-stroke hornets doing hot laps in the industrial area streets and parking areas...) Looked at a scribed line on the sfat to see how much it twisted in a slow turn. The 20-inch long shafts would do almost a full wind. We were all more than amazed.

Summers had no problem making a set of larger half-shafts, splined to match the CV joints. The metallurgy was still quite similar to what was in there already, BTW. make 'em too stiff and they fracture, too springy and they twist more and fracture. Make 'em bigger and they twist less, and don't fracture as easily.

Cost of a piar of 'custom' one-off half-shafts? IIRC, it was about 400 1985 dollars. It would probably cost at least 4x that today, just accounting for inflation. More if you add in liability insurance. But that was for 2 axles... If the torque shaft failures become a serious problem, especially on the pressurized cars, there is an option.

One thing that the Summers guys harped on-- When do the axles fail? At the end of a wet burnout, when the sticky tires finally grab on dry pavement. Does this sound at all like 'drive through a puddle or wet gutter in the recent rains, making a quick left turn into a parking lot, up over the dry sidewalk'?

Bill Ball

This is a later shaft that had a neck-down, and that is where it broke. We are hip to the early shaft now and that's what will go back in, with the end shim kit too.

Glenncal1

Bill, Matt etc., Ifyou do end up having to drop the rear end/tranny see if you can borrow a motorcycle/atv jack that is available at most auto parts stores or sears. I have one but am in Colorado so it would be a bit of a drive to pick up . These things are cheap (I paid about $60) and have a bunch of uses besides jacking up the dirt bikes I am always fooling around with. The only downside is they take up a bit of room when not being used. A lot of the 911 guys use them for engine drops.

nicobel

OH! the torque tube blues!
Mine went borke TWICE!
The rear flex plate broke, leaving the TT barely engaged...bad TT...Replaced (the TT)..then another damaged TT (insert nighmaress about TBF)..took car to Precision Motorwerks..they cared to look into the tranny connection...replaced rear flex plate (front flex plate was perfect, no overpressure, no more nightmares) replaced the TT and the blues are over..
The symptoms included rough running, 4 cyl running (it really felt like so), noisy gear changes, headaches, stomachaches, belly aches, cold sweat...etc)
Good thing is that neither the transmission or the thrust bearing suffered at all.
By the way: LSD is WAY cool!

Bill Ball

Glenncal1: My neighbor has one of those. Will borrow if needed. Thanks for the suggestion.

nicobel: We will take a good look at the rear flexplate once the TT is out, but it looked OK through the inspection plug, so I'm expecting it to be OK. The front looks fine except for the nasty snapped shaft. Greg at Precision is The Man.

ew928

This is the take on the TT from Jay (Mr. Timing Belt Tool) Kempf:
-------------------------------------------------------------------
My guess is improper heat treat after cold forming. Or a
lack of heat treat. The longitudinal fibers are a sign of grain flow in
the long axis which is a sign that the work hardening from cold forming
had not been removed completely. But it is just a guess. A proper
Rockwell hardness test and microscopic evaluation of the fracture sites
would tell all.

Another sign of the failure is that the fibers are still in their spiral
orientation. In otherwords they didn't spring back much. A black and
white sign of taking the material past it's elastic limit.

jfk
-----------------------------------------------------------------

Be cool to have a pinhole camera, strobe matched to engine revs (like timing light)
snuck at the rear of the TT to capture how it twists and screams in terror.
2 or 4 white stripes painted longitudinal should show how much twisting.

Where's the MythBusters when you need them.

So all that 5-speed LSD sharks with the rear wheel hopping isn't going to help that
poor TT much, is it?

Ernest (NYC) Tires make much better fuses than TT's.

jon928se

What JFK said.

Is this a clue as to why the front clamp migrates forwards and causes TBF? Only just read this thread lookin for clues as to the simplest route to TT removal to do a TT bearing change so will have to sleep on this but it looks to me like this failure mode would encourage forwards moevement of th front clamp.

byrdman454

Bill,
Tony's writeup is very good. I was looking for some insight on the shortcuts you come up with and in particular the things that have to be done to change the later model TT to the early model TT.

Bill Ball

We just checked the flexplate 3 weeks prior, probably for the first time in its life. The clamp had migrated only 2mm. Three weeks earlier, everything looked perfectly normal to the naked eye. There may have been some microscopic stress or a Rockwell hardness test may have shown something, but there was no visible indication of any issue whatsoever.

Bill Ball

Oh, yeah, Tony's write-up has been printed out an in a folder marked TT in my file cabinet in the garage for years waiting for the right opportunity.

My shortcut idea is simply not dropping the rear suspension and tranny, but still servicing the TC bearings as well as replacing the tube. It seems doable and, to me, simpler than removing all the rear end crap. The bellhousing must be removed so the front of the TT can drop out, and there may be a few tricks there. I've done it and will capture that.

The TT shaft itself... THIS GETS A BIT CONFUSING AND MY INFO MAY BE SLIGHTLY INCORRECT AS TO YEAR EVENT. From 83US/84ROW-88 the shaft for automatics was 25mm and had a groove on the forward end. From 83-84 that groove was occupied by a shimmed retainer mechanism (shims, bushing and circlip). The shim retainer was not used after 84. The shaft changed in 89 to the 28mm with no groove. So, used shafts from 83-88 can be used in rebuilds and provide the opportunity to shim the end.

Greg Brown and others say it is very difficult to properly shim the TT with the drivetrain in the car. I suppose Porsche does it at the factory with the whole chassis together but not yet fit to the body. I have read over the WSM but am not yet sure how hard it will be.