Adk46

It occurred to me this morning that a more flexible flex plate would help. It would reduce the steady thrust on the engine from a displaced drive shaft, or prevent the displacement in the first place by reducing peak thrust.

The bending stiffness of a plate varies greatly according to its thickness (a 2 mm plate is 8 times stiffer than a 1 mm plate). I believe the flex plate is already made up of two thin plates rather than one thick plate, probably for this reason - stiffness is reduced by a factor of 4 on this simple basis (not quite so simple, however).

So, you can guess where this is leading: How about three even thinner plates? Some details would have to be addressed (e.g., buckling). Could a replacement flex plate be installed easily?

Forgive me if this has already been suggested. And no shouting!

dr bob

You'd need to prevent the plates from breaking from the flexing. The Pre-tbf setup had a splined sleeve and used circlips to limit relative axial displacement. I'd be tempted to clamp a larger splined sleeve on the end of the shaft, and let it float in a more substantial plate that doesn't flex.

Meanwhile there are existing solutions like the Constatine clamp, or a clutch disk clamped between a pressure plate and ring gear.

Adk46

The maximum (outer fiber) stress for thinner plates would be lower for the same displacement, so they would be less likely to crack from flexing on this basis. Some careful engineering of the attachments would be required. Buckling failure from the torque they must carry must be considered.

I agree that the elegant solution to the problem is a sliding/floating spline joint. But if we consider that the flex plate alternative *almost* works, then perhaps the easier/cheaper/conservative solution is to improve it a bit - make it a bit more flexible.

If the better clamping systems work, peace. But I wonder if the situation is that transient axial loads are exceeding design loads in normal service. That is, it is *not* that the standard clamps aren't strong enough - they are acting somewhat like a mechanical fuse link. I just have this nagging feeling that a stronger clamp is like putting in a bigger fuse. I've seen the pictures of failed shafts and failed flex plates, and someone has mentioned trouble at the transaxle end of the drive shaft. Where is our devotion to extreme German over-engineering? Again, if stronger clamps work (long-term - do we know? do we care about reaching 200K?), peace.

docmirror

Treat causes, not effects(symptoms). The cause is the driveshaft twisting and untwisting causing shaft helical changes in length. The effect is the bowing of the flex plate. The flex plate has sufficient take-up for normal helical length changes, but when the spline clamp migrates, it puts pressure on the flex plate in a pre-loaded state. That overcomes the design goal of the plate, and subsequent bearing(or flex plate) damage.

The best solution is a flex plate which is flat during unloaded moments, and it allows sufficient movement fore and aft to accomodate the helical changes in shaft length. The spline clamp needs to be fixed, and cannot decrease with repeated stress placed on it.

TheoJ

The clamp was supposed to keep the a firm grip in the drive shaft, but the design was such that it can't keep up with the power of the engine and twisting/torsion of the shaft. It looses grip on the shaft. That makes it crawl (ratchet) towards the engine and pre-loads hundred's of Kg's on the flexplate.... possibly killing the thrust bearing of the engine. The flexplate bends, sure, but try to bend it by hand and find out how strong it is. A more flexible flexplate would work (as long as it does not break), a slipping coupler that allows movement without skipping over splines (rubber spider coupler perhaps).

Personally I go along with the original idea Porsche had: keep the clamp firmly on the shaft. I use a replacement clamp that has lots more clamping force. Constantine has a nice alternative that would also work.

ps: I'm not so fond about the additional clamp next to the flimsy original clamp. Better use a proper clamp!

regards
Theo Jenniskens
1992 928GTS

Black Sea RD

The original clamp was not designed to work by itself, but instead it was a system which also used parts at the very front of the drive shaft which did not let the drive shaft get pulled out through the clamp.

These front parts were to set a specific distance between the flywheel and flex plate. Once set, the flex plate was bolted to the flywheel. If one reviews this procedure carefully and as outlined in Porsche WSMs, it would be seen that the flex plates would actually be pulling back on the flywheel just a bit. Porsche wanted to be sure there was absolutely no forward thrust on the engine thrust bearing.

When Porsche stopped using the parts at the front, they should have changed the clamp, but they didn't.

That's what we did and glad we spawned so many other devices to follow that stop one of the main causes of engine TBF in 928 automatics. Now owners have a few different options to choose from.