sublimate

I don't think a torque tube/rear tranny is inherently more expensive to build. It does require an IRS - which is certainly more costly and complex than the more common live rear axle - and so far that reason it hasn't been used often. But it's also difficult to package because you need room for the rear tranny (hence the almost unusable rear seats in our 928s) and the remote shifter location is complex (and sloppy) for manual trannys. Overall, not worth it except in a high performance vehicle.

Note that the older cars with TTs that I mentioned up in post #14 didn't have a rear transmission - they used the TT instead of an open driveshaft to conect the tranny to the rear diff. Many cars used this configuration from the Model T up until the mid-50s.

There's a reason why if you go to www.torquetube.com you don't see a picture of a 928.

Emickelsen

at the extreme least, it sounds cool.

John Speake

I think another reason for the torque tube is that this drive shaft is rotating at engine speed, unlike a final drive propshaft from a more conventional engine/gearbox unit to rear axle.

They had lots of unwanted resonance issues with the 928 drive shaft, and the location of the bearings in the torque tube were critical in solving this problem. The torque tube was a good way of locatring these bearings.

sublimate

A normal propshaft usually turns at engine speed when in top gear, and sometimes faster (overdrive).

Luckily our TTs don't need to transmit near the torque of a conventional propshaft of else it'd twist up like a pretzel.

John Speake

Ah ! Silly me :-(

So why did Porsche make such a big deal of the propshaft design ?

According to "Project 928" the biggest issue was vibration.....which could result in a permanent kink in the drive shaft.

dr bob

Mid-'70s MB cars (and I'm sure others...) use a rubber donut at the front of the driveshaft for vibration isolation, to help with slight misalignment issues, an to manage axial thrust/displacement issues at the same time. Reinforced ring with six holes spaced around the ring. Three holes bolt to the drive plate on the rear of the engine. Three holes accept a drive yoke from the front of the driveshaft. Sound familiar? We have all the right stuff except the rubber ring and some spacers. And a ring that will take over 300 lbs/ft of torque.

-----

For manual-shift cars particularly, having a large rotating mass between the clutch and transmission would make shifting very slow as you would wait for the heavy driveshaft to spin down, and would beat up synchronizers quickly. The dual-disk clutch is one illustration of how sensitive the P-car engineers were to the problem. Better/stronger synchronizers allowed them to go to the larger-rotating-inertia single-disk clutch for the same reason. This isn't nearly as big a problem on the automatic cars, of course.

Vilhuer

Design used in 928 is very effective in isolating engine and entire drivetrain from car body as there are only four rubber mounts in entire system. If TT wasn't in between both engine and gearbox would need at least one more mount I think, most likely two.

There are obviously problems in Porsche design. Sliding coupling on automatic TT would have saved many owners wallets byt not allowing TBF to happen.

Other lesser problem is bit uncomfortable feeling in manual cars when clutch is pressed and released. I think TT brings all drivetrain looseness out. If there was some rubber in somewhere it would take out large part of it when clutch is released or engaged. Something like rubber donut which is used in many FR cars between front mounted gearbox output shaft and driveshaft.

Vilhuer

This is exactly what I mean. Its used in BMW's also. Having donut in 928 would take out some of the heavy feel TT brings with it.

IcemanG17

Is the torque tube a weakness of the 928.....I don't think so....does it contribute to the 928's excellent handling dynamics? 100% yes......sure it requires more maintence than a standard driveshaft.....& thrust bearing issues are out there (but easily remedied with annual checks)..... If the 928 was with a forward tranny & standard driveshaft....I think many of us wouldn't be there today!

Jim bailey - 928 International

The 944-968 auto runs a large rubber damper as a "flywheel". The 968 clutch disc has a flexible rubber center as I recall much like several later 911 variants.

F451

Excellent info guys! Thanks for all the thoughtful replies, I'm learing a lot.

Ed

GT Jackson

John Speake wins all the marbles! After just reading Project928, there is much discussion during development of the drive train layout and the "fast shaft" which needs bearing stability, but also creates a hell of a racket unless dampened. I think all the other issues mention here also come into play, but it seems the dvelopment team were most concerned about noise.

F451

"learing" Ha ha...

I meant 'learning'!

danglerb

What sort of practical limit does the 928 TT have in RPM?

Very educational post, laid out all the issues nice and tidy.

dr bob

Limit isn't on the driveshaft itself, but the bearings. FWIW, the engine mains and rods would likely give up long before the ball bearings in the torque tube. front of the gearbox, torque converter and front pump if you have an automatic, etc.