Ok in marine I have laid more than a few shafts between the engine output and the transmission. None of them had a torque tube like on the 924/928/944.
So what gives? Is the car not rigid enough? Are they trying to sidestep a center drive shaft support? Is it just the state of the art at the time and can be circumvented?
Learn me GRM you are my only hope.
Chassis rigidity of the 924/944/928 is quite good for the time. The engine and transaxle are connected and the movement of the chassis has little effect on them.
The use of the torque tube on the 924/944 allowed them to make use of Audi FWD transaxles mounted in the rear and avoided having to create a new transaxle. This is especially handy for VW as they were the originators of the 924 project and wanted to use as many parts from the existing VW and Audi parts bin as possible. This is why the front suspension is from a MK1 Golf, the rear from a Super Beetle, engine and transaxle from the Audi 100LS, etc.
The bonus being that it allowed for a better static weight balance without large compromises.
If you think about it, how would you make use of a transaxle like the Audi 016 in a front engine RWD sports vehicle? Porsche is an engineering firm and the engineers found a solution to that question.
For the Corvette, Alfa and other vehicles that used a rear mounted transaxle, it was done to help with weight balance in mind.
The Ferrari 275 doesn't use a torque tube...
In reply to Stefan:
How would I do it?
Like I have always done it.
I would bolt the transmission to the chassis in one location and then bolt the engine to the chassis in another location.
It is the torque tube I don't understand. It seems superfluous given they also decided to bolt the transmission and engine to the chassis as well.
That is the crux of my question.
Since the spindly drive shaft rotates at full engine RPM, the "torque tube" protects it, and, in the event of a failure, would serve to contain the pieces.
In reply to Danny Shields:
So it's a scatter shield/really wide driveshaft loop.
I have found where it held the rear axle as a part of suspension. That's makes sense as there it is a structural member.
But with it in a rigid mount (both ends fixed) it just seems a bit much.
It holds more bearings to allow the higher speed rotation of the shaft. It also keeps everything in line which removes the need for CV or universal joints and the difficulty of balancing those at higher RPMS. It also eliminates driveline "Take up" between the engine/and diff by solidly connecting them together(you have to compress the motor mounts before the driveshaft turns, etc.)
Ok, now the spins faster than the driveshaft argument doesn't make sense.
I just did some calculations and the driveshaft in a new ZL1 Camaro spins faster than the motor does at 170 mph.
7800 rpm on the drive shaft vs 6300 redline for the LT4 in the ZL1. Believe it or not, it has the old two piece driveshaft set up too.
On the 944 Turbo, at the factory claimed top speed, it too is spinning faster than the motor. 7100 rpm vs 6500 rpm redline.
I am wondering if it is simply packaging from assembly, a "Well we have done this-this way so why change now on a price point vehicle answer"
Germans don't make superfluous decisions...this is bugging me.
There was also Pontiac's Rope Drive which connected the front engine to rear transaxle, but not exactly rigidly...
I don't think its safe to assume the engine and transmission are rigidly mounted and certainly not relative to each other. Between motor mounts and transaxle mounts flexing, the chassis flexing around, and then the suspension shaking everything up there is a lot of relative motion. And thats an ideal case...as mounts wear out who knows how much the alignment will change. The torque tube would seem to serve an important role in keeping it all squared up.
In reply to HippieWagon:
So a solution to tolerance stack up...that seems reasonable. It would mitigate flex, but I can't see that flex being more movement than what happens to a live axle car.
I wonder if it is a tangentially similar reason. NVH mitigation.
In reply to FlightService:
Your calculations all include top speed. Your calculations say just because a car can survive the driveshaft RPM needed to go 170mph it is fine to repeatedly have those same driveshaft RPM on a 1-2 redline shift(I know, slightly lower because yours have OD). The average RPM over the lifetime of the driveshaft should be markedly higher in the torque tube vehicles. Is the torque tube absolutely necessary? No, but neither is having the windows automatically go down slightly every time you close your door so the doors close more easily.
FlightService wrote: Germans don't make superfluous decisions...this is bugging me.
In my opinion they love to engineer in "better, but not very necessary".
maybe unwanted flex is the reason. The Vette isn't the only modern car that uses it. So does Mercedes on the SLS AMG.
From their press release, "The advantages of this (torque tube) sophisticated solution are associated with the rigid link between the engine and transmission and, in turn, the optimum support for the forces and torque generated."
That reads like it is helping mitigate torsional wind up from hard launches.
In reply to FlightService:
Like a Miata! 
In reply to MrJoshua:
I went with top speed because that is how you design to a performance point vs to a lifecycle point. This is a "never fail" item so the FS is higher.
In reply to FlightService: I wonder if the name of the item is indicative of its purpose
In reply to jfryjfry:
maaaayyyyyyyybbbbbeeeee
GM used it for years on all models. One less axle locating problem with the then new coil springs.
One less U joint.
In reply to iceracer:
$ U-joint > $ Tube?
Torque tube transfers the wheel's reaction torque through the tube to the front of car. This means that reaction force travels a longer distance(to the engine mounts). With a standard driveshaft, the reaction is only carried up through the differential mounts, shorter distance=more force.
In reply to Stefan:
The transaxle mounts are further spaced apart than a standard differential mount. Where is there a magnification of torque to cause the need for the extra leverage beyond what is given by the transaxle structure itself? Power to the wheels is assumed to be the same and therefore rotational torque caused by the wheels is the same in both setups.
The Mercedes AMG GT has a torque tube with a transaxle too. In some varients it's carbon fiber.
I'm not really sure what the advantage/disadvantage of a mazda style power plant frame (Miata, FD, RX-8) is vs. a full torque tube. To quote Mazda's Yamaguchi RX-8 book "The PPF endows the powertrain with a remarkably accurate engine-attached-to-the-toe feel, allowing it to respond to the driver's commands for acceleration, deceleration without hop, snatch or shudder."
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