FRporscheman

I've always wondered exactly why 944s have torque tubes. Every other car I've every owned or worked on has had a traditional driveshaft (2 mercs, a mazda, a chevy). I mean, I sort of figured that since the torque tube encloses a very thin driveshaft, that the advantage was less spinning mass. But then a torque tube weighs a lot more than a traditional driveshaft overall. And then I wondered, why not just use an unenclosed thin driveshaft with some suspended support bearing somewhere in the middle? And then I wondered, why does it need bearings if traditional driveshafts don't have any?

Someone please shed some light on this for me...

njwilser

Only guessing here but I'll go for it. A driveshaft spins much slower than a tourque tube due to the fact that a driveshaft is driven by a transmission. A TT spins at the same rate as the motor and therefore must have a more sophisticated or stronger design. I'm not sure I'd like to be sitting next to a traditional driveshaft spinning at 5 or 6k rpm. W/ the trans in the rear of the 944, the near 50/50 weight balance is achieved. The other cars have the trans up front and the driveshaft is spinning proportional to the drive wheels. With the 944 the tourque tube is spinning at engine RPM, I I believe the difference is substantial. Suspended bearings from the body would be just that, suspended, and not actually rigid to the system.
?????? No automotive engineer hust kinda guessin.
Mike

bader$

You could run a drive shaft of sorts to the trans in the rear however IMHO they used a tube so they could distribute the weight over the length of the car rather than have to support the engine and transmission separately.

944-LT1

I believe the best part of the design is that the 44 has no u-joints thus no angles in the drive line. That alone sets-off a load of advantages to the system versus a conventional driveshaft set-up. Whats more, from the very tip of the crank to the very end of the gear cluster is a straight-continous line of rotating mass in drive. Ill abstain from the math/geometry here but you can just imagine the advantages of this kind of set-up when in motion.

Disadvatage would include minor impacts that shock the entire drivetrain whereas a driveshaft has 1.5-2 (sometimes more) inches of play. You really rely on the mounts at that point to soak up the energy. And this may (or may not) be the reason why so many 44/24/68's have frame damage after an incident.

chrly924s

It also eliminates axle hop coming off the line

Scootin159

The torque tube is nessesary due to the rear transmission (and thus high driveshaft RPM's). The C5/6 corvettes with the similar drivetrain layout use a torque tube as well.

Crunky

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.

Some Hotchkiss systems do, its called a carrier bearing. Used on multi-piece driveshafts where the two sections come together.

On a standard driveshaft layout, the drive shaft spins faster than the engine rpms in overdrive. Also, domestic car makers back in the 30s used TTs with the tranny in the front and a rear diff.

M758

The reason for the torque tube is simple.

Back when the car was being designed they wanted to put the tranny in the back to help weight. This mean that the drive shaft running from front to rear would be turning at engine speed.

Porsche was not the first company to sperate the tranny and engine like this and experience from other attempts lead them to small diameter shaft supported by a large diameter tube with multple bearings. Durign testing and evaluation that felt that this was the best way to transmit power efficiently, smoothly and reliably between the two units. I would need to read some of my books on the subject to come-up with more specifc reasons, but it was a MAJOR designed element and did undergo alot fo testing to settle with configuration they used now.

FRporscheman

It all makes sense now... I guess it serves multiple purposes. The rigid tube acts as a support for both the engine and the tranny (I've see the engine bob back and forth when the TT was out). I don't completely buy the argument of 'it needs to be lighter because it spins at engine speed' since in overdrive a conventional driveshaft would be spinning even faster... but the argument still makes sense.

Out of curiosity - does anyone know how much the TT's shaft weighs, and how much the whole TT weighs? I've always wondered how the numbers compare with a conventional driveshaft.

TheRealLefty

FWIW, the spine of the 944 is so rigid and strong that you could drive around without the unibody if you had points for steering column, top of the front struts and the rear trailing arm mounts to anchor. The torque tube was independently chosen by Alfa, Corvette and Porsche to accomplish the same end...50/50 weight distribution and an efficient rear transaxle.

MAD MAC

i also thought is forweight distribution. It keeps the car nice and balanced.

hosrom_951

Problem with driveshafts is that they have (at least) 2-3 universal jointsm over time (not as long as you suspect), they fail, causing vibrations and noise.

BMW's are very known to have this problem, i actually have this problem in the E36. The good thing is, i have taken out the driveshaft in 30 minutes, so it is easy. The bad, they cost a lot.

Now, the 944 torque tube could be a PITA to remove, but at least the bearings inside could be replaced. Cheap parts, lots of labour for the Porsche, cheap labour and expensive parts for the BMW.

RMills944

How's the efficiency of power transfer compare in a torque tube to a drive shaft? Would it be a noticeable difference?

TheRealLefty

I'm just the PR guy, never had the "The Knack" disease bad enough to be considered an engineer, but I'm thinkin' there are gains in every drive shaft CV joint that it avoids, plus a gain in the trans over any set up that offsets from the crank center.

Or as any good engineering drop-out could guess after the first day of Strength of Materials lab "It's the area under the curve"

JimV8

My 1 1/2 cents. The torque tube is a structural member that simplifies mounting points of the engine and transmission.