F451

I've been wondering about this for a while now.

Is there some reason why Porsche did not go with a driveshaft arrangement like those seen on typical American cars, you know the large hollow steel tube driveshafts?

Are their significant benefits to the shaft drive being inside the torque tube, or its just a different design that is not necessarily better?

Does anyone know?

Thanks. -Ed

JHowell37

On any discussion forum you'll find people who think it's a great idea and others who think it's a bad idea. The one certain thing about the torque tube is that it is mounted high because it comes straight out of the back of the engine and goes into the transmission in a plane. Keeping it inside the tube can help protect it from the elements. The downfall of the torque tube is they wear out due to some other poorly designed features. If the plane the engine, torque tube, and transmission sit in is disrupted, excessive pressure is placed on various bearings accelerating their wear. Porsche should have figured out a way to incorporate U-joints on the ends of the actual driveshaft that would allow some movement without damaging anything.

F451

Very interesting. I had not even considered the lack of U-joints.

Daniel Dudley

Or a slip joint.

GlenL

The purpose of the torque tube is to connect the engine and transmission directly and ridgedly. This arrangement has all of the torque that is transfered through the drive shaft counter balanced with torque in the outer tube. There is no bending or twisting on the frame. This allows the body to be lighter and the engine and transmission mounts to be smaller.

Now whether that's an improvement or not is another question. The torque tube is a heavy piece. Two CV joints would be extra weight. A regular RWD car has a crossmember to support the back of the engine/tranny. There'd need to be something for that at both ends if the tranny was at the rear.

While it's easy to criticize when bearings fail I think the overall design is good. Very strong and keeps the body from twisting under load. Something around a draw weight-wise.

H2

I just wish it was easier to check for thrust bearing damage....to not be able to test for this easily hurts the saleability and value of the automatics, I feel. And with more high mileage 928s entering the market, many of which have mixed maintenance records, the cost of replacing an engine with TBF is an extraordinary expense that no one wants. Hence, there are some GREAT DEALS on good automatics out there....and a few real costly purchases ready to blindside someone. I'm not sure what the odds are though.

Harvey

fabric

How does it being a torque tube affect the measurability of TBF? It seems the problem with measureing thrust bearing play is that it's an inherently difficult thing to measure precisely unless you have a dial indicator that can be easily mounted, and then even then since you are shifting the crank in the engine, there's some room for imprecision. But I don't see how the torque tube enters into it.

Normy

Porsche did not go with a typical driveshaft arrangement because of weight distribution issues.

Porsche used this transaxle arrangement versus the more common "Hotchkiss drive" found on your local Ford F150 because this places the weight of the transmission at the back of the car. This makes it very easy to build a car with a perfect 50% front/ 50% rear weight distribution. This is the only reason that Porsche did this...although you have to admit- this makes the front legroom approximately the same as a front-drive car. Nice idea.

N!

Jim bailey - 928 International

A larger diameter drive shaft would have had far more flywheel effect and made shifting worse. The engine trans and central torque are solidly bolted together so there is no potential for"If the plane the engine, torque tube, and transmission sit in is disrupted, excessive pressure is placed on various bearings accelerating their wear." short of a major accident bending something. And "perfect balance " is NOT 50/50 but more like 55 rear 45 front.

Dennis Wilson

Think this also helped the 924 and 928 meet the proposed 55 mph DOT collision standards which were never implemented.

Dennis

RyanPerrella

torque tube is required for a rear transmission front engine layout, ferrari does this, porsche obviously, chevy does this with the corvette. Its pretty conventional for this type of engine layout. BTW, anyone that knows anything will tell you the rear transaxle is a superior design for a well ballanced car. Typical american cars have the conventional driveshaft cause they either dont care, or its just too expensive.

Who really cares about weight in a ford mustang, they overcome that with more and more power, still not very well engineered from the initial design but thats the way americans do things as it relates to cars.

Airplanes however are another story, i think we build those the best! Cars, ehhhh...crap!

Dennis Wilson

Ryan,

In 62 Pontiac had the Tempest and later Cutllass with a rear transmission and conventional (but flexible) driveshaft. Much earlier Buicks (up to 53) had torque tubes but no rear transmissions. Think it really had more to do with rigidity/safety and longevity.

Dennis

RyanPerrella

Dennis,

I am sure there are many reasons for using this. In my mention goes for modern cars, or current cars. The torque tube design was actually at one point in the 928's life used as a sales pitch for safety, so yes your correct that there was a safety aspect to it. I am sure there were many reasons why Porsche used this layout, safety being one of them. My point was mainly that its a contemporary design for front engine rear trans layouts. Its pretty conventional at this point. As to the original posters idea that it was maybe a hassle and over engineered.

sublimate

Many older cars had TTs because they had a single tranverse leaf spring that couldn't handle the torque on the rear axle that you get with a normal drive shaft. It was cheaper to use a torque tube than to upgrade the suspension.





Is there any reason the 928 couldn't have a slip yoke at one end to eliminate the TBF issue? Seems like that wouldn't be too hard to engineer.

Normy

The torque tube/rear transmission design is considerably more expensive to build. That's why this basically superior design is not on every front-engine rear-drive car.

N!