17 hours ago in Articles
Tips from a Skip Barber Racing instructor.
So last night a few buddies and I were bench racing and talking about front engines with rear trannies. Conceptually that's cool as it distributes the weight better but where we got hung up was the actual mechanics of it.
I know the Porsche 924/944 and the Corvette do it differently but they both have a torque tube. So how does that torque tube attach? It would seem that you'd have to pull either the tranny or the engine to remove a torque tube?
Also which end has the clutch, flywheel and starter? Is the bell housing still in the engine along with the clutch/starter or has it all been moved out back?
The torque tube spins faster than a driveshaft so we are presuming there is some kind of outer tube to contain it, right?
Could you easily do this with say an Audi or VW tranny out back and your engine of choice up front?
On the '61-'63 Pontiac Tempests, the flywheel/clutch/starter are all mounted as usual behind the engine. I seem to recall the 924 has them in the rear though, but I may be wrong. On both of them, the shaft is solid steel held on bearings inside an outer tube.
I do know the 2.5L Audi turbo-5 will bolt up to a 924(non-S) bellhousing, and that early 924 front control arms are the same as an A1 Rabbit.
Hey Per, weren't you looking for another VW project....
The 'vette has the flywheel and clutch on the engine side. The torque tube is a solid tube that contains the drive shaft inside.
notice the slave cylinder in the picture here.
The c4's had a more primitive setup, they had a torque tube of sorts but the transmission was in the normal spot. The tube (much like a miata's) attached the trans to the diff and eliminated the need for transmission mounts.
The coolest thing I have seen is a LS1 with the torque tube eliminated and the trans axle bolted directly to the bellhousing for a sweet mid engine setup.
My GTV-6 has the flywheel and bell housing up front a two piece driveshaft and the clutch in the rear.
ONce you remove all of the bolts there is enough play (just) to get the DS out but it is a real bear. On the Milano Alfa helped this somewhat by making the support for the middle bearing removable but with my car it is a big prybar pain in the butt.
So the torque tube is a solid steel shaft like an axle inside another tube with bearings to support it. I was presuming a hollow driveshaft type of arrangement.
I'd never seen the Vette drivetrain out of the car. Pretty slick and low tech.
I dont' see a slave cylinder on the vette unless that's it on the front of the tube.
i wonder why more don't do this, it looks so simple and has real world benefits.
carguy123 wrote: So the torque tube is a solid steel shaft like an axle inside another tube with bearings to support it. I was presuming a hollow driveshaft type of arrangement. I'd never seen the Vette drivetrain out of the car. Pretty slick and low tech. I dont' see a slave cylinder on the vette unless that's it on the front of the tube. i wonder why more don't do this, it looks so simple and has real world benefits.
Trunk space issues?
Not like there's all that many RWD cars anyway...
On the 924/944 the clutch, flywheel and starter are mounted up front as normal.
Some of the Alfa's used a rear mounted transaxle.
The biggest drawbacks to the TT solution are weight, complexity and expense. They aren't as light as a standard driveline. They are fairly complex in their manufacture and are difficult to repair when the bearings fail. They are expensive to build or repair as well.
A torque tube is just a steel tube with a series of bearings inside and a set of mounting flanges on each end. An extension shaft runs the length of the tube and rides on the bearings. The shaft fits into the splines on the clutch at the front and attaches to the nose of the transaxle at the rear with a splined coupler.
BTW, here's some information on rebuilding a 944 torque tube.
FYI: the later 924 and 944 used Audi FWD transaxles and Porsche motors where as the early 924 used a Porsche 915 based transaxle and an Audi/VW motor. They are bizarre parts bin cars to say the least.
fiat22turbo wrote: Some of the Alfa's used a rear mounted transaxle.
Correctamundo. They would be the Alfetta GT and sedan.
the A series Fiat 124s used a torque tube rear axle. While the transmission was up front and bolted to the engine, it used a single U joint in the middle of the shaft. The front was hooked to the trans with a rubber donut and the rear half of the shaft and differential were all one rigid piece.
Unfortunately, rough roads and misalignment meant broken torque tubes, even if they had excellant handling in that configuration
It would seem that you would need a very good or even special bearing since the torque tube is spinning at engine speed. There's also the extra mass of the long shaft which could make a lightweight flywheel a must. Or would that just balance out the driveshaft that's missing?
So looks like you'd have to make a plate at each end of the torque tube to attach to the rear of the engine and one for the front of the tranny to seal it off.
I'm betting the LS1 bolted directly to the transaxle (not really) is what they are doing here: http://www.factoryfive.com/table/ffrkits/GTM/GTMkit.html
One factor the designer must account for is the torsional fexibility of the shaft, undamped driveline harmonics can eat your lunch. (thus the slang term 'speedometer cable' for the Tempest unit) Going from a solid shaft to a somewhat larger hollow shaft helps a bit, at the expense of greater rotational inertia. (Note that the Corvette TT housing is rather large) If you put the clutch/flywheel at the front, then you get wind-up between it and the transaxle. If you put the clutch/flywheel at the transaxle you get wind-up between it and the motor. Either way sucks.
In contrast to the four cylinder (water-cooled) Porsches having the clutch at the front, the 928 put the clutch at the rear. I never understand why Porsche did it two different ways.
mel_horn wrote:fiat22turbo wrote: Some of the Alfa's used a rear mounted transaxle.
Correctamundo. They would be the Alfetta GT and sedan.
Alfa 75/Milano and GTV6, also.
It would seem that the shaft would be subject to twist as well (hence your term windup?) That's why I used the phrase "solid shaft like an axle" thinking that axle material is designed to counter twist.
Driveshafts should be subject to the same torsional inputs as the shaft in the torque tube should they not?
So you'd need a short wheelbase of about 89.2" which would give you a shaft length of not much over 4' and have it supported in 3 places.
One more question. When you say torque tube is that referencing the shaft inside, the tube it's encased in or the whole set up?
For GRMers this isn't sounding like an impossible project. It might not make for a $200x project, but then again, who knows.
Yes, there is typically some windup in the shaft. Though I would expect it to be much more noticable on my '62 tempest(they even called it a "Rope" shaft) than on a modern Vette. Length would play a big part in how noticable it is, though - just like a swaybar - the diamater of the shaft would effect how much flex it actually has: d*4th-power
"Torque-tube" usually references the whole assembly connecting engine to transaxle.
The 944 has dual pinch bolt sleeve at the transaxle end to facilitate repair /replacement of the drive train, and for engine mounted bell housing movement(slide it backwards) for clutch work.
7pilot wrote: The 944 has dual pinch bolt sleeve at the transaxle end to facilitate repair /replacement of the drive train, and for engine mounted bell housing movement(slide it backwards) for clutch work. m
I'm not quite picturing that.
Carguy: Number 14 on this chart.
carguy123 wrote: For GRMers this isn't sounding like an impossible project. It might not make for a $200x project, but then again, who knows.
Not sure what you have in mind, but you could find rusty, non-running 924s for free(or possibly even get paid to haul it away). A good running Audi turbo-5 isn't that much money, or find a rusty running car. Then you just need your body of choice to install it in. VW Rabbit/Jetta/Golf/Fox are all easy choices, but a Superbeetle would be sweet too.
Oh yeah, there's at least one person stuffing a Z06 drivetrain under a '62 Tempest LeMans that I've read about!
Yep, the 924/944 use Rabbit front suspension and super beetle rear suspension (they aren't exactly the same, but extremely close)
The Porsche 924/944 torque tubes are steel and are relatively easily modified (and have been to adapt other bell housings to the front) The later 924 turbo and 944 used a larger spline for the shaft that is the same as many Fords and Chryslers.
Using another transaxle out back would be tricky since there aren't very many units like the one's used in the Porsche's. I guess you could make it work with a Subaru unit....
fiat22turbo wrote: I guess you could make it work with a Subaru unit....
It would seem that with a cover plate and a shaft that matched the size and splines of the input of the tranny you could do this with about any longitudinal mount FWD unit mounted in the rear. Of course some would be better than others.
Better being defined as cheaper, more available or easier to adapt. Por$che is always spelled with a $ and the 924/944 units are so old it makes we wonder about the availability, condition and price. Plus IIRC not many came with an LSD. Of course there is the "it's been designed to work this way" factor.
What size is the shaft itself on the Porsche unit? Still wondering if an aluminum driveshaft might not be stiffer and work just as well but at a cheaper price point. I'm worried the bearings might be an issue there tho.
The issue with the Subie unit is that the FWD portion isn't available with an LSD in the states, which is why I mentioned the VW/Audi longitudinal units in the first post. I don't know how many have the LSD and how easy they are to find, but at least they sent some to the states with an LSD.
I have been playing with building a Locost. My only issue is that I didn't want to build the typical front engined RWD mostly due to space concerns. All the ones I've spent any quality time with leave you with a cramped cockpit due to the transmission tunnel. And the tranny brings a heat source into the cockpit with you. I got burns from just tracking an Ultralite on a 100 degree day.
I had decided to build a middie and wanted to do a Subie because of the flat 4 until I found out about the LSD issue. A VW build didn't interest me due to all the issues with VW engines and their weight. It could also be the fact that in the past 2 years I've watched 4 VWs spontaneously combust on the roads - 2 Beetles, 1 Jetta and 1 Passat. I couldn't find a way to adapt a rotary, another very compact motor, to a FWD tranny.
I don't want a V8 so there goes the LS1 bolted to the vette transaxle and besides I'd really like to keep the car weight down to near the 1,000 lb mark. And since I'm not the best driver in the world if I get an engine with too much torque I embarrass myself when I get excited and accidentally give it too much throttle in the midst of a corner.
Now it just so happens that I have a spare S2000 motor laying around. I have a 2.0 in my S and I have a new 2.2 sitting in the garage. I could put the 2.2 in my S and use the 2.0 in a Locost or leave the S alone and use the 2.2. The S has some very special concerns in that if you get the wrong transmission ratios you could be going in and out of VTEC when you shift. Right now once you are in VTEC you stay in VTEC with the stock tranny.
I'm not brand or a specific motor loyal, I just want a great power to weight and good CG (hence being enamored with the Subie engine and the rotary). The S is a fabulous engine, but it's tall when compared to the Subie or Rotary.
This seems like it could be a good compromise, provided I can find a tranny. Does anyone know which VW/Audi longitudinal mount trannies would hold 250ish hp, comes with an LSD and can be had for a reasonable price? I haven't researched it yet cause we were really just BSing when this came up, but nothing anyone has said so far makes it sound like a horrid idea.
carguy123 wrote: I had decided to build a middie and wanted to do a Subie because of the flat 4 until I found out about the LSD issue. A VW build didn't interest me due to all the issues with VW engines and their weight. It could also be the fact that in the past 2 years I've watched 4 VWs spontaneously combust on the roads - 2 Beetles, 1 Jetta and 1 Passat. I couldn't find a way to adapt a rotary, another very compact motor, to a FWD tranny.
You do it like this
One question I've had about these setups is how rotational inertia affects performance. With a "standard" driveline setup, the driveline speed is directly proportional to the vehicle's speed, and under normal driving there aren't any necessary rapid changes to the rotational speed of the driveshaft. In these torque tube setups, though, the "driveline" speed is equal to the engine rpm, which means that it's rapidly changing every time you shift. This just seems like it'd have the same effect as having a big 'ole heavy flywheel, and really kill how fast the engine is able to accelerate/decelerate rpm-wise.
What am I missing here?
...the polar moment of inertia. A shaft has all it's mass much closer to its center-line than a flywheel. Therefore it takes less energy to accelerate/decelerate than a flywheel.
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