Torque tube advantage

[TMAPV]

Can anyone explain how a torque tube differs to a 'normal' setup when we we have a front engine vehicle and transmission on the rear axle? It seems to be used on a few cars such as Astons, so there must be some benefit and trade off somewhere, different load path etc. Would be interesting to hear your thoughts

 

[BrianPetersen]

Torque-tube of the backbone-chassis independent-rear-suspension variety (Miata, Corvette C7, Pontiac Tempest "rope drive"), or torque-tube of the Ford Model T through 1950s solid rear axle variety (I think Rambler was the last mass production manufacturer to use this)?

In the modern era (with the IRS design, and the torque tube being bolted solidly to the final drive and transmission and engine etc) it allows the torque reactions from the final drive and transmission etc to all be contained in one "drivetrain module" that goes all the way from the front of the car to the back underneath. You don't get the engine and transmission trying to twist the bodyshell anticlockwise up front while the final drive tries to twist the bodyshell clockwise out back, because that torque gets transmitted through the torque tube (opposing the driveshaft torque) without having to go through rear-end and engine mounts. I suppose it means you can use softer engine mounts because you won't get the engine trying to twist itself out of its mounts whenever you punch the accelerator.

The bad? It's expensive. If you have to change the clutch, the whole thing has to come apart. (Friend of mine is facing this with his C7 Corvette right now.)

It's quite apparent that for the normal run-of-the-mill vehicles that most people can afford, any advantages don't outweigh the cost.

 

[BrianPetersen]

Here's the Model T through 1948 solid torque-tube rear axle variety. The connection to the back of the transmission has to be, in effect, a ball joint. I don't know how that joint was built.

https://www.youtube.com/watch?v=z1uVNECo0AY

 

[BrianPetersen]

Here is the Corvette variety, with the engine and clutch bell-housing bolted together at the front and the transmission-input-shaft-speed driveshaft inside a torque tube to the input of the transmission and final drive at the rear.

https://www.google.ca/imgres?imgurl=https://

http://www.lsxmag.com/image/2014/01...hUKEwjYuLOr9fL9AhVjDWIAHV9LC-4QMyguegUIARDnAw

 

[BrianPetersen]

The internet rabbit-hole investigation prompted by this post revealed online videos about how to replace or service the spherical joint between the back of the transmission and front of the torque-tube in the solid-rear-axle 1950s-Buick variety. Interesting design. There is a single universal joint at the back of the transmission between the transmission output shaft and the splined end of the enclosed driveshaft, surrounded by a spherical joint in a rather complicated bolted-together housing with multiple fixed and moving seals to keep the transmission oil, rear-end oil, and the outside world all separated.

I suspect this was done because there was concern about U-joint life if the U-joint were exposed to the outside world as opposed to running in an oil bath, but that spherical joint looks like a nightmare.

 

[LionelHutz]

As posted, it makes it so all the reaction torque of the engine driving the wheels lifts straight up on the front end. There is no rotational twist of the drivetrain or rotational twist put into the chassis.

 

[TMAPV]

Thank you for your inputs. You have to excuse my ignorance as I'm trying to learn: a typical powertrain layout without torque tube, as it applies torque onto the driveshaft, will have a reaction torque on the engine mounts in opposite direction. A torque tube connecting this engine to the rear would stop the engine twisting from this reaction, and transmit it to the transmission/diff casing on rear axle? Doesn't this mean that this reaction is simply moved to a different spot, so in this example the transmission casing ,and would this not require mounts there?

 

[BrianPetersen]

The difference is that the torque from the diff housing goes through the torque tube to the engine block without passing through the bodyshell ... or the rubber diff mount bushings or the rubber engine mounts.

 

[3DDave]

One can make this experiment. Get a paper tube, two short dowels, and some rubber bands of suitable length - somewhat shorter than the tube, but not very much.

Make holes near one end of the tube so one short dowel will pass roughly through the center. Push a dowel in one side, loop the rubber band(s) over it and then through the other side.

At the other end, cut similar slots to position the other dowel across the center. Then take the second dowel and loop the rubber band(s) over it and through the tube. Now hold that second dowel and turn the tube to apply a torque to the rubber band(s). Build up however much tension and torque you like and then set the second dowel into the slots.

This is the arrangement of the torque tube in the car. You can set that cardboard tube down and it won't be driven anywhere by the torque in the rubber band. You could make a thin and delicate structure on that tube and it would not be affected by the torque of the band being entirely reacted by torque in the tube as long as you apply the torque to the rubber band by reacting it through the tube.

In the car this arrangement would not be entirely useful - instead of the dowel there is a gear set that transfers the twist to the tube, but it also creates a torque perpendicular to the tube via the axle(s). That torque is absorbed as bending back through the torque tube to the body and suspension at the front of the car and is reacted by the weight of the car.

 

[LionelHutz]

You have to excuse my ignorance as I'm trying to learn: a typical powertrain layout without torque tube, as it applies torque onto the driveshaft, will have a reaction torque on the engine mounts in opposite direction.

But how can you have torque in the driveshaft unless the differential end is resisting the driveshafts rotation? There must be an opposite torque being applied at each end of the drive shaft to get torque in the driveshaft. So, the engine torque is cancelled by the differential torque with the use of a torque tube. Then, the only rotational torque put into the drivetrain is around the output shafts of the differential.

 

[GregLocock]

You'll notice most RWD IRS cars with normal drivelines use an isolated rear subframe, this is needed to get some low pass isolation from diff whine etc. That adds dollars and weight, and compromises vehicle dynamics and secondary ride if you mount the suspension to it. All these flexible systems in series can engage in all sorts of nonsense in high powered cars, with say 300 hp/ton being where things usually get interesting.

The immediate advantage to me is that a full structural torque tube reacts the contact patch forces over the entire length of the engine to axle assembly. This means I can use as few as 3 bushings with fairly low rates and so get good NVH and good control at the same time. The designs that use a CV joint at the back of the gearbox aren't quite as neat from a refinement point of view, but retain the main advantage. Your problem is that you've got three shouty things you need to mount to the torque tube, directly or indirectly (engine, trans, diff), and they will make it sing. So I suspect the torque tube to trans joints are somewhat isolated, but that means you might get some angular motion at the tailshaft to propshaft joint, and you might do the same at the diff, so you end up needing 2 or 3 CVs anyway.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MC6]

This ends up destroying the differential to transmission mating point (when power and traction are high enough). The whole differential tries to roll forward and breaks the case, and trans output shaft. The zr1 vettes have significantly stronger differential cases as a result, at least for the 6th gen. I would imagine the c7 zr1 diff case is beefier too.