I have drawn two simple diagrams using EXCEL. I don't have a CAD package on my home computer.

The first drawing is what I believe the C5 Powertrain looks and acts like at idle.




The next drawing is how I believe the C5 Powertrain reacts to a significant torque input from the engine.





Comments and discussion?

 

That is how I originally saw things, but Eric D posted the torque cancellation through the torque tube so the engine mounts only react inertial loads. Torque tube bending would be about the half shaft axis of the car. (This is if I understand your diagram)

Of those who have installed this brace to date, all have said they saw a noticeable improvement in the car.

 

I agree to some extent with Eric D about the torque tube cancellation theory, however that does not explain the rotational up and down strain put on the tranny and differential joint.

Maybe GM should have had a third mount under the tranny to relieve the rotation forces (up / down motion) created by this mounting configuration.

The DTE brace helps prevent breakage caused by the rotational issues mentioned above and I feel the Pfadt piece further helps by limiting the upward and downward motion of the differential.

I'm sure there is some side to side movement, but it's probably nominal, except if you're drag racing and launching the vehicle hard.

 

Eric,

My drawing is trying to depict the differential from a rear view. I see the differential rotating clockwise, just as the engine does under the application of torque.





Here is another drawing that is how I believe the Pfadt mount is helping our Powertrains.





Comments and discussions ?

 

That is the direction that is canceled by the torque tube.

 

We are actually on the same page. We all agree that there is 'up and down strain' on the diff. Any drag racing pic of a C5 lifting it's wheels proves this. The DTE brace is supposed to address that issue. As for the 'left to right' rotation that the PFADT trans brace is advertised to counteract... well the existence of that force is what this debate is about.

 

The other question I would have is the counterweight is left off when the new mount is installed. I am not sure of its function unless it is just for vibration.

 

Counterweight? Are you referreing to the round weight on the left side of the diff some call a resonator? If so, that is to remove some possible drivetrain harmonics. I removed it from my diff when Pfadt brace was installed as it is not needed anymore.

 

No, I don't believe that it cancels any Roll Axis movement.

My understanding of the Torque Tube's function is to connect a front mount engine / clutch assembly to a rear mount transmission / differential.

In fact, I believe that the Torque Tube ensures that the entire Powertrain (Engine - Clutch - Bellhousing - Torque Tube - Transmission - Differential) rotates about the Engine's Roll Axis.

 

There's only one way to install it, the stud spacing only allows it to fit one way. I'm going to get under the car today to inspect.

 

I have put together another simple diagram that I hope will help explain what the torque tube is meant to do:





I believe that the Phaft Differential mount helps to limit the rotation of the Powertrain about the Powertrain's Roll Axis.

 

The engine generates torque that is applied to the pinion gear, which in turn is reacted to the differential housing. The housing is bolted to the tranny and then the torque tube, which is bolted to the engine block. So the torque about the crankshaft axis is reacted out by the torque tube to the block.

 

What? False. Even Da Vinci knew, a little over 500 years ago, that 'in theory' such a machine could work.

As for everybody else -- there's been 6 pages of people arguing about whether or not the torque tube somehow magically buffers out all rotational load transmitted from the engine... You all do realize that the torque tube (I MEANT PROPELLER SHAFT) itself is inside an aluminum tube (I MEANT TORQUE TUBE) that is solidly fastened to the engine, and that aluminum tube is also solidly fastened to the transmission... so when the engine loads itself along the centerline, so does the transmission.



Earlier somebody mentioned that any of this rotational inertia would manifest itself in the cockpit with the shift ****... and I'm just going to say that before I installed the VB&P mount I previously complained about, I got much more movement in the shifter, and exponentially more movement in the shifter under wheelhop conditions (in the wet or what have you).

I seriously have read like 12 of these posts and I'm really confused (irritated) about how the few "engineers" are confused about whether or not the engine loads the transmission... it's pretty clear to me that the transmission and differential does load itself along the crankshaft's axis.... its not rocket science, and even if it was, some engineers ought to be able to figure it out without 5 pages of deliberation and constantly rephrasing the same post from the same 3 people drawing excel diagrams that needn't exist (no offense Pumba, I realize we're on the same page).

The first image the PFADT guy, Josh, posted clearly answered anybody's questions... which is why they put the pads so far out, to get more of a force moment to resist the rotational load, while also limiting any vertical movement that might occur.

 

That's the question, there is no "force moment" (your words) to take out.

You do realize that the torque tube your refer to is the "propeller shaft" and the "aluminum tube" is the torque tube or as GM refers to it the "Driveline Tube".

 

http://en.wikipedia.org/wiki/Torque

Fair enough with the whole disambiguation of the "torque tube" vs the propeller shaft inside of it, my points remain. I'm not going to claim to know what every factory wants to call something. ECU, EMS, PCM, its all the same thing.

Regardless, my point was that the torque tube (the housing), and thus the transaxle, will load itself along the same axis of the engine, as it is solidly fastened to both the transmission and the bellhousing.

 

Which brings you back to the question as to why the bar "appears" to be designed to take out this load that has in effect been transmitted back to the engine block. This being the case, I think everyone is trying to figure out the benefit of the brace. All who have installed it seem happy with the results though.

 

Yeah I guess the conscious being controlling the distribution of torque while contemporary physics is sleeping silently whispers into the rotational load's ears and coaxes it to just distribute itself over the engine mounts and not get off at the closest stop (the transaxle mount).

I think you and a few others might be the only ones trying to figure out the benefit.

If the torque tube functioned like you think it does, there would be no need for a soft transmission/differential mount/bushing at all from the factory - there would be no rotational load to speak of that would need to be distributed while allowing for some movement.

On a further note, the design of the PFADT mount itself, because of the center bolt/hinge (red circle in the diagrams, in case I'm not using legit vocabulary), would be a very poor design without the stabilizer arm (Fig.1). A different way to do this would be to have basically two of them adjacent to each other, in the end resulting in a more complicated version of the VB&P mount (Fig.2), using round pivot-style bushings instead of a solid urethane block.



Did this with MSpaint, I don't have photoshop on this computer, sorry for poor quality.

 

OK, I am going to assume an automatic - do the following;

Block the rear wheels so they can not turn (say strap them tightly to tie-downs in a cement floor)
Disconnect the outer tube from the engine so only the inner tube is going to the transmission.
Get your friend to put the car in low gear and step on the gas pedal.
Stand at the back of the car and watch the drivetrain.

When you do the above;
The engine will twist clockwise.
The transmission/differential will twist counter clockwise.
The torque from the engine is being transmitted down a shaft to the transmission so the torque trying to rotate the engine clockwise is the same as the toque trying to rotate the transmission counter clockwise.
Since neither part is connected directly to each other the torque is transmitted to the rubber mounts for each part.

Now, connect the engine and transmission together once again with the outer part of the torque tube. The torque twisting the engine clockwise and the torque twisting the transmission counter clockwise both act on the ends of the outer part of the torque tube. There is no twisting torque along the axis of the engine or torque tube as long as the outer part of the torque tube does not twist (or give or deform). If that outer part of the torque tube is twisting enough to matter then we all have bigger things to worry about than this mount. LOL.

The ONLY force is the torque force trying to pick the front of the car up (stretching the motor mounts) and pushing down in the rear (compressing the differential mount).

SO - I believe this mount fixes the transmission firmer in place and removes the "springiness" of the rubber mount to up and down differential movement. Similar to using poly suspension bushings or poly motor mounts or spherical rod ends in different applications.

I'm not at all sold on the "wings" of the mount doing much assuming that center bushing is a high durometer (if that is the correct usage??) poly bushing that has little give.

I believe the feedback has been from people who have changed from a stock to this mount. In that case, I would expect to have a reported difference just because the compliance of the rubber mount was removed.

Peter

 

The force picking up the front of the car and loading the rear of the car is due to weight transfer induced by acceleration (and massive traction, in the instance of popping wheelies), not engine load. The picking up of the right-front more than the left-front of the car, or vis-versa, however could be attributed to engine load.

Without the 'wings,' as I attempted to portray in my diagram, the transmission would be allowed to rock slightly from side-to-side, putting too much load onto the engine mounts, as they would be overstressed from attempting to damper an object that is 5-6 feet away from it.

You almost made my point in your post -- The engine reacts to rotational load by rocking from side to side, as you would hopefully agree. The engine is solidly fastened to the torque tube. The torque tube is solidly fastened to the transaxle. As you said yourself, if the torque tube was twisting in a torsional manner, we would have something to worry about. How can the engine shift, and be solidly fastened to the transaxle via the torque tube, and yet the transaxle does not shift relative to the chassis in the same manner that the engine does?

High-durometer is indeed the correct terminology for a stiff bushing.

 

I guess your explanation was better then mine, but I agree with it.

I don't have the instructions, but there is some shimming involved. Wondering if the wings just fix it against the rear sub-member.

As for the bending of the torque tube, that goes right through the tranny/differential joint and is what the DTE brace is designed for as per Phil that is the weak part of the case for transmitting this load.