Doublers, cross members, and busted transmissions

[az-k5]

Russ, I think you are looking at it slightly backwards (or I am reading more into it). The tranny is not turning on the t-cases, the t-cases are turning on tranny. That was the reason the strut was put in from the factory. The 205 also had a rubber sandwich mount from the front ouput side to the frame rails. I would think about fitting something simlar to that using soft bushings, and/or valve springs, to allow for a little extra torsional stress since that mount is farther out of center.

Using a stock design strut. Lucky for us I was too lazy to remove my 203 strut when I went doubler. It is still hanging off of the bell housing as we speak. The strut is too short by about 3". I can get the mounting tab to touch the mounting face on the 205 with just a little push (it sits 3/4" too low when hanging). The rear hole on the strut will fit into the front hole on the 205. You could eaily fab a longer tab for the strut. I think that would be the fastest way. My x-members are lowered about an inch though so this may make a difference. That difference will most likely negated with some attention to the mounting foot extention.

 

[BadDog]

I've looked at it both ways. Torque input from the engine, and torque input from the case turning the shafts. I guess I'm not sure which is the bigger culprit in broken cases. With the axle gearing, and tire slippage at a certain point, I don't see the torque input to the assembly coming from the case as all that big a deal, but I could certainly be wrong. If it the breaking force is coming from the back, I would expect to see a lot more broken cases than we do, due to the soft and narrow factory mounts that are frequently kept even with doublers.

And the 203 brace is a good idea that we kicked around for a while. Same for the old style case to frame mount. But my shifter bar pivot/hinge mounts there and it would require a rework. Could be done certainly, but I don't see that strut to bell housing helping much for that, seems it would offer very little restraint to torque, but more to keep that end of the case from moving front to rear. And I can't help wondering why GM removed that outboard frame mount from all later cases. If it's that important, wouldn't we see more broken cases with and without the doubler? The vast majority of cases out there have neither (never there, removed, whatever), but haven't seen that many breaks. would redo it without hesitation if I were convinced that would end my worries, but I don't see either of those two approaches as helpful, based on my potentially flawed mental model.

Can you offer clarification? If I'm my mental model is wrong, I would much appreciate correction. And if that is the case, it is still not too late to switch from the softer factory mounts to poly (and engine as well).

I also realized that there is a lot of weight/mass trying to make the engine/trans/case assembly go sway/arch backed when bounding around in the boulders. The engine mounts are toward the front, and my only rear support was back at the doubler adapter. That force would seem to be at least part of the problem with broken bells, stripped rear adapters, AND broken cases. More so that torque in my current mind. And of course add the hammering from my square shafts when the springs bend. That's the reason for the whole "new and improved" cross member and cradle. That and getting a bit more clearance on bottom (though not much)…

All I know is that I'm getting tired of working on this thing. Just not enjoying it much any more and I have less and less motivation to overcome my desire to do other things. One more broken/bad trans in the near future, and I'll probably just find something else to play with.

 

[BadDog]

BTW, I guess maybe it became clear, but didn't start that way. My biggest focus on this was not to control torque. As I've said before, IMO, as long as you don't let frame flex become an input, and let large amplitude vibrations and such from the engine be damped by the mounts (i.e. no poly mounts out by the frame for either goal) then torque is not the main input into my breaks and most other breaks I've seen (mostly on the web, though a few in person other than my own).

This effort was mainly aimed at avoiding introducing new/more torque problems (by keeping stock mounts (or possibly) inboard, which follows my current opinion) while removing some of the up-down flex in the center. The shape and location of many of the brakes I've seen (including mine) indicate that it is vertical forces, primarily causing a sort of "sway back" (where the transmission wants to go down). That includes breaks at the rear and at the bell. So I moved the mount/pivot point forward with the cradle and also plan to add the upper strut to stabilize that same force.

Am I barking up the wrong tree?

I think the rear links and lubricated shafts are going to likely eliminate the dramatic problems I've had, but this is just an extra measure and addressing something that has bugged/worried me for a while…

 

[Keith in CO]

bushings and doublers

If I don't change my mind, I'll be using the stock rubber "C's and blocks", only flipped upside down. But if I decide to go with poly spring bushings, I've got those sitting out there too.

I'll wade in here as I am about to launch into installing a GenII 203/205 behind the TH400 in my 1 ton. A friend recently witnessed four separate broken auto trans rear cases on auto trans doubler set ups in Moab. From what I can gather, the problem may be the use of a stiff, poly doubler mounts with softer rubber engine mounts. The engine twists when torque is applied, but the rear of the trans is held solid by the doubler poly mounts, and the give point is the rear of the trans case. Having used poly bushing inserts in the clam shell mounts in one of my trucks with a big block, I know that the engine will not rock/twist with those engine mounts. So I figure that if I also use stiff poly trans/adapter/t-case mounts, that the torque will not allow the engine to twist proportionally more than the t-case, thus reducing the stress on the trans case. Whachathink?

 

[BadDog]

I agree totally. I think it's basically a given that if you run poly in the rear, you MUST run it in the front.

 

[ntsqd]

I'd be more inclined to leave the rear in rubber. Let the engine mnts deal with the rxn torque, but let the doubler mnt soft enough to allow for any chassis flex or other mis-alignment.

 

[BadDog]

That's been my thought all along, even including thoughts of a single 2WD type rear mount so that you have a 3 point mount system to eliminate ANY torque stress (from engine reaction or frame flex) on the transmission housing other than driveline reaction torque. And it even meshes with a lot of what I've seen IRL. Such as people using poly trans mounts with stock engine mounts and busting the back of the transmission.

But, several people have been telling me that the driveline reaction torque at the back of the tcase is the much bigger problem due to the tremendous torque coming out of the tcase, particularly in low with compound reduction of the cases. But seems to me that with the additional axle reduction further increasing the torque to the tires, the rotational input to the tcase at the output would never be very high before the tires slip, even if you're vertical or wedged or whatever. <shrug>

I'm not really convinced one way or the other. But if I were, it would have a huge impact on my current effort. Poly or rubber? How far in or out? Quad/Tri mount at the cases so that it restrains rotational forces as well as those rocking the case back and forth? Or OEM type pair so that it can rock on them and may "sway back" or arch the trans case (reduced by the struts I talked about)?

Hopefully I'll be back out there working on it again tomorrow, certainly by the week end I think… Maybe I'll just flip a coin...