Adapting an Audi AWD torsen output shaft to front driveshaft for simple AWD Taco

Posting for entertainment value, but I picked up one of these things to play around with
https://www.ebay.com/itm/335651373451
and this one
https://www.ebay.com/itm/335651373567

Figure with some meddling I get a housing machined, find some universal shaft and seal, and can link it to transfer case, and then modify my front driveshaft to link to the output shaft for selectable AWD. Melt together a frame for support.

These come from the factory with a 60/40 rear/front bias, all the way to 75/25 split either way. Since the output shaft is facing the front, the 60/40 bias is reversed.

Feel free to tell me how this could go horribly wrong

 

more or less what I purchased:

 

An easier way to achieve this might be to just get the transfer case out of a 4runner limited with awd. I was also seriously considering doing this and almost pulled the trigger on buying one from a junk yard. The one I was considering is the VF4AM which has a torsen center that is lockable. and pretty close to drop and swap along with the control module and button. I found this cool engineering document on it too:
https://www.jeepolog.com/atachments/5/8/111299.attach

Some people have done swaps and theres this thread that probably has some good info
https://www.tacomaworld.com/threads/transfer-case-swap-multi-mode-2wd-awd-vf3am-vf4am.627123/

Ultimately I decided not to do it because it felt weird to swap a high mile tcase into a low mile vehicle and didn't really have the excuse of regular snow or etc to justify it as the best option even though it would be sick. Its a really cool technology and they work super well. My thought process was just get it in either the front center or rear. I eventually decided to get an eaton true trac rear diff and its really really good.

https://www.eaton.com/us/en-us/skuPage.913A541.pdf
https://www.eaton.com/content/dam/e...c/eaton-truetrac-differential-brochure-en.pdf

Heres why I thought its a better value

• Its super fun to drive, in the same circumstances where you'd want awd this adds as much fun just in a different flavor
• I wasn't expecting it to add as much snow/sand/low traction performance as it does but I think its better there too
  • 2wd + true trac rear isnt as hooked up as awd, so your default mode is in between stock and awd capability and rwd fun (donuts)
  • but 4wd + true trac rear is super hooked up in low traction, definitely more than awd
  • Its synergistic with the toyota vsc and traction control system. This gets you compounding gains in yaw stability and forward grip that are not as pronounced with the awd option
  • Its better than a locker in every way except for when you get a wheel in the air. Lockers are not fun to drive in comparison and are gonna be helping you as frequently
• Its also fun to drive in all conditions, even if it rains a bit I can tell its working and making more grip
• It improves your ability to not get stuck more than awd does. Not everyone is crawling, but in overall value this is a plus that cannot be said about the awd option
• Its also good for crawling (not as good as a locker) but still I'm impressed, watch this video: https://www.youtube.com/watch?v=gk9FhqMS96g
• Its always on. This is a big plus in value I feel but also its what I want from a vehicle that is not gonna let me down

Downsides:

• Awd/4wd braking is way better than 2wd and by having awd I'd probably use it quite a bit and have the improved stopping distance availible if an emergency braking event ever happens. Can still use 4wd to get this tho
• Major updside of awd option is high speed stability improvement of being able to drive all tires on dry pavement, which I'm not really driving close to the limit on dry pavement anyway so I wouldn't notice it as much.

They also make one for the front and I think someone made a build thread on here and did all 3 mods

 

I guess I'll check and see if this is going to go horribly wrong to make sure you're not wasting time.

Is your plan to just put this inline between the front transfer case output and the front driveshaft? That sounds like what you're saying and if it is I'm afraid it won't work and we need a differential lesson.

 

Yah that's what I'm sayin
https://www.youtube.com/watch?v=B5wiJ10PYXY

 

This is what I'm after

 

Gotcha. So, unfortunately, that won't work. I'll do my best to explain why without getting too deep into the weeds of diff dynamics.

A differential operates by taking a single driving input and creating two driven outputs. Those two driven outputs have a coupled relationship in torque and speed based on the type, style, design, and setup of the diff. It's important to realize that all three of these input/output components are required in order to have differential function.

In this case you're trying to operate the diff with a single input (front tcase output) and single output (front driveshaft). This leaves a free output which means no torque can be supported across the diff and therefore no torque can be applied to your single output (front driveshaft). Just this alone means we're cooked and this can't work. This is the torque problem.

Also, no where have you decoupled the direct drive to the rear wheels. The coupling of the outputs in torque and speed that go to the front and rear axles is the defining characteristic of an AWD vehicle and by just trying to apply a differential in line with the front drive that relationship is not created and the result is not AWD.

I think you may be misled by the cutaway drawing of the transmission you posted. If you look closely you will see there are concentric shafts entering the front side of the torsen center diff. One is the transmission output shaft that is the input into the diff and the other shaft is the front output drive. The rear outputs from the back. So both front and rear axle drives exit this diff and I'm guessing you maybe thought just the rear drive went through the diff?

 

Honda does something like this for their older AWD stuff it's bolted in place of a center support bearing. it's not very robust though.

 

Ahh I think I see what you're saying..

I did assume that the input shaft was coupled to the FWD shaft/differential, and that was touching the first gearset, providing coupling as both input power and drag. Then I assumed the output shaft was simultaneously coupled to the RWD differential providing both load and momentum. I suppose this doesn't make sense as you could never drive the rear wheels more than the front.... (but perhaps this would still work and not be "true" AWD?)

So the outer sleeve of the "input" concentric shaft is actually where 1/2 the load goes due to its attachment to the foremost set of planetary gears? Then the output shaft is driven by the ring gear?
I guess I am still not seeing how the input shaft is detached from its outer sleeve allowing them to move at different rates but still apply force to each other. In the attached pic, the inner splines do not appear to mate with anything..

Nvm, they came! It makes sense now. However, "partial AWD" still seems possible in my head even after playing with them if I just ignore the FWD/housing? Or will it just freewheel because of no load.

 

In the cutaway you have a Torsen C type diff. The inner splines (input) are driving the planet carrier. So in a type C the planet carrier is driven and the sun and ring gears are the outputs. Since the number of teeth on the ring and sun are different it creates a nominal torque distribution that isn't 50:50.

You can't ignore the front output of the differential. The bias ratio tells us that the max torque the output with high traction can get is some multiple of what's available at the low traction output. With no connection, the 'low traction' output can only handle zero torque and thus any multiple of zero is still zero.

 

Guess I'll be perpetually on the lookout for a multimode 4R lol. No sense in my making a DIY AWD transfer case. I don't regret this purchase though.. These things are cool