Torsen front diff and two piece axle?

[Captn. Crunch]

So I’ve been wrestling with this for awhile now. Can a Torsen Carrier live in an S-10 style two piece front diff. The original was an open diff setup so when the axle was disengaged the front diff basically goes into sleep mode
The torsen is a different animal and distributes torque through worm gears and friction.
I asked an engineer at a Torsen manufacturing facility and here’s The conversation ..

Hello
I have a hypothetical question regarding using a Torsen unit with a disengaged axle.
Gm’s S-10 4x4 system has an open front diff with an axle front disconnect. Is there any reason why I couldn’t run a torsen in place of the open carrier whilst maintaining the stock 4wd setup that has the front axle disconnected until 4x4 is selected and the axle is engaged.
Will there be any sort of overspeed or uneven wear concern with only one side of the carrier being driven?
Thanks

Thanks for your interest in Torsen. Under the conditions that you are describing, the differential gearing would spin a high differential rate, but under zero torque load. As long as it is well lubricated, it should be fine. However, I am not aware of ever being paired with this axle configuration in a production application, or if it has been tested. My concern is that the axle disconnected and the transfer case disengaged, the axle's ring gear will not be spinning. Axles typically rely on splash lubrication to distribute oil, and the ring gear is essentially a paddle wheel that flings oil all around. Without it moving, there would be little to actually move the oil into the diff gearing. If this is done, I would recommend making sure that the oil fill level is up close to differential centerline (some axle have the fill level low enough it does not touch the differential), so that you can be assured that oil will flow into the differential without the ring gear moving it. If oil can get into the differential assembly, then the diff gearing will move it around internally.

With all of that said, however, it something of a moot point. We have not produced a differential for the 7.25" axle used by the front of the S-10, so you couldn't run a Torsen in your truck anyway.

Regards,

Little did he know we had gone through one of his competitors and had these designed and built.
I couldn’t get hold of anyone where we had them built, and I just needed to talk with someone who knew Torsens inside and out.

[MountainManJoe]

NOTE: This conversation carried over from this topic:
http://astrosafarivans.org/bb2/viewtopi ... 59#p100759

[MountainManJoe]

OK I'll bite. I had a deeper think about this. It's hard to wrap your mind around it because there's a lot going on.
I think the Torsen rep did a good job of boiling it down.

First lets talk about the vans.

AWD:
You can not use a two piece axle on a true AWD system (ie one with center differential).
With an open center diff, you wouldn't go anywhere. The disengaged axle would just spin. If the center diff has form of limited slip (ie a viscous coupling) then it will be triggered all the time. That's why it's been repeated in the forum many times: do not remove the front driveshaft on the old AWD vans with BW transfer cases. It won't park on a hill and you'll ruin the unit. It needs all the wheels engaged.

ACTIVE:
The later Astro 136, and Blazer 236 units are not true AWD transfer cases. They are their own animal. It's more like a part time automatic, or active, 4WD. There's is no center differential. It is monitoring input/output speeds, and ramping a clutch in and out with a motor, instead of you with a lever. It can also feather the clutch.

Can you use a two-piece axle with active transfer cases? Not really.
They are on-demand systems, designed to react seamlessly without driver input. Every time you lost traction in the rear, and the system tried to engage the front, the power would just be lost to the disconnected axle and you would stop dead in your tracks. MAYBE, the transfer case module could be wired to engage the axle and the shift motor at the same time. However, in 2WD, the TC preload would still be spinning the front driveshaft, and the ring gear, so the benefits of the axle disconnect are negated anyway.

Part-time 4WD:
The 231/233 transfer cases are plane jane 4WD and have a simple mechanical clutch. Engaged or disengaged. There is no "differentiation" happening between front and rear axles. No electronics... no active response... It does not care what the wheels are doing.

I think the axle disconnect is only meant for part time manual 4WD applications, where you engage 4WD and the axle at the same time. They work together.


An open front differential has three connected parts: two wheels, and a driveshaft. I'll call them interfaces.

2WD mode/disengaged axle:
- one driven interface: the engaged wheel being rolled along the ground
- two "floating" interfaces: One free half axle and the driveshaft. They are free to turn in proportion to each other.
(in reality the transfer case may put some preload on the driveshaft (slightly driven), so the half axle may be turning in normal proportion to the other wheel. I don't know if it's the case)

4WD mode/engaged axle:
- all interfaces driven: two wheels being rotated, one driveshaft connected to engine. When all interfaces are driven in correct proportion, everything is in sync.

OK, now lets throw the worm gear limited slip differential into the mix. Torsen, zexel, detroit trutrac, Haldex etc... whatever the brand name. I will call it LSD for simplicity. Essentially the LSD restricts two of the three interfaces: the wheels. There must be torque on both sides within a defined ratio. Unfortunately zero torque is still proportional to zero, so if one side gets no torque, neither does the other.

As mentioned above, AWD , and on-demand 4WD are not compatible with 2 piece axles. That leaves only part-time 4WD (231/233):

Part time 4WD system, two-piece axle, LSD

- 4WD mode/axle engaged: Everything works as expected

- 2WD mode/axle disengaged:
One rolling wheel, one disconnected half-axle, and a free spinning driveshaft (again, assuming no preload)
My understanding is that with no torque on the free axle, there's no friction/binding force, and the worm gear LSD behaves like an open diff. The email is absolutely right, the ring gear doesn't move, and you get poor lubrication to the bearings. Over filling might help for when you're in 2WD, but once you get that ring gear moving again, then you run into the aeration problem I mentioned before, and you'll quickly destroy the diff.

My conclusion: Not worth the 0.5 MPG savings from the 2-pc axle.

[Captn. Crunch]

Thanks timeless, I appreciate the time and thought that went into your reply. This thing has been my Schrödinger’s cat. I’ve thought it through from every angle but without actually watching it do it’s thing and monitoring torque output I don’t see how we will ever really know. And with a price tag of almost $700.00 I’m not taking stupid chances with my MFactory Torsen unit!

[Astrophysics]

Hi Everyone.

Very interesting. I have 2003 Astro with np231 and manual shift lever.
The OEM 3.42 axle is still in even with 30 ‘ tires. I plan to get 3.73 gears and perhaps rear LSD.

Astro Physics

[Captn. Crunch]

AP how’ve you been?!?
When I did mine, I found a front diff from a Zr2, they all came with 3.73’s. Find a Zr2 with a manual and np233 and it should be a cast iron unit. Swap axle tubes and axles. Also swap front prop shaft collar, Easy Peazy If you’re going to swap, might as well upgrade. forestmccarthy followed my build thread with the np236 and twin Torsen. He wheels the snot out of his rig out there in Utah/Montana region and his stock alloy front diff failed him so he also went cast iron.

[Astrophysics]

Hi Capt,

Been doing great.
I am happy that the Astro forum is back. My Astro does pretty good with np231 manual shift low range. I do fire roads And hills and still am a newbie to creek crossings, etc.
I would like to try Mendocino or Tahoe dirt trails this fall.

I should search for zr1 S10 and get the cast iron front diff. That is a great idea. I guess it is best to install 3.73 instead of 4.10. Since 3,73 has one more tooth and the front diff at 7.2 is a weak diff. S10 forum says 3.73 more reliable than 4.10.

AP

[Captn. Crunch]

I agree with the 3.73 for strength. 4:10 would be as far as I’d go but it’s not worth the work if you’re already running the 3:73’s. I’d love to run 37” tires and 4:56 gears but we all know THAT ain’t gonna happen.
I used car-parts.com and searched S-10 front diff then look for the np3 + MT. That’s the magic combo, three button tc and manual tranny