Thread: BW Viscous Coupling differences to CDL

Well sorta .
The VC has a silicon fluid in it.
Here is an explanation for a Ford XR4 diff

Viscous Diffs
The viscous differential also has interleaved plates, and in fact works because of the viscosity (internal friction) of the silicone fluid that fills the gaps between the plates. The plates therefore do not actually touch one another. The percentage of torque transfer varies with the difference in speed between the two wheels, partly as a result of the plate design and spacing, and partly because of a physical property of silicone fluid that it experiences virtually no change in viscosity when heated. If one wheel is rotating only a little faster than the other, only a little torque transfer occurs, when there is a big difference between the speeds of the two wheels, the viscous differential will transfer up to 95 percent of the torque to the wheel with traction. Because of the difficulties in handling the silicone fluid, servicing a viscous differential can be accomplished only with special equipment. The silicone fluid is under pressure, and there is an air bubble of known (and controlled) volume included in the differential casing. Any deviation from the correct amount of pressure or air bubble volume can radically change the characteristics of the differential.

It is often stated that RRC Vcs can divert 100% of drive to the front axle when heated , but probably only 95%.
The BW transfer case also has a differential which outputs to the back axle output shaft. The VC acts to automatically "lock" the diff to provide drive to the front when the front spins.

I would think that Torsen diffs would assist the TC to operate, as the Torsen will apportion drive according to its gear friction. The TC will load the Torson if a rear wheel is off the ground.
I had a rear Maxi and front Quaife on my RRC and IMHO it was pretty ideal, but with TC on the rear a Torson would be almost as good.
Regards Philip A

Somewhere in this monolithic site is a discussion or two on this subject and I recall the comment that LSDs and traction control worked well together. I think the TC took over where the LSDs left off.

I've got Truetracs at both ends, (without traction control or anything else), and they're fantastic - except for some weird tyre wear on the front axle I really should investigate further.

Originally Posted by Davo

Somewhere in this monolithic site is a discussion or two on this subject and I recall the comment that LSDs and traction control worked well together. I think the TC took over where the LSDs left off.

I've got Truetracs at both ends, (without traction control or anything else), and they're fantastic - except for some weird tyre wear on the front axle I really should investigate further.

Remind me Davo - RRC or Disco?

Sorry, I'm afraid it's too long ago for my coal-fired memory to dredge up, the size of this forum being what it is these days. However, I know I found it when researching Truetracs, so if you start with that you'll probably end up at the same thread. (Or threads.)

Originally Posted by Davo

Sorry, I'm afraid it's too long ago for my coal-fired memory to dredge up, the size of this forum being what it is these days. However, I know I found it when researching Truetracs, so if you start with that you'll probably end up at the same thread. (Or threads.)

Nuh! I was asking if you have trutracs fitted to a range classic, or a discovery - could not remember, and if my memory serves, you are into classic RRCs like this idiot!

Oh, Rangie cuz dey rule boi.

Originally Posted by gavinwibrow

As promised from my post in the very useful D2 discussion on traction options with CDL, I'd like to query the effectiveness (whilst still working) of the VC approach.

As I understand it, the VC can roughly be compared to a LSD in an axle - yes /no?

Yes. In the most basic sense it is a limited slip differential. It simply uses a silicone fluid coupling to achieve the engagament properties. "Viscous coupling"

Originally Posted by gavinwibrow

I have the working VC in my 95 LSE, and have subsequently after purchase fitted trutracs front and rear (Ashcroft ATBs weren't available at the time).
So, does the trutrac wipe out and/or replace the rear axle TC that was fitted as standard equipment to late model RRCs?

Partially yes, with a partial caveat of no.
The short answer is there will be an overlap between the LSD starting to allow wheelspin and the traction control engaging.
If everything is working "as intended" then the LSD will limit the rotational speed difference between the halfshafts. The idea is that it is an 'attempt' to equalize the drive to each halfshaft, similar to how the viscous coupling works, except instead of using a fluid, it's using another mechanical method achieve the same or similar desired effect.

In a conventional clutch-type LSD, how tightly the clutch pack frictions are 'shimmed' determines how much torque is required to overcome the friction limit. There are other factors - stuff you don't want to worry about, like LSD friction modifier additives and specific types of limited slip differential oil which have certain characteristics - stuff which deserves a separate discussion in its' own right.


A Helical ATB differential (Quaife / Ashcroft/truetrac) uses a number of helical gear pairs which sit in a parallel arrangement with the driveshafts. The torsen-gleason uses the helical gears in parallel with the pinion and crownwheel. There are some slight differences in the behaviour between the designs, but the principle is the same - the helical gears work in a contra-rotating manner to limit the freespin on the halfshaft with more rotation. But there is a zero-torque issue with ATB differentials - easily overcome with a non-TC stab on the brakes while applying power, in other words "How it used to be done" or Fred Flinstone traction control method. (use your feet). It's only an issue when one wheel is in the air.

There are other designs of limited slip differential centres, e.g. torsen-gleason, as well as some other clutch-pack style units (O.S.Giken comes to mind) and wedge /ramp type (drexler motorsport / wavetrac, hewland, hallibrand, xtrac among others), which have different customizable ramping slip profiles for load and coast, as well as electro-hydraulic activated control (F1 / WRC etc). Each of these have their design and cost limitations, and intended application. Some also have the ability to completely lock the driveshafts, but not through a physical dog/spline/collar engagement mechanism as found in a "locker" like the maxidrive/grizzly/detroit/arb/ashcroft/eaton e-locker.


The caveat: Traction control
The traction "control" is the bit that will (and is supposed to) take over where the LSD starts to slip. The intended purpose is to measure the difference in wheelspeed via the abs/wheelspeed sensors and when a predetermined slip threshold is reached, apply braking force via the ABS electrohydraulic controller pump and solenoids to rapidly limit the rotation of the free wheel, through a cyclic pulsed halt-release application which allows the differential to redirect the torque through the other halfshaft until the 'balance ratio' is achieved. When this is achieved, the TC disengages.

I'll elaborate a little more later if req'd.

Originally Posted by gavinwibrow

I like the trutrac on the front in lieu of a dedicated locker - the few times when a front locker would be necessary, I'd use an alternative route.
Again as to my limited knowledge, both the VC and the trutracs are a softer, easier on tranmission option for most scenarios.
Would appreciate any comments from the wise! Cheers Gavin

Looking at the transfer case from an Electrical Engineering viewpoint, I would see the TF's centre diff (solid gears & cogs) as being 'wired' in parallel with the Viscious Coupling.(slippery/sticky fluid changeable grippiness).

Both are two halves of the same intention, to properly and fairly apportion the 'drive' sent to front/rear diffs... - where the local diff...be it open, ATB, Tru-Trac or Locker, can decide for it's own axle, which side gets what.

Think of the TF case as having Yin and Yang elements. (or male and female, if SWMBO doesn't read these posts!) One of the pair is happy to 'diff' the load, even to the point of ALL in one direction.... whilst the other one gets more grumpy and 'grips' more with increasing inbalance.

Problem is, the VC and CD are tied together at the INputs and the OUTputs, which means what one side of the marriage does... affects the other, But when push comes to shove... the VC locks up and over-rules the wishy-washy differential...The front/rear propshafts are now locked together. Automatically.... which I reckon ws the underlying Design Imperitive, seeing that the LT230 centre diffs are fragile in the hands of incompetent drivers.!
So someone had a brainwave to make a TF case that could think - and act - for itself, locking up as-much and when required without waiting for the idiot-driver to reach down and move the appropriate lever. - before his lead-foot has smashed said centre-diff into pieces...

Great idea that works a treat in sand more than dry rocky stuff, but the execution left a bit to be desired. Like durability of the VC and, the drive-cogs & chain or Output shaft, depending on Classic of P38 installation.
Some owners get a lifetime out of the TF case, many have failures under 150K. - Pot luck.

The odd thing being, other vehicles which use the VC idea, and even a BW Transfer case, do not seem to have this problem of it being a Consumable item.