Modified Centre DIff

[Robbks]

Ok, Just after some opinions/ thoughts and Info.
Long story.

My last car (2006 Liberty H6 with 6MT) had a stuffed centre diff when I bought it, I went to my trusted local mechanic to have it sorted and he put me onto another local tranny specialist (long time and respected box builder for street, race and rally cars)

After a long discussion back then his explaination about the standard VC unit is that it's only designed to provide a precise amount of "drag" on the planetary gears under slippage conditions.
And after a looking at a dismantled one, I agree with his assessment.
Problem with the stock unit is if the seals let go in the VC unit from being overheated, air and visco goop escape, then the plates mash together and effectively lock the two sides of the centre diff together.

His solution is to modify's the stock centre diff to effectively delete the Viscous Coupling with a machined "shim" (about 26mm thick) that is splined to the transfer gear shaft (where the inner hub of the VC coupling would be
And the addition of precisely selected thrust washers on either side place a certain amount of pre-load between the Planetary gears and Housing to retain the "drag" that is needed to provides drive under slippage conditions.

Standard VC is part#5 below

I'm thinking of going this way with my Fozz as the VC is stuffed in this one too.
After another conversation today we believe the effect is sufficient that it provides ~90% as much torque transfer as a standard centre diff would, without the future issues associated with the stock VC failing again.
It's also adjustable by means of changing the washers to provide more or less pre-load on the planetary gears.
however too much pre-load and lots of slippage can cause premature wear of the centre diff gears.

in 99% of cases, I'd probably run with the VDC on anyway, which is going to automatically take car of braking any spinning wheels to provide drive elsewhere,
I figure the only time I wouldn't would be in mud or on sand where you want the wheels to keep spinning to maintain momentum and grip onto anything they can.

Just wanting others opinions and thoughts on a different piece of gear than is usually on offer.

 

[kiwifoz]

Seems like good theory - but having less slip in the centre diff would give you more understeer onroad, correct?

I'm curious as to how you diagnose a dead VC.

 

[Robbks]

Nah, as is with the standard VC in place when there is only a low speed difference between front/ rear (cornering) it's effect is very little and operates essentially like an open diff.
But if you're climbing up some loose stuff with the fronts spinning fast, it comes into effect (the intertia of the steel plate acts on the washers which act on the planetary gears)
Same as the stock VC, the harder you "shear" the Viscous goop, the harder it resists.

Dead VC is easy,
Get the car hot (40-50km of driving)
then do some full-lock turns at slow speed, you'll hear and feel the diff's and driveshafts got thunk, thunk, thunk.
I can feel it on the road at speed now i know what i'm looking for, more than 90 degrees of lock at reasonable speed, the car feels very understeery and you can fell that thunking (as above) fighting back through the steering

Another way is to jack up one front corner of the car in Neutral and rotate the wheel,
when the VC is locked up/ failed you can hardly turn that wheel

 

[kiwifoz]

Right, thanks for that, I'll do it the next time I go for a decent drive.

From my point of view though, so long as the VC locks up when it's dead that's actually not a bad thing for 4wding, but I can see why it would be a pain on road. What tends to kill them?

 

[Robbks]

overheating of the visco goop is what kills them,
Fitting new tyres at one end of the car only is enough to do it, and is what was done by the first owner of my Fozz

when the goop heats up it expands, if it expands too much then it gets pushed out past the seals,
overheating it also cooks it and it goes hard under normal operating temps.
so as soon as the box warms up, it's locked 100%

 

[nipper]

Right, thanks for that, I'll do it the next time I go for a decent drive.

From my point of view though, so long as the VC locks up when it's dead that's actually not a bad thing for 4wding, but I can see why it would be a pain on road. What tends to kill them?

A viscous coupling has a special fluid that gets very thick as it heats up. There are plates in the VC that sheer the fuid. This shearing heats up the fluid and the ties the plates (front/rear) together.

What kills a VC is that this fluid is not designed to be that hot all the time. Eventually you bake the fluid, which clogs up all the plates, and it locks up the AWD.

https://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

Whent hsi locks up it can make for dangerous handling on dry pavement.

 

[taza]

Same as 20kg centre diff

 

[Robbks]

worse than a 20kg....

 

[Dedman]

Bit of a dead thread but I just found it and though it was an interesting idea so I would express an opinion.

Assuming I understand the concept correctly, I do not think this idea would be a good option without complete redesign of the whole centre diff for the following reasons:
- The idea applies a load from this spacer through the spider gears in the diff causing binding and will remove and backlash in the gears. The gears a carefully setup with a certain backlash and any change could lead to excess wear and failure. There will also be a notchy movement of the diff as the teeth move past each other if they are to close.
- The thrust load will also be retained by the large circlip. This is not designed to take this load so diff may need to be welded up instead.
- The viscous unit supports the rear output shaft to some degree so this would have to be incorporated in the design.
- Unless the was some kind of friction plate in the arrangement I do not think oily metal rubbing on oily metal would provide much grip.

Just some thoughts but I would still be interested in seeing effectiveness of the end result if it was done,

 

[Robbks]

I queried the same thoughts with the builder the other day actually.
The pre-load actually comes from the the mass of the spacer.
So its actually resisting turning the gears by use of inertia, not physical pressure on the gears as I first thought.

But mine has to be repaired by the dealership anyway do it'll just get another factory unit.