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vLSD workings

Ratbag

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Joined
Jan 1, 2012
Messages
7,417
Location
Bayside, Melbourne, Vic
Car Year
MY06, MY10
Car Model
Forester SG & SH
Transmission
5MT/DR & 4EAT Sports
Gidday KF


I think I read somewhere that the vLSD spins opposite ways like the open diff, as it needs the oil to warm up before it kicks in?
Without power through it, and a differential power balance, a vLSD will behave exactly as if it doesn't exist ... :iconwink: :lol:. Just like an open diff, as you have said.

The actual vLSD is a sealed unit inside the diff housing. It contains a non-Newtonian fluid - i.e. instead of becoming thinner as it gets hotter, it gets thicker and stiffer.

Inside this sealed unit, there are many asymmetrical plates that are about 5-8 thou thick. These are in pairs, but always with an odd number in the stack.

One member of each pair is connected to each of the output shafts.

The 'stack' is tightly packed, the drive plates only being separated by a very thin layer of the fluid.

As the fluid heats up from the different rotation rates of the two shafts, it locks the plates together, giving the limited slip effect. As the two wheels start to rotate at the same speed and loading, the fluid returns to its static state - i.e. one of no resistance to the now non-existent shear forces.

If the vLSD is abused by being worked too hard, the internal plates can come into physical contact with each other, grinding against each other instead of the fluid causing them to lock. The result of this is to shed very fine metal particles into the non-Newtonian fluid, turning it into a Newtonian fluid - i.e. its ability to behave as a LSD is destroyed.

The same can occur with the centre diff when it's a vLSD one.

I have seen one of these diffs disassembled, and have photos of them in both new and buggered condition. The damaged diff "oil" is disgusting black muck. A freshly re-conditioned vLSD has fluid that is almost clear (just cloudy). It is clear enough that one can see well into the stack of asymmetrical plates.

These diffs are not really all that different from a "conventional" LSD. These latter can also be destroyed by abuse (but will withstand more abuse than a vLSD). They also require adjustment and maintenance, neither of which is necessary with a vLSD.

IOW, a vLSD is an ideal device for many applications. It has none of the vices of an AWD vehicle with "conventional" LSDs. It requires no special driving techniques. If it is not abused, it will last for a very long time. As an example of this, the centre vLSD in my Impreza gave no symptoms of degradation after nearly 18 years of normal use. I think I would have noticed if it had gradually turned into a FWD car! :poke: :rotfl:.

This is how failure of the centre vLSD diff presents. The front output shaft is fixed, and the rear output shaft is locked/unlocked as required (as I understand it! Someone please correct me if my description is wrong or flawed ... ). In most of the MT cars, the diff is set to 50:50 F:R as the default condition. It can vary between 40:60 and 60:40 as required (assuming that the centre vLSD diff is working as designed).

The range of distribution between F:R varies a little from model to model. The above is true for my old '93 Impreza, and for Foresters (both with MTs).

The ECU manages the F:R torque distribution by some arcane magic that involves the angle of the car, along with other things ...

The rear vLSD is entirely 'mechanical'. It is not controlled by the ECU or TCU.

Hope this is both accurate :iconwink:, and of some benefit to you and others.
 
Thanks for the explanation, really helpful! Clears up any questions I may ever have had about the vLSD. :)
 
I believe RB is right about a viscous LSD acting like an open diff until some power goes through it, so the wheels will spin in opposite directions with a vLSD. Also, in Oz, AFAIK all SG Forries had a vLSD.

However, AFAIK the onlt MT centre diff that has electronics is the DCCD. All viscous centre diffs work entirely by differential speeds as RB described. There is no TCU input for the way a viscous centre diff works.
 
My SG ran an open rear diff, although this may have been fitted to the car when the rear diff was overhauled under warranty when I purchased the car second hand. They said they put a brand new crown wheel and pinion in it. I had to wait for those parts to arrive from Japan. Perhaps they fitted them to another centre.
 
As far as the vLSD rear is concerned, I've heard conflicting reports. The USDM SG X trim got an open rear, whereas I'm pretty sure (thanks NL and various threads I've read) the ADM/NZDM got the vLSD rear in X trim. I've also done a bit of driving on a very slick muddy road, I started with the right wheels on firm road and the left in mud and it seemed to hook up and take off alright, but I don't know whether this was due to the diff or not. The owner's manual is no help, it simply states the characteristics of the diff "if fitted".

Rally, I'm pretty sure the JDM SG base models didn't have vLSD rear diffs - they had drum brakes on the back! Only good thing about them is the 1.47 DR.
 
Thanks for that, NL.

I believe RB is right about a viscous LSD acting like an open diff until some power goes through it, so the wheels will spin in opposite directions with a vLSD. Also, in Oz, AFAIK all SG Forries had a vLSD.

However, AFAIK the onlt MT centre diff that has electronics is the DCCD. All viscous centre diffs work entirely by differential speeds as RB described. There is no TCU input for the way a viscous centre diff works.

So you put in a grounding kit, good work :biggrin: I've been meaning to do that myself! Where did you get the kit from or did you make it up yourself?

That's why I said "... some arcane magic ... " in my OP. There is patently some way that Subaru set up the centre vLSD diffs differently in different models. Most are 50:50 F:R (default value; but can vary within the limits of 60:40 and 40:60), but some are 45:55 and some are 60:40 (IIRC ... ).

They must have some method of controlling the minimum/maximum and default torque/power split values. Physical design and initial setup, perhaps?
 
As far as the vLSD rear is concerned, I've heard conflicting reports. The USDM SG X trim got an open rear, whereas I'm pretty sure (thanks NL and various threads I've read) the ADM/NZDM got the vLSD rear in X trim. I've also done a bit of driving on a very slick muddy road, I started with the right wheels on firm road and the left in mud and it seemed to hook up and take off alright, but I don't know whether this was due to the diff or not. The owner's manual is no help, it simply states the characteristics of the diff "if fitted".

All Foresters are Japan Domestic Manufacture. They were different specs and equipment in different markets though.

Rally, I'm pretty sure the JDM SG base models didn't have vLSD rear diffs - they had drum brakes on the back! Only good thing about them is the 1.47 DR.
Some of the early X model SGs (base models) had rear drums and no vLSD. Not sure if we got any of these here in Oz or not. The 2003 brochure I have for the SG states 4 wheel discs on all models. The SF brochure also lists 4 wheel discs for all models.

I actually prefer front discs and rear drums ...
 
All Foresters are Japan Domestic Manufacture. They were different specs and equipment in different markets though.

The "M" stands for Market. All Subies except the Baja are built in Japan but for different markets...JDM, USDM, AUDM, NZDM, etc are all the markets in those countries & have different builds, both stock & option.

 
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