Andrew Dixon

As far as I know, the std. UK car has ...

A normal diff between the front wheels. A limited slip diff between the rear wheels. And a viscous coupling between the front and back.

The front diff allows the wheels to turn at different speeds whilst maintaining drive. The rear diff sends more torque to the wheel with most grip. And the centre diff sends more torque to the end with most grip.

I'd assume that there would never be a situation where the front/rear split was 100:0 or 0:100 though.

I think it's all mechanically controlled. (I really would like to know more about how my car works!)

Please someone, enlighten use!

Andy

JamesS

I am curious as to the 4WD system we have on our lovely Scoobies.

As I understand it there is a front and rear differential on the axles (between) wheels and a centre diff.

Now what is the tq split on a flat level road (I thought 40:60).

I assume the centre diff is `active`, anyone know what the min/max front/rear split can be? Can it go 0:100 or 100:0.

What about the axle diffs can they shift tq between left and right, again by what amount?

Finally what controls it all; a mechanical system or electronic control?

If electronic could it be `chipped` to get an STI (type-R?) centre diff control?

On a sub note how should the scoob be towed (if it ever needs it!!!!) - all four wheels on the raod or flatbed truck only??????

coolhand

The std car has rear LSD n centre diff lock to a certain percentage. RA model has a manual control of the centre diff lock to adjust trq split to his preference. The rear LSD is viscous coupling while the centre is probably gear type. Trq split is set in factory to provide friendly n possible daily drive.

AlexM

Just my tupence worth.

AFAIK, the UK cars have viscous LSD at the rear and the middle, and a free diff at the front, with a 50:50 torque split front to rear.

MorayMackenzie

As I understands it, STI cars get an LSD up front as well as the rear one, the front LSD would help to reduce understeer but changes the steering feel when it's active.

Moray

mike_nunan

Wow, talk about confusion. Funnily enough, I've been doing a little straw poll asking various experienced people what their understanding is of this subject. I seem to get a different answer every time. Here are the few things I'm reasonable sure of:

* The 2-door cars have a free centre diff that is mechanically lockable.
* The locking portions torque to the front, until at 100% lock you have a 50:50 split.
* The 4-door cars have a viscous coupling in the centre. I still don't really understand how this works or how it differs from the Torsen diffs that Audis have or a standard LSD (or is that exactly what it is?)
* All the cars have free front diffs and limited-slip rear diffs.

Some people have told me that the 4-door cars have 60:40 split, while others say 50:50. Some people have said that the 2-doors have a certain fixed ratio too, but this is incorrect if the centre diff really is a free diff (in unlocked mode).

Aaron Payne

So why does this happen ?

On a dry day with under a 1/4 tank of fuel I have on 2 occassions pulled out of junctions and managed to spin the 2 front wheels for around 2 seconds. This was not intentional as I was pulling out into heavy traffic and a I needed to pull out quickly. Being so light on fuel I would have expected the rear tyres to spin if any - but as its AWD I expect none to spin, I do not have bald tyres either.

MY99 4 Door

Diablo

My tuppence worth:-

Viscous Diff - has plates which rotate in a fluid which increases in viscosity as one output shaft rotates faster than the other, thus providing infinitely variable locking of the dif.

Torsen - Audi's own electro mechanical limited slip diff which senses torque differences between each output and tightens the diff as required

Mechanical LSD - has a predetermined amount of limited slip, factory set to specified amounts.

As far as brief front wheel spin, this is normal as the front wheels are unloaded under acceleration due to weight transfer. If the MY99 uk cars have a free centre diff (ie with no limited slip capacity) it will be possible to spin the wheels at either end indefinately. If there is an LSD in the centre (which I think there must be after what happened leaving a wet roundabout on sunday (understeer + MORE power = oversteer ), you may get a bit of wheelspin until the diff transfers torque to the rear axle.

The turbos do not have an equal 50/50 torque split (non turbo's do) as (according to the UK website) the final drive reduction ratio is different front to back:

FINAL REDUCTION GEAR RATIO
FRONT AXLE 3.900
REAR AXLE 3.545

Cant remember which way round it goes, but I think higher numerical = lower ratio.

If the front had a lower ratio, would this mean more torque goes to the front?

D.

[This message has been edited by Diablo (edited 11-10-1999).]

andyp

So as not to re-invent the wheel, have a look at this URL: