[torsen] RE: Torsens in the new milenium...

jeffrey.goggin at cox.net jeffrey.goggin at cox.net
Wed Jun 29 16:38:02 EDT 2005

> ... I can't comment too much on the A4Q and lurking
> spiders ... but I suspect you'll be able to get some good
> info from folks here ...

I don't know.  It's been 36+ hours since my post showed up and so far, nothing.  :^(
> I actually appreciated some of the info you posted ... I
> was not aware of the interchangeability of the center
> diff.  

Although I haven't confirmed this yet (but will soon), I've seen enough collective evidence to suggest there's indeed a strong possibility that it's true.  As soon as I found out one way or the other, I'll post back.

> I've got the bits in the garage to continue my
> investigation ... but what I am looking to do is to adapt
> the V8's rear Torsen to a locking urq rear diff ... I
> doubt it is possible, but I figured I'd put the bits next
> to each other to see what issues there'd be ...

It's definitely possible to install a V8 rear torsen diff into an UrQ diff case, but without an awful lot of machine work, I don't see how it would be possible to also make it lock and unlock upon request.  Besides, for performance driving on dry pavement, I'm not convinced you'd ever want to lock the rear diff.

> I also know the inside rear lifting phenomena well ... my
> old Fox used to do it all the time.  The thing was that
> it never really seemed to upset the balance of the car
> that much ... although that was the only time I ever got
> that FWD car to oversteer ... it was a situation not
> unlike an off camber turn ... and I got to experience the > old adage about oversteer scaring the passengers ... ;-)

The suspension design philosophy at VW/Audi is such that they don't provide their cars with much droop travel hence the wheel gets yanked off the ground when the shock runs out of travel. Depending upon how the car is setup, this may or may not be problematic given how much weight is on the front end and how little work the back wheels do when cornering, even with quattro.

The bigger problem, as I explained in my post, is that because the Torsen center diff sends a multiple of the torque produced at the wheels with the least amount of resistance to rotation to the wheels with the greatest amount of resistance to rotation (the Torsen perceiving resistance to rotation as a proxy for the amount of traction available) and anything multiplied by zero is zero, it behaves exactly like an open diff under these circumstances and allocates what little torque the engine can develop absent any load equally to the front and rear wheels.

For all practical purposes, the net result is that AWD effectively becomes ZWD and the car behaves exactly as it would if it was coasting.  Anyone who's ever tried to corner hard or accelerate out of a turn with the car in neutral will immediately understand how devastating this can be to a car's lap times.

In an attempt to band-aid this design characteristic of the Torsen (read: flaw), Audi uses EDL to brake the spinning wheel and thus allow the non-spinning wheel to support more torque.  While this works in theory, in practice, EDL is too slow to respond, works by soaking up power that could otherwise be used to accelerate the car, wears out brake pads and rotors quickly, and cuts out above a certain speed.

A better (but more costly) solution is install a limited-slip rear diff, which will prevent the lesser loaded wheel from spinning in the first place.

Anyway, you get the idea...


Jeffrey Goggin
Scottsdale, AZ

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