WHAT's Haldex was: mixing and matching tires on a Quattro?

[KUNZ,BOB (HP-Boise,ex1)]

Hairy green toads made SuffolkD say:
> 
> TT's and VW's use this as awd/4 wheel drive.  Roughly said:
> Its front wheel drive until slip is detected then the rear wheels get
> power.
> I asked a TT's owner if it was front wheel drive even though the quattro
> badge stared me in the face.
> They have a nice little habit of darting right on acceleration on snow &
> ice....just like fwd.
> HTH - Scott by BOSTON
> 

There are two control modes for the Haldex. One is mechanical and the other
electronic. There are pistons that operate when the input shaft and output
shaft start to get out of phase (i.e. slippage). These pistons move the
fluid to get the wet clutch to engage and lock the input and output shafts.

The electronic mode comprises a pump and a set of valves and can control
both engaging the clutch or denying the mechanical system. One might want to
disengage locked shafts if ABS or ESP is currently trying to control the
car.

It's correct to state that the Haldex basically keeps the car in FWD for
most situations. The mechanical system reacts to slippage within 20 degrees
of rotation of the wheel. There are aftermarket controllers that can adjust
the degree of torque split front to rear but this is likely detrimental to
the system. It is designed as a part time AWD system.

However, my TT which has the Haldex system doesn't dart anywhere except
where I point the front wheels on snow and ice. And that's even with summer
tires in the winter.

--bob

'86 5Ks Avant
'02 TTQR