Subject: Re: Question: 80Q "limited slip" center diff doesn't limitslip...

[C1J1Miller@aol.com]

"Avram Dorfman" <dorfman@est.org> wrote:
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>> He went into a 2 level parking lot with a steep ramp, and drove up on 
>> the edge of the ramp so that the right rear wheel came off the ground, 
>> but all others were firmly planted. The wheel in the air spun free as 
>> the care came to a stop and started rolling *backwards* until the fourth
>> wheel hit the gound again (and then made a lot of noise and smoke 8-)

>> This doesn't match my understanding of "limited slip" which is this: It 
>> allows only a certain percantage difference in rotational speed between 
>> it's two sides. Namely, for the rear shaft to spin at 100 rpms, the 
>> front "shaft" must be spinning at some percentage of that, e.g. 60 rpms.

>> Well, if the rear axle is spinning forwards and the car's not moving,
>> that's pretty much full slip, right? And if the car can go backwards, 
>> then it's really full slip, right?

Phil Payne <quk@isham-research.demon.co.uk> replied:
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>The Torsen senses _torque_, not rotation.  It was probably applying more
>toque to the front axle than the rear, but you were artificially not
>applying enough to overcome gravity and so didn't see the fact.  The
>Torsen Audi used in these cars doesn't go as far as a 0-100 split, so
>there's always about a fifth going even to the slipping axle.  The
>rear differential is (relatively) normal, so it spins the free wheel.
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"Avram Dorfman" <dorfman@est.org> next posted:
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::Hmm... I'm still not sure if I understand. You're saying that 4/5 of the
::engine's torque was going to the front wheels, and that wasn't enough to get
::up the hill? He gave it quite a rev while the wheel was in the air, I don't
::think it's a question of not being enough torque.

::The question remains, can the "limited slip" mechanism wear out and start
::allowing full slip?
================
Can they wear out? yes, I suppose they can, although it has not been reported 
on the list or elsewhere to my knowledge.  Can they transfer full power? Not 
likely.  

Phil's right.  If the car was in gear, and you hit the throttle, the engine 
will quickly rev up, and not produce much horsepower.  If all four wheels 
were in the air, you'd have more resistance, and the engine would produce a 
couple more HP, but still quickly rev up.  Now, you're on the ground, the 
engine's producing more hp, but most of it goes into spinning the wheel with 
no resistance.  So, as the engine quickly spun up, you put out a couple hp, 
20% of the torque went to spinning the rear wheel, and the rest was applied 
against the ground through the front wheels; he didn't move because the 
torque wasn't enough to go up the ramp.  Again, not enough to move the car.  
Now, try locking the rear diff; different story altogether.
chris