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Re: Torque vs EDL

In a message dated 9/13/99 11:56:57 AM Central Daylight Time, 
Robert.Houk@East.Sun.COM writes:

> My understanding of EDL is that it simply selectively applies brakes
>  to individual wheels, thus directing power transfer (note I don't say
>  "torque") to other wheels. Carried to its extreme, you completely lock
>  up three wheels, directing all power transfer to the remaining wheel
>  allowed to spin.

Carried to the extreme in a awd application, means that 3 wheels can lock, 1 
carries traction.  FI, audi hasn't ever used 4 wheel EDL, it either uses 
front or rear wheel (meaning it can't cross over the center axle, 
specifically, in the extreme, it can only lock 1 wheel per axle).  IT doesn't 
really direct power, it raises Trg to the ground.  Normally, (lets pick rwd 
with EDL, for ease of example), in an open diff (50/50 R/L distribution all 
the time), 100% of available Trg (albeit only enough to spin the faster 
spinning wheel) goes to the wheel with the least traction, car doesn't move.  
What EDL does, is brakes the spinning wheel, raising the Trg to the spinning 
wheel which also raises the Trg linearly to the wheel that is not spinning. 
Poor mans LSD.  In terms of torque, we can say, for example, that if you 
apply braking force to the spinning wheel of 200ft/lbs, the spinning wheel 
(with EDL braking engaged), can support 200ft/lbs of Trg.  Which means the 
non spinning wheel also transmits 200ft/lb of Trg, but since it has traction, 
that 200ft/lbs moves the car forward.  The 200ft/lbs of torque on the 
braked/spinning wheel, is lost energy in braking heat.

What this means is, that ANY EDL intervention, cannot, by definition, allow 
100% of Trg to go to the other axle.  Any EDL intervention means that the 
braking torque to that wheel, is equal to the engine torque trying to spin 
it, which lowers the total amount of trg transfer accross the axle.  In the 
example above, the highest amount of Trg that can go to the non spinning 
wheel, is maximum, only equal to the amount of braking torque that can be 
applied to the wheel with the least traction.

The only true 4wheel EDL as of today is the Mercedes ML SUV's.  They have all 
3 open diffs and 4 wheel EDL to control wheel slip of up to 3 wheels.


Scott Justusson