[torsen] LSD in a rear wheel drive

Orin Eman orin at drizzle.com
Thu Sep 25 19:55:02 EDT 2003

> The 944 list I'm a member of on Rennlist (Porsche board) is delving into 
> the realms of a junior Torsen type thread...
> A couple of people seem to have slightly wrong ideas, but strong opinions.
> Can I get the experts here to give me a simple outline of open diff vs 
> LSD (assume clutch type).
> Nothing serious, we're looking at straight line effects and the classic 
> one wheel on ice scenario.
> One comment that has disturbed me so far is:
> -
> What you are saying (effectively) is without resistance there is no 
> torque.  I don't buy it.  If for no other reason than there is torque 
> input *to* the differential, so therefore there must be torque (less 
> frictional losses) torque preserved *out* of the differential.  It 
> simply *has* to go *somewhere*.

They are neglecting angular acceleration.  The above statement is
only true if the input shaft to the differential is rotating at
constant speed.

If the input shaft is accelerating, then a torque of
<moment of inertia of diff guts> * <angular acceleration>
is required to accelerate the guts of the differential.
This accounts for output torque being less than input torque.

What happens when you lose traction on a wheel with an open
diff is the whole drivetrain accelerates up until frictional losses equal
the input torque, the engine reaches its rev limiter, blows up or the driver
backs off the gas pedal (the usual case).

Have fun with this one...


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