Miata Mailing List: February 1994, Message #125
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   From: Andy Poling 
Subject: Re: How LSD works?
   Date: Tue, 8 Feb 1994 10:51:59 -0500

On Tue, 8 Feb 1994 mac.avid.com!Michael_Wissner@avid.com wrote:
> > The viscous type LSD simply involves having a fluid coupling between
> > the halves of the differential which is filled with a very thick,
> > viscous silicone oil. This limits free wheelspin by imposing a
> > viscous drag on the spinning wheel. This is a fixed effect, so one
> > can't use too much drag without effectively locking the differential.

The critical missing piece here is that the fluid's characteristics are
dynamic.  With little or no speed differential between wheels, the fluid
has little viscosity.  As the speed differential increases, the shearing
forces applied to the fluid by the differential cause it to effectively
thicken and demonstrate increased viscosity.  This is when it begins to
limit slip. 

Thus, under ordinary circumstances, the differential acts like a
non-limiting diff.  Only when speed differential increases (i.e. a wheel
must be spinning/slipping) does the slip limiting come into play.  I
believe it can only transfer about 20% of the available torque to the
slower (non-slipping) wheel... 

> > The Torsen or Positraction differential uses a friction clutch
> > somewhat similar to the synchronizers in a manual transmission. This

Posi-traction differentials do use clutches, but the Torsen does not.  It
uses the one-way torque passing characteristics of worm gears to limit
torque transfer to a spinning wheel.  No clutch involved.  No chatter, no
clutch parts to wear out.

-Andy

Andy Poling                              Internet: andy@jhunix.hcf.jhu.edu
UNIX Systems Programmer                  Bitnet: ANDY@JHUNIX
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