Re: Snow Tires

[Eliot Lim <eliot@u.washington.edu>]

On Fri, 12 Jul 1996, Sean Reifschneider wrote:

> >there are 2 very different ways of using a VC:
> >1) as *the* center diff/transfer case  
> >2) as a slip limiter on a conventional diff.
> >those examples you list are (2), while the 911C4, volvo, vw syncro are
> >(1).  
> 
> The examples I listed were NOT limited slip diffs on 2WD cars.  Perhaps
> I misunderstand what you're saying, but these cars have a viscous coupling
> center differential which then sends power to the front and/or rear diff
> which may or may not be limited slip.

the differences are very subtle but very significant.

the term "viscous coupling center differential" does not distinguish
between two very different implementations.  this is a completely
vague description but unfortunately used quite often.

> A 2WD with LSD helps a bit, but not a whole lot.  Perhaps it helps more
> in a FWD, but my RX-7 with LSD still would spin the wheels at every
> opportunity, EVEN with snow shoes on.

if you can picture your RX7's rear diff and VC LSD as the center diff
setup in the first list of cars, then that's half of the picture.
i.e. that's conceptually what the mitsus and the bmw use.

now, what the volvo, vw and 911C4 have is entirely different from this,
even though the term "viscous coupling" is used.  there is no mechanical
gear based differential.  the viscous coupling in this case *IS* the
center differential and transfer case.  that's where the 95/5 split
comes from.  

in the first instance, the system is sending drive to both axles all
the time, even during zero-slip conditions.  this is true "full-time"
4wd.  this is the key difference between this and the second
implementation.  in the second implementation, it doesn't become 4wd
*UNTIL* slip occurs. if you only had 5% engine output on an axle i
argue that that's not sufficient to influence the proceedings in any
way.  and that's why i label it "part-time automatically engaging 4wd".

i would suggest the book "AWD performance handbook" by jay lams if you
still cannot picture this.  he has nice diagrams detailing the
differences.  there is much subtlety in terminology but vast
differences in each implementation, which is unfortunate.  most of the
magazines don't understand nor appreciate the difference either.


eliot