But I once
>had the occasion to jack all four wheels off the ground, and sure
enough, when
>you spun one wheel, the other wheel rotated the opposite direction.
This I
>have always been told is indicative of a positraction rear end.

I don't know about that, but it IS indicative of a differential.  Just a
differential.

 I also started
>it up, and put him in gear. Both rear wheels rotated.

Yep, and if you grab one wheel and hold it, the other will keep
spinning.  This is why you can have problems with 2WD cars with regular
differentials (like our vans) when you have one drive wheel on ice and
the other on dry pavement.  The one on ice has an easier time of turning
and so the differential gives all the power to that one wheel, and you
don't go anywhere.  This is why some cars have locking differentials, so
that each drive wheel will spin at the same rpm all the time.  This,
however, creates problems if you try to turn the van, as both wheels
need to spin at different rpm to complete the turn, which is the whole
reason for having differentials in the first place :-)

Someone correct me if I'm way off base on my understanding of how a
differential works :-)

Sean Bartnik
Fairfax, Virginia
============================
'81 Vanagon L Westfalia
'74 Karmann Ghia convertible
http://www.type2.com/bartnik/tech.htm

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