Re: Quattro vs Volvo V70 AWD

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

the reason why volvo wants to reassure you that AWD works in reverse
is because of the viscous coupling based implementation that they used.

viscous couplings are pretty dumb devices, and don't know the difference
between accelerating and braking.  they lock when they sense a discrepancy
in axle speeds.  the locking effect during braking is very undesirable
because it makes life really tough on ABS, which needs wheel speed
independence to work properly.

so, those makers who use this particular implementation have to come up
with a secondary means to disconnect 4wd (this is after all a part time
automatically engaging system) when the brake pedal is applied.  i have
been told that chrysler's minivan uses a freewheeling device (think of a
bicycle crank) which means that there's no 4wd in reverse.  volvo is
probably using a secondary clutch that is electrically disengaged when the
brake pedal is stepped on.  that way you have 4wd in both directions. 
that is what they are trying to reassure the customer about.

of course those makers who use torsen differentials or computer
controlled clutches do not have this problem.

it is pretty unfortunate that porsche also went this route, but
i think they put in extra smart ABS brains rather than do any
kind of disconnect.


eliot