Re: ABS 'bugs'

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

On 9 Dec 1994, David Robbins wrote:

> As a side note, part-time four wheel drive functions completely different
> than full-time AWD systems.  With the former, the wheels are locked in sync
> (side-to-side).  While this is great for off-road/deep dirt/snow traction,
> it's less than optimal (and much less effective than AWD) in hard packed snow
> and ice.  It's also unusable in the dry.  That's why I love my AWD (sorry,
> but it's not a Quattro).
> 
> -DMR.
> 
> 1991 Mitsubishi Galant VR-4


not quite.. we just had a lively discussion not too long ago about
porsche using part time 4wd in the latest carrera 4.. it used to be
that part time 4wd were mostly trucks based on 1939 (?) willys jeep
technology (many of today's macho trucks and SUV's which i dispise
still have not progressed one iota from this) while the other group
of full time 4wd cars were represented by audi and the wannabes..

the picture today is much hazier.. some of the more progressive SUV
trucks have seen the advantages of going full time (range rover was
the first) by fitting a center differential with various methods of
locking.  the humvee is full time with torsen diffs all over. e.g.  i
believe one of the jeeps also offer full time on-road 4wd.  i believe
automatically engaging 4wd is also starting to appear on trucks.

on the other end of the spectrum, many cars today are part time 4wd,
the porsche carrera 4 and the lamborghini diablo VT being the most
exotic.  the difference between these cars and the trucks based on
1940's technology is that these cars have automatically engaging 4wd
while the trucks are manually engaging.  also, the same mechanism that
is used to transmit drive to the idle axle also serves as a center
differential.  these are the well known viscous couplings.  

unfortunately there is tremendous widespread ignorance (helped no
doubt by an equally ignorant US auto media) about the two completely
different ways that a VC can be deployed.. in the case of the
mitsubishis, they are used as limited slip devices, where speed
differences cause the differential that they are attached to to lock,
while in the case of the porsche, the various VWs, the lambo and many
minivans, they *are* the center diff as well as the drive engagement
device.

anyway, when one talks about 4wd, one needs to figure out which of
the categories the subjects fall into:

1) part time, manually engaging 4wd.. the most primitive of all

2) part time, automatically engaging 4wd.. many minivans and cars

3) full time 4wd, manually locking diffs.. 1st generation quattros and
some others like the mazda 323 GT, some toyotas.

4) full time 4wd, automatically locking diffs.. 2nd generation quattros,
mitsubishis, newer subarus, the killer lancia integrale

5) hybrid switchable systems between 1 & 3 or 1 & 4.. 


only the 1st category do not use some form of a center differential
which renders it unusable in the dry.

also, it is no longer easy to generalize 4wd systems based on the
vehicle's off road capability.. i.e. a old subaru is far more
primitive than a range rover for example.

i might also add that the 3rd & 4th categories is the type of 4wd
system that has seen the greatest competition success which is
testimony to its superiority.


eliot