From: eliot@engr.washington.edu (eliot)
Newsgroups: rec.autos
Subject: the truth about the Nissan Skyline GT-R's 4wd system
Date: 22 Jan 1993 23:55:04 GMT
Organization: skulls 'r us
Distribution: world
Reply-To: eliot@engr.washington.edu (eliot)
NNTP-Posting-Host: stalfos.engr.washington.edu


over the past couple of days there has been some discussion about
Nissan's hot-rod killer car.. even comparisons made to the Porsche
959 where the nissan proponent claimed had a superior AWD system
to the 959's.  i have done some looking into this and here's
what i found:

while it is somewhat subjective whose system is better, nevertheless
there are facts by which we can evaluate the systems.  a recap
of the skyline for those who are not familiar:

a 2 door sedan/coupe (with a conventional looking trunk) boasting 4WS,
4WD and a twin-ceramic turbo blown straight six pumping out an
official 280 bhp.  by most accounts it is a great car and it is indeed
unfortunate that it is not imported to the US.

the skyline is a part time 4wd car, being 100% rear wheel drive until
the speed sensors indicate wheelspin, whereby a computer controlled
multiplate clutch gradually transfers excess power to the front
wheels.  under maximum slip conditions, the torque is split 50-50.
under no circumstances will the computer send more than 50% torque to
the front wheels.  the claim that the skyline will deliver 100% of the
torque to any one wheel is absolutely false.  in fact the skyline does
not even have a rear limited slip differential, nor a front one.  what
this means is that if one side of the car runs onto ice or a ditch or
whatever, one becomes stuck, as all the power will be sent to the side
with the lost traction.  the spinning front and rear wheels would be
spinning at the same speeds, though, because the computer would have
split the torque 50-50.

while this AWD system can be considered somewhat sophisticated, it is
far from state of the art.  (nissan doesn't claim it to be either) a
twelve thousand dollar subaru legacy equipped with automatic
transmission has the exact same setup and characteristics, except that
the automatic legacy is 100% front drive instead of rear drive.  i.e.
the car is 100% fwd until speed sensors detect wheelspin, whereby
power will be gradually sent to the rear via a computer controlled
multiplate clutch till a maximum split of 50-50.

the mercedes 4 matic system (to be abandoned soon) has the same setup
as the skyline, except that in addition to automatic engagement of
front drive, it also has a computer controlled rear limited slip
differential.  in terms of specification, the mercedes system is
already clearly one up on the skyline's.

now, the 3 systems described above can best be generalized as
"automatically engaging part time 4wd systems".  there is another
category of AWD cars called the "full time 4wd systems" where power is
sent to 4 wheels at all times, even with zero slip.  these systems
differ in terms of how the power is distributed as well as how the
system reacts when one or more wheels start slipping.

it is somewhat debatable whether a full time system is superior to a
part time system.  the issues are that the full time systems will show
ugly fwd effects with very powerful engines and there is more inertia
in the driveline.. on the other hand the cars tend to be better
balanced and surefooted in slick conditions because the tractive loads
are far more evenly distributed.  a part time car will attempt to send
all its power to one set of wheels, thereby depriving it of turning
grip until the computer or viscous coupling intervenes.  these cars
would be crossing to and fro the grip threshold of its permanent
driving wheels many times, and it is the transient behavior when it is
in the process of transferring power where one needs to consider.

when you are cornering at your tires' adhesion limits and your awd
system starts redistributing power and in effect the cornering loads
as well, there is a potential for upsetting the entire manouver.  i
found that this is indeed possible in real life when i test drove a
subaru svx, which boasts variable torque splits under slip conditions.
in the wet i entered the curve slowly and past the apex i gave it a
good dose of throttle.  the nose started sliding so i throttled back a
little while keeping the same steering lock, in the hope that the
understeer would go away.  to my surprise, the tail suddenly whipped
out without warning and i had to apply frantic opposite lock to catch
it.  while i have no proof that the awd system was entirely
responsible, i couldn't help but think that it was at least partially
responsible, given its specifications.  i though that it certainly was
quite unusual for a 4wd car to behave like a 911.  (without the
seat of the pants warnings, though!)

with a full time system, you are always assured that all 4 tires will
share the tractive loads and that the remaining grip available for
cornering will always be more evenly divided than a part time or 2wd
car.  thus i would say that its grip threshold is higher.  the system
will be dealing with transient situations less often.

we come now to the system found in the porsche 959, which has been
called the most advanced system found in any car ever built.  with
the exception of when the car is operating at parking speeds, the
car is a full time system with a twist.  drive is disengaged to
the front wheels at very low speeds for the trivial reason of 
lightening up the steering wheel.

the 959's system varies the torque split *before* wheelspin occurs.
this is the difference between it and everything else.  all other part
time and full time systems *react* to slip.  in the part time systems,
typically drive is engaged to the other axle.  in the full time
systems, typically differential locks start redistributing power.
in both cases, the car has to cross the threshold where there is
no more grip on one or more tires before they act.

the 959's system is somewhat analogous to a braking system that has
forward sensing radar that intervenes and activates your brakes for
you if it decides that you do not have enough stopping distance before
you hit the guy in front.  the others would fit the model of abs
braking, which reacts after you have gotten into hot soup.  i know
many of you are going to say "i can judge for myself stopping
distances, thank you very much".  this is where the analogy ends,
however.  the 959 distributes power in a way to push the grip
threshold even higher.  it will not cut back power like traction
control systems; it will merely make decisions as to which end should
get more power so that the car will remain completely stable and that
available adhesion for cornering remains evenly distributed over the 4
wheels when part or all of its 450 horses are unleashed.

for those who are numerically inclined, the 959 has center as well as
rear computer controlled clutches for differentials.  the car was
programmed to split the torque 20-80 under full acceleration, 34-66
(not 100% sure about this) under normal driving and 50-50 when the
driver selects "rain".  there is also a "snow" selection that the
driver can make and this turns the car into a 4X4 style system where
the center differential is locked solid.

so, going back to the skyline, it is quite clear to me (at least) that
the skyline's AWD system is not even close in terms of sophistication
to the 959's system.  the skyline is by no means a crude or bad car;
it simply was never as ambitious as the 959.


eliot