Expanding on Drew's reply:
The original quattro system was full-time four wheel drive, with a 50/50
torque split between front and rear wheels. Audi engineers were convinced
that there were so many advantages of 4WD, and so few disadvantages, that
there was no conceivable reason that a driver might want to switch it down
into 2WD mode, so they didn't offer the option.
Later in the 1980s, the Torsen diff started to be used, which allowed a
variable torque split between front and rear wheels, varying beween 25/75
and 75/25 bias. In an interview at the time in one of the UK car magazines,
an Audi engineer said that this change was really made so that they were
seen to be updating the technology in the vehicle, and that virtually all
the time the torque split would still be 50/50, just the new setup has the
possibility to distribute torque differently if it felt it was required.
The Haldex system in the A3/TT is part-time, in that the rear drive is
brought in as required.
Related Quattro question: My 2001 A4 (1.8t, Tip, Quattro) got stuck
in the snow twice during last years eastern snow storms. The snow was
fairly deep with an icy layer under the snow. A push got me going,
but while I was stuck, only the rear wheels were turning / spinning.
Is my Quattro bad (no known driving or handling issues) or is it
normal for only the rears to spin during certain situations? Thanks.
I don't know the definitive answer to your question, but I do know that with
2 wheel drive cars, its common for only 1 wheel to spin. And it that case
you don't conclude that its broken and only 1 wheel is working properly. Am
I right in thinking that a limited-slip differential is supposed to help
prevent this effect? By transfering torque to the wheels that aren't
spinning? Well so much for the theory. Anyway, from the sound of it, I
would guess your car is behaving normally, but I am no expert.
A full time 4WD performance car, as mentioned earlier, needs 3 differentials
in order to operate properly. A differential is basically a mechanical
device that allows the wheel which sits on the outside (longer radius) of a
road bend to spin faster than the wheel on the inside (shorter bend radius).
These devices are used on the axles that hold the wheels that are driven by
the engine and, in the case of a 4WD car, between axles. If no differential
is present then the driven wheels would spin at the same speed in a turn
thus rendering the handling of the car very unpleasant. Self-locking
differentials add to the classic "free" differential the ability to lock
(drive both wheels at the same speed) under certain conditions such as when
wheel spin occurs. For instance by locking itself, the differential, allows
to avoid the immobilization of the vehicle in situations such as when a
wheel sits on snow while the other sits on dry tarmac. In this case, the
absence of a locking device would send all engine torque to the wheel that
spins faster (the one on the snow) and the car would not be able to extract
itself. Locking the differential would split torque distribution on both
wheels thus allowing the car to move forward.
Taken from a website
in theory the quattro has 3 diff's so you should see it spin all the wheels,
or it will spin each wheel in turn until 1 wheel stops spinning then it will
apply max power to that wheel to get you out and when that one starts
spinning agin it changes to another wheel, untill you are free...
Thats whats suppose to happen I think
In contrast if you were taking a fast corner and needed more purchase on the
rear of the car, it should transfer power there to stop the car from sliding
out, and then varies between front and back.
A good test is to find a quiet roundabout, when its raining, so slippery and
drive flat out round the roundabout, if the back end slides out before you
hit 60mph then somthing is up :)
<Warning I take no responsibility if you write your car off> :))
Also try driving on the beach, on the sand, and floor it <accelerate hard>
from standstill, you should'nt get any spin, and should pull away nicely
unless you are driving an s4 or a chipped up 1.8T with 200bhp.
The car should pull away fast like your on the road, and not spin like the
S4 would as all 4 wheels put down so much power that the 4wd system acts
like a 2wd car and digs all 4 wheels into the ground :)
My first Quattro was also an '01 A4 1.8tQTip and I never had such a problem
in that car or my current A6 2.7tQTip. It comes to mind that perhaps the
snow buildup had "lifted" the chassis so that the tires were no longer in
firm contact with the roadway and a push was all that was needed to get you
over the hump.
That's possible, but on two occasions I found my car to act differently than
I had expected. First one was in the snow, I can do a spin-a-roony if I
really want to, the wheels do slip, even when starting from a standstill.
But that was no biggie, more concerning is the next case. My A4Q went off
the road once, into a ditch. Three wheels were firmly planted on the (hard)
ground, while the fourth didn't have as much weight on it, that wheel kept
on spinning, no matter what.
I thought with the Quattro system, even if only one wheel had grip, it would
be enough to move the car, not too sure I believe that anymore...
Totally agree on the spin-a-roony, which I've also done on both snow and
fine gravel. I can't tell you about any "ditch" situation as you depicted,
but I suspect the EDS, i.e., the braking of the wheels is only effective up
until a certain amount of torque. If that threshold is exceeded on a
low-grip surface the wheels will slip-spin, no matter what.
So, the golden rule of smooth and very gentle acceleration prevails when
pulling out of slippery surfaces, I guess.
By the same token, I would hazard an engine with a comparatively low
torque - say an 1.8T, might in good hands do better than a mightier one,
because it's easier to apply just only the necessary torque so that the
wheels won't lose grip.
These are only assumptions but they're based on my experience. I would be
curious to know which the torque values are that have been used in
pre-programming the quattro-brake-assisted system.
My two cents,
In theory, no. With the rear wheels unloaded (as in not having any
grip) the Torsen centre differential should have transfered 75% of the
torque to the front wheels which should then have moved the car, or
started spinning themselves. A wheel on one side should be prevented
from spinning by the electronic differential lock function implemented
via the ABS system.
All this assumes that your car does have a Torsen centre diff, and open
front and rear axle diffs. I think that some of the automatic cars did
have a slightly different arrangement, relying more heavily on the
ABS/brakes for transfer of torque. If your car is one of these, then
your experience could point to an ABS problem.
Peter Bell - email@example.com
I'm quiet unsure about what type of traction control a 2001 A4Q actually
has. I'm assuming here: TorSen center diff, open (manually) lockable
diff. And: NO EDL's!
If it is TorSen, it is not able to split more than say 25/75. So: 0 grip
at one axle is 0 x 75 at the other end and that is: 0! Spinning tires.
In your case the solution would have been to apply "grip" to the
axle: use the handbrake to transfer torque to the rear output shaft of
TorSen. Then 75 x something = something for the front: You would have
Unlucky if you have a front tire spinning: No lockable diff and no
*separate* brake for the fronts only either...:( So always point the
REAR to the more slippery patch on the ground!
Summary: That problem occurs, if the grip-difference between front and
rear is over a specific limit.
P.S. If the car is equipped with EDL, that *should* have braked the
spinning wheel(s) to let the TorSen transfer some torque to the other
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