blabla on AWD

[Jens]

Hi there!

No, I don't want to tell you what is better FWD/RWD/AWD, I had a very
exhausting discussion in alt.autos.audi about that. (As some of you
might have read.)
Reason is simply that I have read a message asking about it (indirectly)
and a friend of mine wanted to know it, too. 
I take this as a chance to improve my technical english.

So what do we have? (Only talking about major passenger cars with
"permanent" AWD here.)
VW: Synchro + 4Motion
Audi: quattro
Mercedes: 4Matic
BMW: iX or something
Porsche: Well, Porsche, what can you say? There was a Carrera 4 but
otherwise I don't know any specific AWD signes.

[From now on:
- means question
- plus -- means answer, plus -- means next question, ....

Longitudinal engine (bad ASCII art)

| | |
                              .
|   |

transverse engine:
  _
|   |
                              .     
|   |   

]


-Different systems?
---Yes! And Synchro is not = Synchro and quattro is not = quattro.

Systems:

1. Hollow shaft in gear box with integrated central diff: Can only be
used with longitudinal mounted engines like pretty old and current
Passat, real Audis ;> , or Porsche. With Audi: First it was a simple
diff, then torque sensing (Torsen) which can transmit power to the axle
where it is needed (not the one on ice ;)

2. Transverse mounted engine that drives the front diff, from there a
special clutch, either Visco or Haldex transmits power to the rear. No
real diff! If the slip (=difference) between front and rear is too
large, the Visco gets locked more and more. This is achieved by a
special fluid between discs mounted on the driveshaft coming from the
front diff and some on the driveshaft going to the rear. The fluid
thickens, if these discs revolute differently. The thicker fluid can
transmit the power between the discs.
A Haldex clutch is like a smart Visco (simply spoken) it is
electronically actuated and controlled.
However, both can only lock the diff, but not transmit more to the rear
than to the front. Remember, the front diff is always driven by the
engine!
(Theoretically, the engine could be mounted longitudinal, but I know of
no car with that system.)
 
While 1. and 2. mostly apply to converting front engine-FWD cars, or
rear engine-RWD cars to AWD, the next group is about converting front
engine RWD cars to AWD.

3. Engine stays as is (longitudinal), in the gearbox is a splitter (a
planetary drive) that also works as a diff: some power goes to the rear,
some over a secondary shaft (running alongside the engine) to the front
diff. Often, the oil pan of the engine is in the way, resulting in very
weird constructions that are by no means high-performance like the
original quattro. This is nearly only used in Mercedes and BMWs (talking
passenger cars here again). It is called Ferguson principle, it was also
used on the Jensen Interceptor and some Fords (even a Mustang) that were
converted, some for the police! (Yeah, I know, a 1970 'Stang has power.)
;)

-What has my car? Can that guy not simply tell me?
---Ok, you won!

Do you drive an Audi:
-Yes
---Does it have a longitudinal engine?
-----Yes
-------Then you have principle 1. If it is built after 1/88, you even
have Torsen. (date noot sure for all Audis everywhere.)
-----No
-------Then you have Haldex= system 2., called quattro nevertheless. (TT
and A3/S3)
-No
---What are you doing here then? ;) Do you at least drive a VW?
-----Yes
-------Does it have a longitudinal engine?
--------Yes
----------Then you have principle 1, a quattro, even if it is called
Synchro (old Passat/Quantum) or 4Motion (newer models)!
--------No
----------Then you have principle 2, either called Synchro (old
Golf/Vento/Jetta...) or 4Motion (newer models)!
-----No
-------Go change it, sell your BMW or Mercedes and buy a '90 200 20V. If
you own a Jensen or a converted 'Stang, give it to me as a bithday
present, I deserve it!

-What is better in 2. than in 1.?
---System 2., especially Haldex, provides a very dynamic system that can
be controlled for maximum ESP and ABS functionality.

-And vice versa?
---System 1. is always there, when you need it and when you don't. There
is no microprocessor that has to decide "Hey, guys, I think we neeeeeed
those rear wheels now!" or a fluid (in the old Synchros) that has to get
thick before the rear wheels can transmit power.

-What should I buy? 
---A '90 200 20V in papyrus metallic of course!
-----I already got one.
-------Take an RS4, the new S6 or even better the UrS4.

-R U stoned?
---No.

This was a bit of blabla on my part on the AWD systems. Feel free to add
to or correct it.
I hope the "---" stuff works on your e-mail viewer. It is plain text, so
I think it should work.

CU
Jens

http://www.audistory.com