4wd Systems

[confusionhunter]

Hi Folks,
I am looking for pictures and schematics of the s4 4wd systems the, biturbo model. I'd like to know more about them before i take the plunge.

Ie what diff do the use? how many ( ie are there right left LSDs as well a front back diff? whats the drive split? 60/40? 50/50?? Are these massively different to the rs4/s6 systems?
Is there a transfer box? or is it part of the front G box? are all the rear diff the same???
Ive done some searching on the net and on this forum but cant really find that info.....

Can anyone help or point me to the right section?
CHeers.
Mark.

[Dippy]

This is what I know from my research:
LSDs at the front
LSD at the rear
Torsen diff in the centre
Drive split is about 60/40 (front/rear)

RS4 has a different gearbox but the rest of the system is the same???

[JonnyX]

Hi Dippy,

I thought we had an open diff rather than LSD's?? (hence the LSD upgrade from Stasis) - where did you find the info on that?
The torque front/rear is via a torsen differential (mechanical diff with planetary gears) - on an S4 it's 2:1 which means that upto
66% of the torque can be split front/rear depending on slip. The split without slip is 50:50 - the new RS4 has a 60:40 rear bias.

The RS4 (2.7T) has a 4:1 torsen centre diff which means that upto 80% of the torque can be split front/rear, depending on the
conditions. (I think)

[confusionhunter]

oooo, Thats the kind of info Im after! I know the S3's have the Haldex unit like the R32 Golfs that you can electronically messa bout with the toque split...60/ 40 to the front sounds about right but just wanted to know more...

Is there a manual for the s4/RS4? I dont think Haynes do one! lol

[PhilT]

Is there a manual for the s4/RS4? I dont think Haynes do one! lol

IIRC there's a Bentley Manual.

[simple1]

Were are you getting your figures from dippy.............torsen diff does not split........... it divides the drive from front to back by sensing torque...........TOR..... and SON......... being the germany for sense so I am told The split can vary from 70/30 to 30/70 depending on what the diff senses and where the most grip is being achived.......... Niether diffs are LSD, this can be varified by turning one rear wheel when the car is in the air, the corresponding axle wheel turns the opersite way, so no lsd, they do however have a torque diff, which will deliver drive to the wheel with the most grip, not much different to a normal diff in function or ability...........
Lesson over,

[confusionhunter]

Ahh.... So..... if I am playing devil advocate here...

If youe were to manage to get wheels spin, the transmission would only need to overcome the grip of 2 tyres... one at the front and one at the back..... but I take it that the torsen diff acts as a form of TC as when it senses that one front wheel is spinning it shove a load of power at the back thus helping launch the car and stopping the front wheel spinning and thus returning the power to he front....ish...
So really LSD's are not neccesary with a torsen unless you do a lot or rallying to work....
Cool.... OK... Where on the S4 is the actual Torsen located... and in the front gearbox is the prop shaft output in the side or the read of the box?
Sorry for the Q's....
I did learn the S4 and Rs4 tranny's are the same except for 4th and 5th gear ratios....

[Dippy]

Right, I may be completely wrong, but you haven't convinced me yet.

Torsen diffs don't have to be set up for a 50/50 neutral split, they can be biassed. I thought I'd read that the S4 had 60% torque to the front.
Also I thought that an LSD will do what you say when the car is in the air - the LSD comes into operation when there is a torque imbalance across the axle. Surely having just open diffs on the axle completely defeats the objective of having a torsen diff in the centre. I can't see what you mean by "the do have a torque diff" - surely that's exactly what an LSD is???

TORSEN is literally TORque SENsing. Nothing to do with Germans. I believe it is an American invention.

Keep it going - I haven't had a good argument for ages

[gavsteryoung]

Hey guys, try looking on http://www.torsen.com/oem.htm

and also read the FAQ, it might give some clues......

[JonnyX]

Right, I may be completely wrong, but you haven't convinced me yet.

Torsen diffs don't have to be set up for a 50/50 neutral split, they can be biassed. I thought I'd read that the S4 had 60% torque to the front.
Also I thought that an LSD will do what you say when the car is in the air - the LSD comes into operation when there is a torque imbalance across the axle. Surely having just open diffs on the axle completely defeats the objective of having a torsen diff in the centre. I can't see what you mean by "the do have a torque diff" - surely that's exactly what an LSD is???

TORSEN is literally TORque SENsing. Nothing to do with Germans. I believe it is an American invention.

Keep it going - I haven't had a good argument for ages

All the evidence I can find states that the default torque split is 50:50 on
the S4. The press are making such a big fuss of the NEW RS4 at the
moment because, for the first time ever, Audi has given the car a 60:40
rear bias.

http://forums.audiworld.com/s4/msgs/2029956.phtml
http://forums.audiworld.com/s4/msgs/1882013.phtml
http://forums.audiworld.com/s4b6/msgs/101968.phtml

The short answer is we get EDL (Electronic Differential Lock) instead of
LSD because it's cheaper. Stasis do nice centre diff and rear lsd upgrades.

[JonnyX]

Ahh.... So..... if I am playing devil advocate here...

If youe were to manage to get wheels spin, the transmission would only need to overcome the grip of 2 tyres... one at the front and one at the back..... but I take it that the torsen diff acts as a form of TC as when it senses that one front wheel is spinning it shove a load of power at the back thus helping launch the car and stopping the front wheel spinning and thus returning the power to he front....ish...
So really LSD's are not neccesary with a torsen unless you do a lot or rallying to work....
Cool.... OK... Where on the S4 is the actual Torsen located... and in the front gearbox is the prop shaft output in the side or the read of the box?
Sorry for the Q's....
I did learn the S4 and Rs4 tranny's are the same except for 4th and 5th gear ratios....

You should be OK as long as all 4 wheels are on the ground. If you have
ESP on and a wheel leaves the ground then the EDL part of the ESP
program should detect this and apply the brake so that torque can be
applied to the other wheel.

The prop shaft exits from the rear of the box.

[Dippy]

I did a quick web search and confirmed what you state, it's a 50:50 split. However I did find some press releases about the B6 which state a 60:40 Front:Rear split, so maybe that's what confused me.

I also found again one article that I read: http://www.audiforum.nl/Technique/Quattro/quattro.html

Note the statement: "In normal condition, torque split between front and rear wheels is 50:50 (other ratios are possible, depends on the pitch of worm gears)."

I also guess the statement "Its Quattro 4WD system uses a pure mechanical LSD, Torsen differential" confused me into thinking the other diffs were LSD.

[gavsteryoung]

I think the B6 S4 still has 50:50....

Since they have developed the new RS4 with the 40:60 they seemed to have transitioned this to the B7 S4 and New S8.....

[confusionhunter]

Right..... let me get this right..... according to that article:
"Except the first generation Quattro system that appeared in the early Quattro coupe, most of the subsequent Quattro systems used Torsen differential in center and rear axles. "
So the S4/RS4 have a torsen diff in the prop shaft and in the rear (diff translating from centre and rear shafts). However I find no iother evidence of there being a torsen in the rear diff... In fact the Torsen site shows just a centre one...... SO....

Thats good!! Torsen IS an LSD in a form.... so Im not sure what the LSD centre "upgrade" is... ? Now it would seem the fron and back biffs are normal diffs but have "Electronic Diff Lock" (EDL)

Which brings me to my next Q..... EDL? How does that work??? Solenoid? Im assuming its activated when the wheel sensors show significant difference in wheel speed..... And does it lock the diff or apply the appropriate brake? I doubt it would do both.....

Sorry for stirring....

[JonnyX]

Right..... let me get this right..... according to that article:
"Except the first generation Quattro system that appeared in the early Quattro coupe, most of the subsequent Quattro systems used Torsen differential in center and rear axles. "
So the S4/RS4 have a torsen diff in the prop shaft and in the rear (diff translating from centre and rear shafts). However I find no iother evidence of there being a torsen in the rear diff... In fact the Torsen site shows just a centre one...... SO....

Thats good!! Torsen IS an LSD in a form.... so Im not sure what the LSD centre "upgrade" is... ? Now it would seem the fron and back biffs are normal diffs but have "Electronic Diff Lock" (EDL)

Which brings me to my next Q..... EDL? How does that work??? Solenoid? Im assuming its activated when the wheel sensors show significant difference in wheel speed..... And does it lock the diff or apply the appropriate brake? I doubt it would do both.....

Sorry for stirring....

The S4/RS4 have one torsen diff in the centre. This is 2:1 and is able to divert upto
67% of the total torque generated front or back depending on the 'slip'. The stasis
upgrade changes this to a 4:1 ratio.

As standard they have open diffs in the front and rear.

EDL is basically a re-use of the ABS hardware, It uses the ABS sensors to detect
if one wheel is turning faster than the other (within certain parameters) and
applies braking to that wheel. It doesn't lock the diff, that is purely mechanical.
Stasis do an LSD for the rear to replace the open diff.