Quattro explanation (few questions)

[johnhenry]

Morning all

Its been a while since i posted on here, worlds going round pretty fast for me at the moment! still not found a car but im pleased as ive read alot more material now and know abit more, so i hopefully can make better judgements about them.

Ive been doing abit of reading about the quattro system, but came across some criticism about the system itself, mainly it being caught out on the little rollers if both rear wheels have no traction all drive is sent to them as they are turning most, so the car goes nowhere.
I was reading rs2 material and that said the difs automatically locked under a certain speed, i believe very low at about 25kph (rings a bell but correct me if i'm wrong).

1) Wouldn't the diffs locking counter this problem?
2) And if so is this a feature of all quattro systems? follow up, how do cars get stuck on the rollers then?!

3) is there a 'better' quattro system used in certain quattro's? the search has been widened abit from just s2's so any models with good quattro systems would be great thanks!

Cheers
John
PS: excuse any huge mistakes: other forums and wikipedia have been used in places, not always the most reliable places of information

[Dan]

Okay, firstly, there are several different generations of quattro with slightly different configurations:

http://en.wikipedia.org/wiki/Quattro_%28four_wheel_drive_system%29#Evolutions

Gen II is what the S2 and RS2 have. Open front diff, torsen centre, open, locking rear diff.

With the rear diff locked, both rear and one front (3 total) wheels must lose traction completely before the car cannot move. The rear diff lock locks together both rear wheels and so guards against losing traction on one rear wheel. It doesn't make a difference if both rear wheels lose traction.

With regard to the 25kph thing, that's when the rear diff lock automatically comes off if you manually set it on under that speed.

The roller "bug" is caused because the torsen diff can only apportion up to 3 times the amount of torque to the front as is going to the rear - and vice verse. Here's the problem: since the rear wheels have almost zero traction and therefore near-zero load, there's almost zero torque required to turn them.

Since almost zero torque is therefore being sent to those spinning wheels, even with 3 times that sent to the other end of the diff (ie. the front wheels), that's 3 times near-zero. Not enough torque at the front to move the mass of the car.

Fortunately, there's a simple solution that's almost never shown in these "QUATTRO SUCKS LOL" videos, which has been known by off-road drivers for a long time: apply light braking (or if it's the rear set spinning-free, handbrake).

With the brakes applying resistance to the free-spinning wheel, the load and therefore torque required increases. Suddenly that 3 times 0 turns into 3 times 1 and you're off. (actually, this is similar to how the later quattro+ESP/EDL systems use braking via ABS to feign limited-slip through an open diff, by braking just the free-spinning wheel)

If one is looking to improve the standard S2 setup, I know some people have replaced the front/rear open diffs with LSDs, but it's not something to do unless you know exactly what and why you're doing it.

[johnhenry]

answered all the questions in one!
thank you very much mate!

[S2central.net]

Isn't funny how those "quattro sucks" vids never show a real world scenario of quattro vs Fwd or RWD on a snowy / wet / gravel road ?

Nice explanation above by the way