Bob

Quattro effectiveness at low speeds is dependant upon ABS-based traction control, namely EDL. This was done to deliver a CHEAP but very effective AWD system to the mass market. EDL stands for Electronic Differential Lock. This is a marketing misnomer - there is absolutely no lock or simulated lock occuring. This acronym was chosen because Audi used differential locks at one time.

EDL works on a per "axle" basis - there are effectively two separate traction control systems, one for the front and one for the rear. Again, this was done to deliver a CHEAP but effective traction control system to the mass market. Having two separate traction control systems allowed Audi to add off-the shelf traction control software to their ABS system. EDL transfers rotation, not torque, from one side to the other if it senses slipping on one wheel of an "axle". If the car is travelling faster than 25 mph, or if the brakes overheat, or if EDL is hunting, the system shuts off. At this point, there are only conventional open differentials on each "axle". Audi shuts off EDL at 25 mph because they are concerned with instabilities inherent to pulsing the brakes at high speed. LAND ROVER has solved this issue, with their version of EDL, 4ETC. They have a "partial mode" which works up to 62 mph.

The center Torsen, transfers torque, not rotation, between the front and rear as well as acts like a differential. The Torsen has a characteristic load biad ratio of 3:1 or 4:1, which means that it requires 1/3 or 1/4 traction available on the "slippery axle". If this ratio is not maintained, it will work like a conventional open differential. Torsen was developed by D.L. Gleason Inc. many years ago and is used in RAV4's, Hummers and in many industrial applications. (Kinda makes you wonder what Audi Engineering did? ;-)

The Haldex system is not a flexible as QIV. The Haldex clutch (equiv. to the center Torsen) can not act as a differential. It merely acts as a clutch. The Haldex system is primarily of FWD system that transfers rotation to the rear when excessive slipping occurs in the front (which has EDL). The Haldex system does not have EDL in the rear.

Bob
96 A4QM
86 4KCSQM

mm

What do you feel the pros/cons, and trade-offs are for the currently available systems? Who has the best systems for off-road applications?

I think I understand Quattro IV (Torsen, EDL) fairly well but am not so well versed on the systems other manufacturers use. (Yes I've read the FAQ...)

Anyway, I would appreciate your thoughts if you have the time/inclination/knowledge... Also, what's your background, mechanical engineer?

Bob

> Haldex uses computer controlled clutches to prevent wheelspin (once detected).

The Haldex system in the TT uses only one clutch between a power takeoff in the front and an open rear differential. I mention this because, I get the impression from what you said that wheelspin is controlled at a per-wheel basis. If I am wrong, please accept my apology. This, of course, is not true. EDL and the front open diff do the best they can. If there is excessive front "axle" slip, the "computer senses this, the clutch engages (yes, variably) and transfers rotation to the rear open diff from a power takeoff shaft from the front. By the way, according to a few sources there is no EDL in the rear. I am not sure if I believe this. Also, the Haldex clutch cannot act as a Torsen _differential_ nor can it _directly_ transferring torque to the front or rear - the computer indirectly senses front slippage and hopes it's better in the rear. IMO this is poor.

> I would describe the Haldex system as more flexible and tuneable.
Flexiblity is not capability.

Bob
96 A4QM
86 4KCSQM

Bob

> What do you feel the pros/cons, and trade-offs are
> for the currently available systems?
> Who has the best systems for off-road applications?

I may be able to help. Any more details? Off-road only 100%?

> Anyway, I would appreciate your thoughts if you
> have the time/inclination/knowledge... Also, what's your background,
> mechanical engineer?

I am an engineer. I have owned, worked on and raced (minor trust me ;-) Audis and other cars since the 80's.

Bob
96 A4QM
4KCSQM

Bob

> If EDL is disabled above 25mph, why were A4's (Quattros) banned
> from the racing circuit due to the unfair advantage.

Pardon me, but I laugh out loud when I hear this. The Quattro IV system has never been raced nor will it ever be raced (pro rally, Touring Series etc.). The cars you are referring to had proprietary electro-mechanical clutches, and driver-tunable LSD's - viscous that it. Surprised! Welcome to AUDI Marketing Machine.

> I assumed Haldex was superior to the Torsen set-up. This
> doesn't seem to be true now. I am confused. Is one clearly
> superior, or are there advantages/disadvantages to both.

Haldex is not superior in this form. The advantages are none, other than it fits in the platform with a transverse engine.

Bob
96 A4QM
86 4KCSQM

Wayne H.

Are front and rear differentials open? Are they limited slip?

Bob

Quattro IV front diffs are open

mm

I'm interested in learning more about the currently available 4/AWD systems for both on and off-road. I understand Quattro IV reasonably well, and also the basics of CV based F/AWD systems. I will search the archives for more info on Haldex, but I understand it is a F/AWD clutch type system (i.e. clutch activates AWD only when needed, otherwise FWD). Just wanted more info on who was using what systems and what trade-offs are involved.

The general populous seems to think that 4WD is the same as AWD, that all "AWD" systems are essentially equivalent, and that 2WD traction control is just as effective as 4/AWD, none of which is, of course, really true in actual fact. Too few consumers seem to ask the "tough questions" regarding AWD, and fewer still know/understand the actual/correct answers...

Bob

Sorry, I meant front and rear are open.

mm

Bob is correct, Quattro IV front and rear diffs open, EDL limits left/right wheelspin front and rear