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There seems to be a lot of confusion regarding the benefits of ESP versus Quattro, so I thought I would include a brief (or perhaps not so brief) write up on the subject.

ESP is Audi's Electronic Stability Program. It is an option available on most MY2001 Audis and also on other makes and models of cars. It basically uses the brakes, the ABS system, and a number of additional sensors to maintain vehicle directional control, particularly while changing direction, swerving, or cornering.

The various ESP sensors basically compare the cars actual yaw (turning) rate against the car's steering wheel angle and speed. From this comparison, the ESP system can determine if the car is over steering (i.e. when the rear end of the car slides/skids out towards the outside of the turn) or under steering (i.e. when the front end of the car slides/skids out towards the outside of the turn). If the ESP system determines that the car is over steering, it uses the car's ABS system to individually brake the outside front wheel to bring the car back into the proper cornering line. If the ESP system determines that the car is under steering, it uses the car's ABS system to individually brake the inside rear wheel to bring the car back into the proper cornering line. Basically, ESP compares where the car is going and how it is cornering (i.e. yaw rate) against where the driver appears to want it to go (i.e. steering wheel angle and car speed). If from this comparison it determines that the car is not behaving/cornering as the driver intends, it attempts to use the brakes to correct the situation and keep the car under control and out of the ditch.

ESP basically keep track of how the car is responding with regard to lateral (i.e. sideways) traction and attempts to correct situations where lateral traction is lost or compromised. This type of system is available on BMWs (they call it DSC-Dynamic Stability Control), Mercedes, Porsches, and Corvettes among others, and now on Audis.

So then, the question is often asked whether or not ESP can provide much benefit for a Quattro AWD car. The answer is unequivocally yes, without a doubt. Quattro does nothing to address the issue of lateral traction directly or in the same sense that ESP does, so the benefits of ESP on a Quattro AWD car are essentially and in most ways the same as they are for a FWD or a RWD car.

The important thing to remember about the Quattro AWD system is that it's primary function and intent is to maintain, balance, and distribute propulsive traction, and not lateral traction (at least not directly), between the four driven wheels. Now then, the since the Quattro AWD system does distribute propulsive force between four wheels rather than just two as in a FWD or RWD car, it does offer some side benefits which can enhance available lateral traction to some extent. Since the propulsive load on a Quattro AWD car is divided between the four tire contact patches instead of just two, the overall tractive load on each driven tire is generally less than it otherwise would be for a 2WD car. This essentially allows each tire contact patch to provide a better balance between propulsive and lateral traction before the threshold where traction is lost is exceeded.

Basically, each tire has a certain limited amount of traction available. If the total force on a tire exceeds the amount of traction available, the tire slips, skids, or slides, it loses traction. For a tire to maintain traction, the sum of the lateral and propulsive (or braking) forces must remain below the tire's available traction. If the lateral force exceeds the tire's available traction, the tire will slide. If the propulsive or braking force exceeds the tire's available traction, the tire will slide. If the combination of the lateral and propulsive (or braking) forces exceeds the tire's available traction, the tire will slide.

This is why RWD cars are generally more prone to over steer at the cornering extremes. The front (non-driven) wheels generally just have to contend with lateral (and braking) traction, while the rear (driven) wheels have to contend with both lateral and propulsive traction at the same time. Thus, at the extreme, the rear wheels on a RWD tend to get overloaded and lose traction first, resulting in an over steer condition when cornering. It's similar for a FWD. The front (driven) wheels on a FWD have to contend with both lateral and propulsive traction at the same time, while the rear (non-driven) wheels are only concerned with lateral (and braking) traction. At the extreme, the front wheels tend to get overloaded and lose traction first, resulting in an under steer condition when cornering. Of course, the overall balance and weight transfer will affect this all significantly too, but that's the gist of it.

This brings us back to my original point about Quattro AWD and lateral traction. The primary function of the Quattro AWD system is to maintain propulsive, and not lateral, traction. While the Quattro system can, in essence, free up more of each driven tire's traction for use in the lateral direction by reducing the propulsive tractive load for the driven tires below what it would be for a 2WD car, it does nothing directly to ensure that lateral traction is maintained. ESP does, which is why it can and will be of benefit even on cars equipped with Quattro AWD.

This is all a bit of an oversimplification, but while it may be lacking in technical detail and some nuances, I hope it helps to explain the benefits of ESP, and how they do apply even to a Quattro equipped car.

skabaru

<p>2001 A4 1.8T QMSX Avant (JUST ARRIVED!)

Steve66

If I remember correctly, the dealer's fact sheet on ESP also states that ESP can reduce the engine power output if necessary to prevent/correct a lateral skid.

audimods

<p>2000 A4 1.8TQMS

daijobu

<p>DJ

USER

Safety wise, ESP is much more desirable than Quattro.

Additional weight of Quattro is more of a detriment that penalizes stopping, road-holding and acceleration. Even in snow just adding winter tires on a 2WD, you are in a safer condition than a Quattro car with all season tires.

Ok, it will out-accelerate any car on slippery surfaces that's it. Very small contribution to safety, for which primary conerns should be stopping and cornering.

ESP systems I have seen are formidable for safety, but they tend to slow down the car.

eharleq

ESP just controls yaw, and in most iterations only oversteer-induced yaw. Ever drive with an esp car? With the exception of the Lotus as design, I want a switch to shut it off.

Is the quattro too heavy? Sure, it could lose some mass. The whole car is too heavy for that matter. Is the mass at the roll center of the car a problem? Nope.

Penalizes stopping? Compare our numbers for any other street cars in the back of R&T. In fact, the awd allows MUCH safer engine braking on ice and snow:no contest here in the mountains.

Lateral traction IS enhanced with awd. Someone just yesterday did a nice job explaining the additional available contact patch with awd. This is the real advantage of AWD; for racing victory is getting all the power to the ground at the exit of a corner, this happens because awd allows the driving wheels to corner and, accelerate OR brake.

USER

Best no-brainer proof. No major league car makers proning safety 1st (except Audi/Porsche) as BMW and Mercedes, Volvo and SAAB have put them in their no-cost top cars, which is NOT to say they don't sell some of lower class cars to people who are scary about "road safety". If it was SO obvious that AWD was such an advance, everybody would have it. Idem for performance with Ferrari. Simple. Clear. No if, but or whatever.

AWD does NOT enhance lateral acceleration, that is pure bull****. Even if the snow, winter tires are much effective than AWD except for acceleration which is FAR from being a concern, safety wise. That was once more demonstrated last year in C&D magazine (May '99)

eXciton

I asked that question about the difference between ESP and Quattro and if one can substitute for another but didn't get a satisfying answer. Your post addresses the issue perfectly.