NWS4Guy

Better than the one without which would be stuck at the bottom

I'm not saying the SD is the end all be all, but what you get for the cost of it, it's definitely worth it - even without the ADS light features that you also get. I used ice as an extreme example for illustration, the same would go for any time traction to one rear wheel is lost (oil, sand, gravel, water, etc).

mellow_sparky

quick question here then.

when I was test driving audis, my SA had me set the car up with half the wheels (passenger side) in the gravel on the side of the road and the other half on the road (driver's side). And then floor it.

the result - the driver's side wheels got the traction and the car launched without any fuss (and without spewing gravel from the passenger side).

what system was controlling that? I did it in a Q5 without drive select.

drrick89

Mellow Sparky was asking a rhetorical question, but I'm going to answer it literally. Quattro, ESP, and the sport diff probably all combined here; a little of this, a little of that; much of it happening simultaneously & faster than anyone could even think about it. But to break it down: Quattro shuttles torque between the front & rear wheels, ESP controls wheelspin with the brakes, and the sport diff moves torque between the left & right rear wheels. Each system steps in when it's called on, hopefully without interfering with the other systems (i.e. adding torque while simultaneously braking at the same wheel). My guess is that the sport diff added a good measure of safety, stability, and speed here.

NWS4Guy

Spot on, yes these all act, you will take off at a much more leasurely pace without the SD since ABS will slow wheelspin rather than moving the power and torque to the wheel on the pavement. They do this demo with all Audi's to mainly showcase Quattro more than anything.

Reds4Phil

Not getting SD is like a football team driving 99 yards down the field and kicking a field-goal from the 6 inch line. Audi does not do a very good job of explaining the features of SD (Comfort, Auto, Dynamic & Custom) It is more than just corners.

mellow_sparky

um - actually I wasn't asking a rhetorical question. NW's example of an icy hill was very similar to a demonsration my SA did for me. So I was really interested to know what was acting to allow the car to launch so well with one side on a low traction surface.

NWS4Guy

The difference is that in gravel, the wheels on that side will still get traction, allowing the ABS in a non-SD car to grab the spinning wheels to slow them down to match those on the pavement with grip. On a SD car, rather than slowing wheels to match, the torque in the rear is moved to the side on the pavement.

On ice, this situation is exaggerated by a total lack of any traction at all while on a hill, thus making the car literally immobile.

mellow_sparky

got it - makes sense.

justhacking

I am going to kick up a controversy by saying that it will be safer without the SD for a normal driver. The idea of the SD is to give more power to the outside back wheel in a corner so that the car will rotate faster. What that does is to induce oversteer faster and more readily. With the normal driver without proper advance driving skills it is harder to catch an oversteer, whereas you just need to back off with understeer, which is what most people will do.

NWS4Guy

I would agree if this was how the car acted all the time. Oversteer is only induced when in Dynamic mode, and if all traction nannies are off.

If you do this and get into trouble, you deserved to.

In Comfort or Auto, with ASR and ESP active, the SD will counter the typical Audi understeer, making the car neither over or understeer right to the edge of the traction limits, once these are reached, the ESP and ASR will kick in to cut throttle input and leverage ABS in every attempt to stabalize the car.

To correct your above statement:

The idea of the SD in a corner is to give more power to the outside back wheel in a corner so that the car will rotate faster.

The idea of the SD overall is to combat loss of traction on any rear wheel in any given situation be that speed during cornering, loose gravel, sand on the shoulder as you swerve to miss a deer in the road, a patch of ice, water, oil slicks, <insert your favorite traction loss scenario here>. This is accomplished by the SD monitoring -

- Steering wheel position to determine if the car is in a turn and how severe of one

- ABS to detect wheel speeds independant of each other, referencing the steering sensor to determine if a rear wheel is moving faster due to a curve, or loss of actual traction

- A Yaw sensor near the middle of the car, which measures the lateral G in order to let the SD decide how hard of a turn the car is making when referencing the other 2 sensors, further allowing decisions for if this is normal or a loss of control situation.