What is the difference between the S-tronic and normal differential?

[ImpartialFocus]

I did a little research on the S-tronic and the engineering behind it. It is nice that it will provide more (additional) power to the appropriate wheels in a turn vs simply 'braking' one over the other. I had an A6 for a few years before moving to the S6, and the cars are both quite different from one another. I am starting to 'push' the limits (for me) on the S6 finally, but now with snow tires have backed off a bit to keep from chewing those up. Here's the question.
In the A6 I could definitely tell the understeer on corners when pushed hard. With the Sport Differential in the S6, does it play a significant role in reducing the understeer? I have noticed how well it feels on longer corners (270 degree on ramps) with a little push of the accelerator, but was curious if there is significant data on the understeer.

[superswiss]

S-tronic is the brand name for Audi's dual clutch transmissions. It has nothing to do with the rear differential. Just thought to clear that up first. The sport differential is a separate component and is located on the rear axle, independent of the transmission. The sport differential is an active differential, that can overdrive one side by up to 10% versus the regular open differential. This allows the car's electronics to do a few things. First off, the most prominent feature of the sport diff is its ability to rotate the car and hence reduce understeer. Depending on the speed you carry through a turn, the steering angle and what the yaw sensor reports, the sport differential is directing more torque to the outer wheel in order to get you around the corner w/o understeer to the point of inducing oversteer, depending on the car. The S6 is not that easy to oversteer, but I can power slide around a hairpin in the RS5. This is just one aspect of the sport differential, though. It also acts as an inverse ESP. If you are hitting a slippery road it redirects torque as necessary from side to side in order to keep the car going in the intended direction. It does this long before the traditional ESP needs to intervene with brake application and allows the ESP to have higher thresholds than on cars w/o the sport differential. Another thing the sport diff does is counteract load reversals. For example taking the foot off the throttle in a turn or stepping on the brakes normally upsets a car's chassis and can destabilize it. The sport differential within its limits attempts to counteract these forces by strategically shifting torque to either the left or right rear wheel. That's a quick summary.

The S-tronic just to bring that back, introduced an all new center differential called the crown-gear center differential. It further improves quattro and allows for 4-wheel torque vectoring using the brakes in addition to the active torque vectoring provided by the sport differential.

Here's a YouTube video that explains the sport differential.

https://youtu.be/rQowh2Kr38s

[ImpartialFocus]

Ugh. I had messed up the original thread topic. I saw the video, hence the reason for my question. How much difference does it make in understeer around a tight corner compared to a normal differential. Just curious if anyone has seen a big difference.

[superswiss]

Driven correctly, you should have very little understeer if any until you get to the limit of tire grip and then it should brake away all 4 tires instead of just the front. A non sport diff equipped car will push over the front tires and once you are understeering, letting off the throttle is the only way to regain control.

Driving a sport diff equipped car correctly is key. If you've ever owned an RWD car, then you should be familiar with throttle steering. A sport diff equipped Audi behaves very much like an RWD car. The more throttle you give it around a turn, the tighter the turning radius gets and if you let off the throttle the turning radius increases. You have to fight your instinct initially. If you feel the car understeer, you will instinctively let off the throttle, but that's the wrong action. Instead you have to give it more throttle to let the sport diff correct the understeer and push the car into the corner.

EDIT: Also, I should mention that this all depends on what Drive Select mode you are in. You get the most rotation/least understeer if you have the sport differential in dynamic mode.

Here's an article (a bit older, but still valid) that explains in some detail how the various modes affect things and how the sport differential works in conjunction with the other subsystems. http://www.audiworld.com/articles/qu...ism-redefined/

[GolfGL]

Nice explanation superswiss!

For winter driving, I've found the best setting to be Individual Mode set to Dynamic Sport Diff, Comfort Engine and Comfort Suspension.

[superswiss]

Sport differential in comfort mode would actually be the preferred setting for driving on the slippery stuff as in that mode it emphasizes stability and its ESP aspects.

[ImpartialFocus]

Superswiss,
Thank you! That was exactly what I was looking for. That explains so many engineering details I couldn't otherwise get. This makes me love this car even more.