NWS4Guy

I thought we could use a centralized thread with this topic since there seems to not only be a lot of minsunderstanding about what it is, but also what it does, when it does it, and HOW it does it.

I'll lead off with a nice video explaining in good detail, the inner workings along with see through vision into the Sport Diff as it works. Remember the Sport Diff is only on the rear wheels only, and doesn't affect the front wheels, likewise, the S4 is a 40/60 power split between front/rear wheels, lending a heavier bias to power at the rear. This coupled with the Sport Diff very effectively combats the known understeer issue with Audi's of the past.


http://www.youtube.com/watch?v=rQowh2Kr38s



Further, here is some more misc. info:

Yes the Sport Diff is new for Audi, no it's not new in the auto world, it is very similar in form and function to Acura's SH-AWD, and Mistu's S-AWC. Sure it helps in a sporty situation, but is also helps when in any sort of scenario where tracion is an issue:
- ice/snow
- sand/gravel
- high speed cornering
- emergency lane change
- even when there is no throttle input or when braking

This makes the sport diff not only a performance feature, but also one of safety, leveraging the existing torque created both by the engine via the transmission, but also through the kinetic enegry and inertia of the car itself in motion.

This system is unlike an ABS system which uses the brakes to slow any wheel losing traction, instead, torque is removed from the wheel losing traction, or in some cases even one which is expected to lose traction, like the inside rear wheel on a tight curve at higher speeds, and that extra removed torque is applied to the opposite wheel. This prevents the loss of speed in a slippage situation, as ABS never attempts to slow the wheels unless the car is pushed past the same limits of what overall traction can cope with (which is when you would want to slow down in any case).

Other uses of this tech:
Audi has stated this will be availible on the Q5 coming soon, lending credence to the fact that this is as much a safety feature for traction in more of an offroad/extreme elements situation as it is a performance feature.

Also of note, the new RS5 announced at the Geneva Auto Show recently will also have this in both the rear diff, as well as a new version of this technology in the central diff. This will allow not only torque vectoring between the rear wheels, but also to adjust the front/rear torque bias, rather than being held to a constant 40/60 split.

Feel free to add questions, comments, and observations as you see fit!

koffeetalk

My S4 with sport diff should be here within two weeks! Thanks for the informative post. I can't wait to be driving it every day.

It looks like the RS5 is using the center diff for splitting power front and rear but using ABS for torque vectoring? Or maybe I'm misunderstanding that in this video: http://www.fourtitude.com/news/publi...cle_5717.shtml

NWS4Guy

ABS cannot torqur vector, it can only slow a wheel spinning.

rktskicar

2nd. It looks it is using braking for torque vectoring form the video. What's different is that the S4 has a torgue vectoring rear diff, but not a front one. This system uses both axles for left to right torque vectoring.

Bruce

koffeetalk

Yes, the video is pretty clear that braking is used for torque vectoring on both front and rear axles on the RS5. So what does that mean for our S4 sport differential? Is it limited to applications below a certain horsepower?

What system is used for torque vectoring on the R8?

rktskicar

As NWS4Guy pointed out, the braking torque vectoring system is lossy. The torque ratio between left and right can be adjusted by braking, but the braking torque on the braking wheel is lost (i.e. lossy). The S4 rear diff is much more effective in terms of preserving total torque (not as lossy as braking method)

I don't think the S4 sport diff is limited by HP. Overall, the braking torque vectoring system is much cheaper than mechanical front/rear diffs as it requires only a SW change.

Bruce

koffeetalk

Makes sense. I also like that the S4 sport differential is effective even when no power is being applied (i.e. coasting). Agree that the brake-based system is lossy. And it seems it would require more frequent brake maintenance!

MattPec

Great post NWS4Guy! Given the passive benefits of the sport diff this is clearly as much a saftey device as a fun toy. I can see it coming into play quite often while braking or coasting as much or more than accelerating (at least in real world/city driving).

Video is great, too, though would be more satisfying if they guy had a German accent...

NWS4Guy

For the RS5, they are using the brakes to slow the inner wheel in a turn proactively, this could be due to the new "crown gear" center diff, which is basically applying the Sport Diff tech to the central diff to balance front to rear. I'm not sure how exactly or why they use this in the RS5, but the S4 only employs braking when ESP needs to kick in ti maintain vehicle control at the ragged edge. In other spirited driving and/or slippage conditions, just the brake packs inside the Sport Diff activate in order to adjust the torque accordinly.

The S4 Sport Diff uses information from 3 sensors:
- ABS sensors to monitor wheel speeds
- Steering sensor to note angle of the front wheels and if you are in a turn or sliding/drifting
- a Yaw/Lateral G sensor to determine how much lateral G is being experienced

Taking the data from all 3 of these, the Sport Diff is able to determine if you are losing traction to a wheel due to a high speed right hand cornering maneuver (high lateral G, the steering wheel is turned to the right, and the right rear wheel is starting to slip, or even is expected to slip due to the speeds the ABS shows the car travelling at, and the Sport Diff calculating how much weight is coming off that tire), or if you hit some sand or ice (steering wheel is straight, no lateral G's but a wheel is slipping).

Until some more details come out on the RS5 I am not really certain how they have built on top of the current Sport Diff implementation, but it appears that they use ABS to enhance the effect of torque vectoring by preemptively slowing the inside wheel before it spins, to exaggerate the extra push of the outside wheel in a corner. This would certainly give an even higher sense of "rear wheel steering" which you can feel with the S4 currently in a high speed sharp turn.

hk007

NWS4Guy -- what would be the proper technique to take advantage Sport Diff on high speed turn? Anything different? When would you slow down? push gas? does sport diff only work when you are pushing gas on the turn?