Glisse

Superswiss, from the detailed description of how the system works, it would always have been in AWD mode in the conditions you experienced on the weekend.

As to proactive vs. reactive, Audi describe the goal as:
The full description is available here (click "full text" to expand):

Audi quattro with ultra-technology ? Audi Technology Portal

The animation shows how the 2 clutches engage and disengage. It is a Torsen style system, and not hydraulic. The role of the clutches is just to isolate the propshaft and quattro gearing to reduce engine load. Audi do not mention whether there is a self-locking centre differential, nor do they mention torque splits.

The last paragraph is interesting - they point out the system is designed for manual and S tronic transmissions, as it requires a directly driven axle. Which means Audi will have a manual transmission option with quattro in the MLB2 platform, which they currently do not.

And the final point is that this is clearly described by Audi as an ULTRA system. Which means they may be replacing FWD Ultra with AWD quattro Ultra - a very smart decision if they do. Some earlier posters seem to think they were dumbing down the entire range of quattro drivetrains, but they are not doing this at all.

SFO Guy

I wonder if this is not Audi's way of getting out from paying Torsen licensing rights for a perfectly simple, proven mechanical AWD system? If it isn't broke, why are they trying to fix it, and really hiding the change in their marketing. No mention whatsoever.

SFO Guy

Further, this is from the Audi AG press release published on Audizine, specifically for the B9 A4 allroad:

"quattro all-wheel drive from Audi represents the ultimate plus in dynamics, traction, driving safety and straight-line stability. It is purely mechanical and thus operates with zero delay. quattro drive is partnered with the updated wheel-selective torque control system, which is active on any surface."

No mention of so-called Ultra-quattro. Where is this nonsense coming from?

superswiss

Audi has discontinued the use of the Torsen center differential for all new models since the RS5. The RS5 was the first model to ship with the 6th generation Quattro system with self-locking center differential and torque vectoring. If the car is equipped with the S-tronic, then it's the crown-gear self locking center differential and if the car is equipped with the Tiptronic, then it's a planetary gear version. The B9 A4 has the self-locking center differential and the higher end models in particular S and RS will continue to use self-locking center differentials. This new ultra-quattro system was just announced and will make it's debut on the Allroad, but as has been said, this ultra system may only make its way into the fuel efficient ultra series line of vehicles.

On a side note, the 6th generation Quattro system is not available with a manual transmission, that's why none of the new Quattro models (A4 and higher) are available with MT. MT is only available for the Haldex (A3 and lower) and FWD drivetrains.

audi8k

Hi, do you have this information from a reliable source? It seems difficult to find out if the B8.5 (S-tronic) models have Torsen or Crown Gear centre diff.
This blog states that B9 will use Torsen:
All-New 2016 Audi A4 Announced - YouWheel.com - Car News and Review

superswiss

Yes, the B8.5 (S-tronic) has the crown-gear with torque vectoring. That's been in Audi literature. Don't have any links handy at the moment, though. This is the first I hear that they are going back to the Torsen C. That would seem to be a step back. The crown gear is superior in every way to the Torsen C.

audi8k

In what way is it superior? I have a 2014 3.0TFSI S-tronic with the active rear sport differential. In that sence I have torque vectoring, but I don't believe there is torque vectoring using the front brakes.

I previously had a 2008 B8 (3.2FSI) (but obviously no active rear diff). I can sense a difference when the sport differential kicks in but it's not a night and day difference. I also have a problem with all descriptions of the Crown Gear differential that reports that the torque can be varied up to 85% to the back but in a following bullet states that it can "fully lock". If it could fully lock there is no reason why the torque distribution couln't go to a full 100% to the axle with grip.

superswiss

Because it is self-locking, it can send 85% of the torque to the rear axle even if the front axle has zero traction and 70% to the front if the rear axle has zero traction. This works fully mechanical on its own without intervention from traction control. It can send a 100% to either axle with the help of traction control. The torsen center differential is based on torque multiplication and its Achilles heal is that if one axle has zero traction then nothing gets sent to the other axle w/o traction control applying the brakes to the wheels on the axle with zero traction. So, basically w/o traction control the Torsen can get you stuck. This can happen on ice or in sand for example. Turning on traction control may not help in those situations either, because then it may just brake the wheels to a point where nothing is moving anymore. With the crown gear you can turn off traction control and then you have a mechanically locking center differential always sending torque to both axles at varying rates, depending on the current traction.

The crown gear is also lighter and smaller, so less rotational weight, resulting in a more responsive drivetrain and it is more rear biased 70/85 vs the Torsen's 80/80. Both diffs can send a 100% of the torque to either axle with the help of traction control, but that's a bit of a marketing gimmick, because if a 100% is sent to one axle that means traction control has to bring the other axle to a complete stop and then you are not moving anymore unless that axle is airborne or can slide on ice or another low friction surface.

The mechanical properties of the crown-gear also allows it to be better integrated with the car's electronics, which makes the 4-wheel brake based torque vectoring possible. By proportioning the brake force it applies to the inner front and rear wheels around a corner allows it to control how much torque is sent to the front and the rear when cornering, overall leading to much better and sportier driving dynamics. If combined with the sport differential, the brake based torque vectoring kicks in at higher threshold to aid with rotating the car when the sport diff cannot send more torque to the outer rear wheel on its own. At that point the inner rear wheel gets a braking torque applied which through normal diff action sends additional torque to the outer rear wheel and the inner front wheel also receives a braking torque to tuck in the nose of the car. This whole spiel allows for much faster cornering speeds, but obviously that only matters on a track or a deserted canyon road where you can explore the car's limits.

audi8k

But the statement "Because it is self-locking, it can send 85% of the torque to the rear axle even if the front axle has zero traction" is a contradiction. If one axle has zero traction (zero torque) and the other axle provides torque this means 100% and not 85%.

(I'm familiar with the rest of your text except some typos .. The rear bias is reported to be about the same for both, 40/60)

superswiss

No, it's not. That just means 15% of the torque is sent to the front wheels but they are just spinning. In order to send that additional 15% of the torque to the rear, traction control has to brake the front wheels. A locking center differential that always sends torque to both axles is superior in low traction situations. That's why offroad vehicles use 4WD and locking differentials so you can force torque to all 4 wheels at all times.

This is a good video showing the new diff in action. Notice how the wheels on the rollers just gradually slow down as the clutch-pack locks up the center diff and more and more torque gets sent to the axle with more traction, but the spinning wheels never completely come to a stop. Traction control doesn't step in until the end where they put three wheels on rollers and then EDL needs to help in order to send torque to the one front wheel that has traction.


Was looking for this video for comparison. This is the classic BMW goof test showing how the torsen quattro is ineffective w/o traction control. They basically disabled traction control to make xdrive look superior.