Found an interesting video talking about Audi's new diff -

http://www.iamaudi.com/2009-audi-s4-...ial-explained/

Just curious, most reviewers rave about it, and the S4 has been killing in reviews and on the track - seems this diff has a very mechanical driving feel to it as well - in addition to Audi's rear biased quattro...

discuss?!

vs. a traditional mechanic also - let's say a cusco 1.5 type RS - since I have one in my car now =D but Gates' X is running one too -
Isn't the only real advantage for active diffs opposed to mechanics is that sometimes they give more or less torque depending on the input, and some can transfer more depending on the strength? While the mechanical delivers a constant amount from side to side?

edit: basically, both active diffs are exactly the same is the answer.

 

The audi diff is exactly the same as the evo x ayc diff. It just uses hydraulic clutches and speed increasing gears, literally the exact same thing.

A traditional differential like the cusco can only transfer up to half the torque to either wheel netting no yaw moment. A differential like the torsen (mechanical) can bias torque to one side and create a yaw moment. However it does this by giving power to the slower turning wheel (assuming this has more grip) which isn't always the correct thing to do. An active diff is basically just a smart torsen that removes the torsen's shortcomings by using sensors.

 

I should also state that the "real" advantage of an active diff is it allows higher lateral g's through a corner because it can combat/cancel the yaw (direction of rotation is out of the corner) created by the dynamic vehicle conditions (bad polar moment of inertia and cg contribute to this).

Not to suddenly turn violent, but I don't give two ***** what everyone says about ayc. OOooohhh racecar drivers turn faster laps without it. Well you know what? That's because they can't adapt their driving style to accommodate active yaw control. The physics and testing on cars with and without ayc show you can run a lower skidpad time, and get wot earlier in any corner exit than you can with a traditional lsd or even a torsen (torsen fails on the skid pad).

 

So when you say a torsen biases the torque to one side, how is that different from the cusco transferring half the torque - are you saying the torsen biases 100% (or slightly less) to one side? Just that the assumption of the torsen may be wrong? But the Cusco biases based on which tire has more grip, correct?

It is true that the goal of traditional LSDs is to help drivers get on the gas as early as possible in the corners - the type RS uses springs that supposedly help for this, and Carbonetics claims their rear diffs allow you to WOT even quicker than most, though who knows if this is true.

I dont know what to say otherwise - all the center diff reprogrammings in japan turn off ayc function - testing shows you should theoretically get on the gas quicker with an active rear differential, but from what I've seen it seems like its more of an error correction/easy mode than full on race function. Though, for rallying it helps immensely for all the inputs that the driver can't really account for and do anything differently about. For track cars mechanical is still king - if not for anything other than robustness.

Overall then, the explicable advantage of active differentials is that it can transfer more torque to one side while analyzing which side is the "correct" side to transfer torque to. I mean the difference/advantage should be there.. honestly I'm just going by what the 9's overseas are doing, can't really compare with the X or any other car. I can't ever put a X's 4wd system onto the IX so I don't really worry about what the X is doing, just what the best option for the IX is - and it's to go mechanical.

One note though, in rallying drivers did adapt to active differentials - they actually weren't any faster, and according to one driver, their driving style was ruined (can't find the article =[)

 

Couldn't the car be a little too rwd-ish, considering the awd split is like 30/70?

 

1. In a torsen you actually set the bias ratio. I've seen 4:1 before. If you look up "torque biasing" differentials (aka the torsen) you will see that they explicitly state that they will help you "rotate" the car aka control yaw mechanically. The difference is that the cusco can never give more than 50 percent of the torque to one wheel while the torsen can give almost 100 percent depending on the bias ratio. The cusco literally is nothing more than a locked diff allowed to slip slightly.

2. I would die to get a good answer as to why everyone keeps pulling their ayc diffs and turning them off. Reliability is the only thing I can come up with... other than that the car will understeer during transient simply because the engine is outside the front axle... which ayc is supposed to bandaid.

3. As far as your 30/70 comment... i remember reading a paper by mistubishi stating they found 30/70 was the optimum split for track times but 50/50 was the safest for the street. Hence we have 50/50... sti's have 60/40... and our insurance rates are lower for that exact reason

 

I also thought from the commercials I have seen about the audi that the active is only on the rear wheels. I thought the X's was at both axles. Please correct me if I am wrong.

 

Ohh that's right - yea I've seen torsens that are like 3:1

are you saying that when traditional limited slips can't move more than 50% of the torque, are you saying that up to 50% each wheel, i.e., they can go 75% to one wheel 25% to the other, or that they are simply locked at 50:50 like an open diff and just lock at appropriate times which in essence transfers the torque?

These theories are good and all, but the results are the results - one is reliability, amount of power it can handle etc. There is very slight lag with the system, not that it matters on more open courses. I don't know off of experience but it was said that the Gates car understeered less with the addition of the cusco 1.5 after pulling the AYC. Don't know why but that's just the result.

Interesting find on the paper - definitely post if you ever find it up - it's interesting because over in the States it's spoken of like a car can't ever handle as well as a car with an active diff, but in Japan the STIs are literally just as fast as the evos using a more rear biased awd system compared to evos 50:50 with active rear

great posts btw

Thinking about why most of the cars out running are using mechanicals.. perhaps the ayc can't yet handle as much power - also it's a much more complicated system, less reliable etc.? Mechanicals are more stable by the very definition that they aren't varying the amount of torque that goes to either wheel.. ie, with the ayc, you go in too hot, and it adjusts/provides more torque or w/e, you go in too slowly and it provides more or less accordingly, so how does a driver hone the sense to figure out when he's traveling the optimum line? If it's always fixing/making minor corrections, wouldn't a driver's sense be dulled as to what he may be doing wrong? Kind of similar to the argument about using r compounds

 

Xs active in the rear and "active" in the center in that it varies the lock, but not the actual torque split

 

STIs don't have a 60/40 front rear, you have it backwards. The 04 & 05 STIs have the 40/60 front/rear split. From 2006 and up the split is 41/59 front/rear, with no ayc or torque vectoring.

 

sorry i didn't mean it that way... just a brain fart. To be honest I thought the 04's had 35/65 and they went 40/60 for 05. I'm almost positive actually.

 

04/05 STi had a 35/65 split. 06+ went to a 41/59 split.

 

Actually you're right. Even as I read my own post you quoted I was thinking WTF, why did I write that, only to see that you were actually in the process of correcting me. Thanks for the heads up. Guess I may be a little gassy in the also.


…and finally, we have a winner. These are the correct numbers. I'd also like to add that stis stock torque split f/r is 50/50 until you start adjusting the dccd which will take it from the 50/50 up to those numbers posted. So for example it can be set to 45f and 55r.

 

This is actually incorrect. The center planetary diff is set at 41/59 when open. By open i mean, when the electronic dccd is switched to open so it does not transfer power to the front wheels. When you lock the dccd, it electronically transfers the 41/59 to the maximum 50/50. So when you have the dccd turned off, the mechanical ratio is 41/59. fwiw

 

This is rediculous. Center differentials CAN NOT change the torque split unless they have gear reductions systems on the side controlled by hydraulic clutches (rear diff on evo x) The torque split is SET by the difference in radius of the front to rear gears. The center differential only controls the differential of front and rear wheel spin. If the diff is open and the rear wheels spin they keep spinning... oversteer. If the diff is locked and the rear wheels spin they will stop spinning unless the fronts are spinning too.

This god damn center rotator knob is not magic... it's not like active yaw control, it does not change the torque split ever.... ever.