[azwillnj]

Post your videos of xi action..

I'll start:

I did donuts in the Jaffa Shrine parking lot in Altoona, PA. (I went to school at Penn State Altoona and will be transferring to University Park in the Fall) It was really tough to overcome the xDrive, even with the DSC completely off.

[reggiecmah]

that is weakk.....

[longodj]

Is that in snow?! I've easily done better than that. Grab some cajones, stick it in first, slam on the gas and modulate the ebrake and watch the throttle. You'll be in a controlled sideways drift in no time.

Dry is a different story...haven't been able to do much (besides spinning out at autocrosses ).

[OverDrive]

Here are mine with Dunlop 3D Winter Sport tires (235/40/18's).
Enjoy.

[longodj]

That's more like it

[OverDrive]

thanx

[azwillnj]

Yeah that's much better. It was a really cold snow when I did it, it was powdery and there were only a few inches, so there was way too much traction. To give you an idea, when I first started the video, I was up around 40 mph and cut the wheel and that's all that happened. To add to it, this video was taken before I found out that simply pushing the dsc button dosnt turn it off, you need to hold it...

[TambourineMan]

Here's the Quattro versus XI one for anyone who has not seen it:

http://video.google.com/videoplay?do...8466&q=quattro

[Phila]

what the heck? That quattro looks like RWD only to me lol.

[TambourineMan]

I also have a Subaru Outback which I got first when I decided to go to AWD from FWD and was researching AWD systems. The Subaru forums have a lot more technical info on this.

Here is a good non-technical overview by one poster:

Audi's quattro system is definitely more special than most retro fitted all wheel drive systems out there. By retro-fitted I mean that the all wheel drive system has been engineered into the package as an afterthought rather than a purpose engineered from the outset AWD system (Subaru). Let's look at BMW's AWD system X-Drive for example, we all know that BMW steadfastly pursue a rear-wheel drive layout for a great many important reasons and this imparts their cars with brilliant driving dynamics and driver appeal. In order to pursue this philosophy, BMW ensure a perfect front to back weight distribution by mounting the engine well back in the engine bay. But when you wish to fit all wheel drive, this is where BMWs get ugly. Heavy componentry needs to be fitted to get drive all the way back to the front wheels - it's an exceedingly inelegant form of all wheel drive: it adds weight and compromises balance.

The biggest drawback with good all-wheel drive systems in the Audi and Subaru is the mass of the engine ahead of the front axle. This also affects driving dynamics as the nose of the car is less willing to turn in quickly and brings about the un-fun phenomenon of understeer.

Subaru use lightweight aluminium boxer engines that are short and sit very low in the engine bay thus improving the centre of gravity to alleviate this problem. Their Symmetrical All-Wheel Drive system is engineered as the whole combination of engine and drivetrain.

Another mechanism by which to get around the understeer problem (particularly in ultra sporty cars like the RS4 and STi) is to employ a rear biased all-wheel drive where more power is either automatically directed to the rear axle (RS4) or can selectively be sent aft (DCCD in STi) by the driver.

I think the following by the same poster explains the BMW versus Audi video:

Audi Torsens work on a multiplier ratio which I think is 2.3:1 . If one axle gets stuck it can only send 2.3 times more power to the rear meaning that if it takes 10 lb/ft for the front wheels to spin up in the snow you better be dammed sure that 23 lb/ft is enough to pull you out. If 23/lb ft is not enough to pull you out then you will continue to spin up the stuck wheels.

The STI electronic clutch system is WAY better as it has locking capability. It also has ways to sense which axles need the most power.

A bias ratio refers to the maximum amount of power that the torsens differential can send to the one set of wheels as a function of the traction on the other set of wheels.

If one wheel becomes unloaded or suffers limited traction the other set wheels will share the same limited traction based on the bias ratio. This underlies the main problem in using torsens differentals in limited tractive conditions. That is if one wheel completely looses traction or becomes unloaded the torque transfer to the other set of wheels (wheel) will be ZERO. Any bias ratio times ZERO is ZERO.

For example: Suppose in your torsens equipped car you get your front wheels stuck in a snow bank and your rears are on perfectly dry pavement on a sunny day. It takes 10 lb/ft to turn the front set of wheels before they spin up in the snow bank. Your rear wheels will get a maximum of 23 lb/ft probably not enough to move your car.

With the Subaru viscous couping system if you were in the same situation spinning the viscous couping up enough will induce a center differential (quote) "lock" and your engine power would mostly be diverted to the tractive wheels. This is because a viscous couping does not depend on a bias ratio. It depends on a speed difference.

Torsens are good but they aint perfect thats why pro rally cars exclusively use electronic diffs. Even most group N cars use viscous diffs. A torsen does not have locking capability... If you exceed the torsen's bias ratio you will spin up the wheels with least traction.

As for the 50mph disengagement of the AWD system, its a necessary safety feature because if you are to apply brake force to one axle at a high speed you are going to have a lot of sue-happy Americans making Audi broke.

The current generation Quattro does not use real diffs to bias side to side power. It uses an open diff that is limited by braking action.

At high speeds clamping onto a rear right wheel while turning left will loop you so fast you will think you are on a merry go around.

[baggerben]

nice video.

[Appropriate]

Audi A4's... today's Accord... with all wheel drive.

[Dubbedoutbimmer]

hey in my XI 335 coupe I can rip nasty doughnuts!! when the new speed of dark DVD comes out you will see the might xi drift/ doughnut these cars are nasty and all my stuff was dry pavement!!!