Awd, X, 4matic, 4motion....

[Snake Vargas]

The man is a genius!

Thanks for the concise summation, Martin. Much better than I could ever have said! Now I'd better commit that to memory should more people ever ask..

As for more questions, if nobody else has any.. do you know how the SUV 4WD systems fit into this scheme? Are they mainly derivatives of the permanent fixed all wheel drive, torque being sent to both axles and both wheels on each axle with an even split? Or as a variation on #2, one open (permanently active?) middle diff and LSD type diffs front and rear?

And I'm always curious about the 'fake' SUV 4WD systems, such as X-drive. I believe they have only one diff in the middle, making permanent 4WD, but to limit the amount a wheel can spin in the air, they have electronic sensors and use the brakes..



Sorry if you guys wanted to ask about car 4WD systems, and this is clouding the issue..

 

[martinbo]

Hardly Snake, hardly...

In truth, I'm no engineer, what I know about AWD I've gained by a little research and a helluva lot of help from the technical manager at Subaru SA.

Snake, you're absolutely correct, AWD is only as successful as the ability of the three differentials to transfer away from from the wheels that slip to the wheels that grip.

An all-wheel drive system is nigh on useless if the central differential doesn't have some form of limited slip locking functionality. Imagine if it didn't and acted simply as a regular open differential? You've probably seen the practicality of this situation before: a car unable to make any forward progress because just one of the wheels is spinning all the engine power away furiously... now instead of mounting the diff tranversely at an axle, mount the centre diff lengthways along the driveline of the car. Front wheels start to spin on pull off and they remain spinning because the open differential has no lock-up functionality to send power rearwards. So, at the very least a centre diff must have some form of limited slip or lock-up functionality.

So, now we have a LSD in the centre, an even 50:50 torque-split and regular open diffs at each axle. Should just one wheel at each axle lose purchase, then centre diff or not - you ain't going nowhere either. This is particularly important in heavy off-roading but less so in on-road cars because of the off-roading phenomenon of cross axle traction loss (i.e. loss of traction between diagonally opposing two wheels, one on the front axle, one on the rear) is a common occurence.

So at best you'd better have another LSD at one axle such as in the case of my Forester.

Or, like in BMW's case, you can use the ABS system to lock up the spinning wheel and send a signal to the centre transfer case clutch pack that better grip is to be found at the other axle. It can be quite a violent and clattery affair with the ABS engaging and disengaging each of the wheels as they begin to spin especially in taxing (yet mild for true 4x4s) off-road conditions.

 

[Alx]

Very informative, after re-reading it several times, its still mind boggoling too me... but hey i get the generality.

 

[Centurion]

Audi are king of Pioneers when it comes to AWD and after seeing a video of the RS4 racing against a professional iceskater in a stadium, I was suprised how good grip the car had. You should really see that video clip.