bltzkrg

Okay, here's one for people who know their stuff. It has been stated here that the Quattro system is 4wd and awd. Now speaking technically, I don't think either one is exactly right (and please set me straight.)

These are the qualifiers to the statement:

1. In a two wheel drive open differential car you can only have one wheel with traction and one will "freewheel". Technically "one wheel drive.

2. Two wheel drive with a lsd or locker slows/stops the "freewheel" as stated in item #1. So this leads us to LSD=1.5 wheel drive and Locker=2 wheel drive.

3. Allwheel drive vehicles use a viscous coupling or electronic gizmos to send power (rear to front) to any slipping wheel which can lead to true 4wd but it isn't all the time but rather on demand. Maybe these could be called "Rear drive with potential for any wheel." (I know the MDX is the opposite-front drive to rear)

4. 4wd vehicles with open differentials will only power two wheels at any one time. True two wheel drive.

5. 4wd vehicles with lsd or locker will power all four wheels at once (this sucks when you are trying to turn in a parking lot.) True four wheel drive.

6. Quattro Operates with the features of #3,#4,and #5 all at once (sort of.) It seems that the quattro splits the available power first front to rear. Then, if any slip is detected by the abs at any wheel, it (the abs) activates and begins to brake the slipping wheel. That in turn sends the available power to the other wheels. Process continues until traction is found. Now what do we call this? It doesn't seem to be true 4wd, it isn't all wheel drive, and it is better than any two wheel drive. Hmm...Opinions?

ryoung

... an open differential ALWAYS splits torque 50:50. An AWD car with open differentials in the front, rear, and center will transmit 25% of available torque to each corner.

You're confused about what seems to happen with open differential RWD when one wheel is on a low traction surface, such as ice. That wheel spins because it can't get the torque to the road (i.e., essentially zero torque), and with exactly the same amount of torque going to the other wheel, the car goes nowhere.

A Torsen center differential can transmit up to 70-80% of the torque to the end with the least traction. Using the ABS system to brake a spinning wheel works because then that halfshaft IS transmitting torque (to the brakes), that same amount of torque then goes to the other halfshaft, and the car moves.

bltzkrg

Also if you are in a low traction situation (ice, mud) and you have your version of AWD then you will slide sideways with the grade of the road. At least this is true when you have a 25% split of all power to each wheel- all of them in ice. Right?

ryoung

.. and I've found that you're more vulnerable to sideslip on mud/ice with all differentials locked.

I used to own a Range Rover with three locking differentials (front and rear were ARB air lockers). Even with open differentials, the RR's long travel suspension makes it more capable off-road than most other vehicles. Since it's difficult to get a wheel in the air
(zero-point-zero torque) that 25% equal split gets the vehicle thru some ridiculous situations.

The problem with a locked diff on a low traction surface is that then all torque goes to the wheel with the most traction; and since you've doubled the torque to that wheel, you can brake it loose also. More than once I've had the rear locked going uphill thru mud, both wheels spin, and the rear is then free to slide sideways wherever it want to go. With the front and rear diff's open, the stationary wheel tends to act like an anchor preventing sideslip.

Shaun

"...The torsen differential will split torque 50:50 in a no-slip condition. However, when one axle slips, the torsen diff will send more torque to the axle with more grip, in other words, it works in an exactly opposite way to a conventional diff. Torque splits of up to 80:20 are available, depending on the pitch of the worm gears. And since it is a completely mechanical device, the locking action is instantaneous and progressive..."

Pretty cool concept. The link below is to the rest of this article, which provides a great intro to different types of awd/4wd systems.

The other thing to note is that quattro does not apply the brakes. ESP will, in order to do it's thing, however.<ul><li><a href="http://www.eskimo.com/~eliot/awd.html">http://www.eskimo.com/~eliot/awd.html</a</li></ul>

bltzkrg

Thanks (Shawn and ryoung), I get it now!

bltzkrg

I owned a Jeep YJ with Detroit Lockers foint and rear. Did some amazing things with that truck but, it was the pits on the road when it snowed. this whole discussion is based on my experiences while building that one truck several different ways. That is what started my love affair with vehicles using more than one drive axle.

Thanks for the help.