powderlvr

Hello everyone,

I'm curious how Quattro works when, for instance, going up a giant speed bump like we have here at my apartment complex (see image).

It would seem that when going up onto the speed bump the front wheels are traveling further/faster than the back wheels. Are the back wheels just freewheeling (i.e. along for the ride) in that case? How does the Torsen differential handle torque distribution when starting to go up such an incline?

Anyone know?

-JT

dvs_dave

In exactly the same way as when the car goes around a corner. The front axle travels further than the rear axle which the center diff allows for, and is it's primary purpose. The only fancy thing about a Torsen diff over a normal open diff is that it's of a limited slip type.

powderlvr

So in the case of the incline does the Torsen just send equal torque to all four wheels, even though the fronts are moving faster? Or is all the torque going to the front with the rear wheels just freewheeling along? -JT

mishar

Middle differential allows 0-100% difference from axle to axle. There is more difference between axles in a sharp turn than in your scenario. Torsen differential just creates some resistance to that difference.

mishar

If there is no wheel spinning more torque goes to the faster axle, so in this case to the front axle. How much more depends on Torque Bias Ratio (TBR). I am not sure but I believe that A8 has 2:1 TBR, meaning that faster axle can get 66.6% while slower gets 33.3% of a torque.

If wheel(s) on one axle start slipping Torsen differential transfers to the other axle TBR times more torque that spinning axle can still handle. Unfortunately that means that if one axle has 0 traction and can't handle any torque, the other axle will get TBR time 0 which is still 0, so in that case Torsaen differential acts just like an opened differential. Not really off road transmission.

bmwm750

Exactly. This is why a true Limited Slip Differential (LSD) has a major advantage over Torque Biased Differentials (TBD, of which a Torsen- typically based on a Gleason design- is the most common type). Serious off road vehicles will use some form of locking differential.

tdawg183

Maybe I'm missing something here but how is the front travelling any faster than the rear when moving in the example you depicted?

Totally understand the effects in a turn but not here. If you were to take the rear wheel from the 3" to 4" mark on your ruler the front wheel will be sitting directly on the 5" (using your pic).

I get your point about the speed bump though in terms of distance traveled relative to the rear wheel. But combine in the fact that tires are forgiving, the distance difference will equate to only a few mm's, and that its only a momentary change in speed... I'd say its really not going any faster.

dvs_dave

High school maths and Pythagoras theorem will give you the answer to that. No I'm not about to teach you....

powderlvr

TDawg, the front wheels move faster upon first climbing an incline (see image here) but as the car climbs further up the incline things begin to reverse with the rear wheels traveling more quickly than the fronts.

In this image you can see that the car's axles (represented by the ends of a ruler) travel different amounts as the car begins to ascend. The rear tire has covered 3 inches while the front tire has covered over 4 inches in the same amount of time (faster).

Like you said, it's all inconsequential for a speed bump. I was just wondering how the Torsen differential handles the situation though. With the rear wheels essentially freewheeling, trying to keep up, I wondered if the Torsen would treat them as though they had no traction. But from the discussion it sounds as though they receive just as much torque as the faster-moving front wheels.

-JT

mishar

That picture with rulers fits better Humvee board, but not good for this discussion. Huge bump would more look like this and red line is obviously longer than the black one.