AWD Systems
[ November 21st, 2008 ] By: Mark Ozimek Posted in » Ramblings
I just came across an interesting video that brings up some curious points.
I would advise everyone who goes to view the video on youtube to take note of the comment, while the experiments are unbiased, the interpretation of results are quite the opposite. Even so, it is fascinating to see how different manufacturers’ implementation of putting power down to all four wheels compares in a synthetic and repeatable test like this.
AWD has always been a tricky situation to deal with. When all four wheels have similar levels of traction, things are easy, and open differentials will handle the job fine and split the torque evenly between all the wheels. But what happens when one or more wheels breaks loose? In an open differential system, all four wheels receive the same amount of torque, which will be equal to the amount of torque the spinning wheel can put to the ground (aka: not much).
The simplest way around this is to just lock the differentials when slippage is detected. This will provide the maximum amount of traction because all the wheels will be forced to spin at the same speed, but this reduces drivability a bunch if you’re trying to turn at all, since the inner tires spin slower than the outer tires in a turn, and the rear spin slightly slower ontop of that.
There are many (enough that I’d rather not take the time to list them all, since I’m sure I’d miss a few) limited slip differential technologies that bias the torque away from the spinning wheel(s) to put the power down to wheels that have traction. This is usually done with some sort of fancy gearing or internal clutches in the differetial. Most AWD systems use limited slip differentials with varying levels of success. As you can see from the video, Subaru has one of the better limited slip systems out there. I really would have liked to see how an Audi with Quattro performed in the same test, but oh well.
There is also what I consider a “lazy-man’s” limited slip system that applies brake pressure to the spinning wheels to increase the amount of torque at that wheel, thus the other wheels too. This is the method of “traction control” my S70 has. I put quotes around traction control because it requires a very responsive system for it to really be of any benefit outside of driving slowly slippery surfaces, something Volvo didn’t quite account for in the design.
Or you can just forgo driving the car with the wheels and use a giant turbine or something, but I don’t think many rally organizations allow propulsion in such a manner.
