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BMW: All 4 wheels spin
https://youtu.be/iSj9UEVYI8o?t=2m1s
Forester: All 4 wheel spin
https://youtu.be/iSj9UEVYI8o?t=2m28s (obviously with too much throttle)
CX-5: Back wheels don't spin, front wheels do
https://youtu.be/iSj9UEVYI8o?t=2m8s
Yes, all 4 wheels spin at 3:34, but why not at 2:08? Looks like the driver starts with really light throttle then increases power gradually, some torque is being transferred to the rear, but not enough to spin.
What is happening? Does the system protect the clutch in this case?
In that scenario the AWD clutch was likely completely locked (no slipping).
The first run the driver left the traction control on. This reduces engine power upon detecting excessive spinning. It is the same thing as reducing how far the accelerator is depressed. If this reduction in power is not enough for the front wheels to regain traction, the back wheels will be getting very little torque because an open differential ALWAYS transfers the torque 50/50. If the front wheels have very little traction (spinning) then not much torque is being applied to the rear wheels. That is how open differentials are designed to work. However, applying the front brakes will automatically transfer whatever power is consumed by the front brakes (still spinning) to the rear wheels. I don't pretend to know for sure why the computer did not apply the front brakes in this video but I suspect it was due to two factors, lack of off-road driving experience and temperatures well above freezing. I'll explain shortly.
Two days ago the driveway to my ski cabin was a solid sheet of wet ice (temperature about 38 degrees). This driveway starts level, becomes VERY steep and then becomes almost level at the top. I drove up it as usual, no slipping, and came to a stop at the top. Thinking about this thread, instead of parking I backed back down to the steepest portion and tried to stop there. But the driveway has a slight slope to the left. All four tires started slipping (because I was using the brakes gently to try to stop on the steepest portion) and I started sliding off towards the edge of the driveway. I immediately released the brakes and continued backing to the bottom. Then I tried to climb the steepest portion at a crawl. I felt the front wheels start to slip and power to the engine was reduced. So, without backing down, I turned TCS off and reapplied power. All four wheels started spinning and the front of the car started to rotate sideways towards the left edge of the driveway necessitating a manual reduction in throttle (because I no longer had TCS). I backed down enough to straighten the car, stopped, and using the two foot throttle/brake technique, applied power without releasing the brake (which had light-moderate pressure on it). The car climbed right up that sheet of wet ice with very minimal wheel slip and amazing acceleration (considering the slickness of wet ice). Then I backed down again and turned TCS back on (the default state). I drove forward and stopped on the steepest portion. I repeated the previous run (but this time with TCS). This was just as successful as the run with TCS off, probably because there was not enough wheel slippage to differentiate the two.
The lesson here is, if you're on a steep low-traction surface and you can't make forward progress, use the two-footed brake/throttle technique. This technique largely eliminates the need to turn the TCS off and, unless you are on a surface where you need tire spin in order to dig down to a higher traction layer, it is best to leave it on. I will say, the two footed technique is only necessary in unusual or very extreme situations and, as much as I am driving in the mountains during all kinds of nasty conditions, I still have to invent scenarios extreme enough to need to do this. In all my regular driving, the car has always been surefooted and had the forward drive to get out of snowed in parking spots, up icy hills, etc. It's only when I purposefully stop on a particularly treacherous spot that I have ever found it beneficial or necessary to use the two-footed technique. The car is simply that sure footed. The AWD programming is very complex and Mazda does not release the specific parameters that trigger specific behavior because it is proprietary to Mazda. But they do disclose that the AWD module takes into consideration such things as steering wheel angle, ambient temperature, vehicle speed, brake application and even whether the windshield wipers are on/off. The complete list is much longer.
One thing I have learned from decades of off-roading is most beginner-intermediate off-roaders get themselves in trouble from one thing in particular. And that is sliding sideways on an off-camber (sloping sideways) portion of a steep/slippery trail due to excessive spinning of the wheels. This is what TCS is particularly good at preventing. So I recommend keeping TCS on unless you are an expert and know what you're doing. There are times wheel spin can help you make forward progress but they are few and far between and you need to insure there is enough available traction to over-come any unfavorable camber to the trails surface. The two-footed brake/throttle technique should be all that's necessary.
Of course the traction provided by your tires are the final arbiter of whether you slide off the trail or not.
Today I was once again delayed on the State Hwy. to the Ski Area about 20 minutes even though the road was freshly plowed and there was only 1/4" of fresh, wet snow on the well-maintained road. An idiot in a Subaru Forester thought he didn't need winter tires because he had AWD and failed to make forward progress after a steep hairpin turn. A steady string of downhill traffic prevented those immediately behind from attempting a pass. I don't know what it will take people to learn that AWD is not a substitute for winter tires!
