Any update???? Never seen anything like that in the video

 

My two cents... My friend has a really steep driveway, so I always put the parking brake on in my car. Never rolled down the hill, even when I slipped trying to get parked off the sidewalk. I've never left my car on his driveway without the parking brake engaged. That being said it looks like the rear wheels are spinning within the diff, not at the transmission.

 

I’m wondering if you did further digging on this, as one year ago my former wife may have been run over and killed by her own car at the bottom of her driveway with a fairly steep incline. I am an engineer who studies failure modes under actual use, and I want to be clear that I do NOT think there is anything completely unique to your car manufacturer or model. There is likely nothing “wrong” with your car, other than that the behavior is likely “as designed,” but without the designers having been fully cognizant of the following failure mode.



The main reason we have not seen it as much in the past may have been due to the more recent adoption of AWD across many males and brands, and the “normal” behavior of full-time AWD drivetrain using three differentials to spread power between all four wheels. When you place your car in “park,” the driveshaft coming from your engine is locked, but the wheels themselves are not locked with respect to the ground, by design of the differential. There is a direct formula. If you put the AWD car up on a Jack in park and spin one wheel, other wheel(s) will spin in the counter direction.



I believe the failure mode looks something like this. After driving around, the car, brakes, wheels, and tires heat up. You park on an incline and the car stays put for a few seconds, minute, or whatever, but the tires pack and heat the slush, snow, underneath all 4 wheels. With the drivetrain as described above, the tire with the lowest static friction, whichever that may be (it prob won’t be a front tire, as that has significantly more weight from the engine) breaks loose from the static friction and begins to spin in the direction opposite the car’s downward movement along the incline. Why? Because the other 3 wheels/ tires have NOT broken free from static friction (recall my statement that one wheel will always have the lowest friction, just by luck of the draw) and the 3 wheels may now move freely with the direction of car motion while the one tire that broke lose spins in the counter direction, just as you so beautifully show in your video.



An additional irony to the matter is that once a single tire breaks free from its initial lowest static friction (compared to the others), it is now spinning under dynamic friction, which is significantly lower than the initial static friction was. After initial break-away, the car’s prospect become worse, not better, fueled by momentum as the car increases speed down the incline.



So there, I have tried to explain this behavior in a nutshell, and I believe this has happened and is continuing to happen to many more people than the automakers would ever want to admit, as it is by design of full-time AWD vehicles with the above-mentioned differentials. For my former wife, it was just a matter of some additional bad luck of having gone down to the bottom of her driveway to tend to her garbage cans, and then being struck and run over by the car rolling in relative silence over a fresh snow, with her back turned.



At least one car company, Audi, who pioneered this technology, I have been told applies brakes to all four wheels when the car is placed in park. I’m betting there haven’t been enough law suits yet for the other car companies to see that this is a dangerous situation, even though the safest way around this, and the way they will all say should be followed, is to always apply your parking brake on ANY incline, rather than relying on “park,” which on most makes cannot overcome this serious failure mode.