# Impressive all wheel drive slip test performance



## JML (Jul 26, 2018)

The people at TFL do slip tests where they put a car on sections of industrial conveyor belt rollers, and see if the car can pull itself off. This simulates having a wheel on ice, or such. They're just down the road from me, and since I got my car I'd been debating contacting them about using it to do a slip test; mostly to satisfy my own curiosity. I never got around to it, and they bought their own Model 3 dual motor, and finally did the slip test. This is the video; skip to the 2 minute mark to see it. The test only takes a few minutes. The other 25 minutes of the video is the aftermath of backing the car into a wall and needing $6000 of repair.

Anyway, the Tesla seems pretty impressive to me. With the front wheels on the rollers, the rears have no trouble pushing it off. With the rear wheels on the rollers, the rear tires spin a bit, but then the fronts pull the car down. They do a cross test with one front wheel, and the opposite side rear wheel on the rollers. A 4 wheel drive vehicle with no differential lockers or traction control that brakes the spinning wheels will do very poorly at the cross test. The Tesla pulls itself off with no problem. Then they do both front tires and one rear on the rollers, and the single rear tire with traction can move the car. Finally, a single front tire with traction is able to move the car.

It seems like the traction control works extremely well, and is able to brake the spinning wheel, so power goes to the ones with traction. Of course, power is sent to the front and rear completely separately, so loss of traction at one end won't affect the other. They've tested other cars in the past, and some of them have lots of trouble with the tougher tests.


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## BostonPilot (Aug 14, 2018)

Coming from a Subaru STi (which has 3 limited slip differentials) I was a little surprised at how rear biased the torque was on the test. Made me wonder in normal snow driving how awd the car really is, vs just for getting unstuck... The STi with it's LSDs is fantastic for bombing around in the snow... Newer awd cars aren't always that great - you can see in these videos that it takes the computer a while to decide to limit the spin... When that happens and you're driving (as opposed to trying to get unstuck) you feel that as a sudden change... almost like hitting a dry spot on the pavement... And sure enough I've felt that when drifting the car on a highway offramp.

We didn't have a lot of snow this year, but enough that I got to play a little. The M3 wasn't great when reaching the limits of traction until I turned on track mode. Then it got a lot better. It seemed kind of weird to hear the chiller running when I turned track mode on, when the outside temperature was below freezing! I guess it still tries to get it to a lower set point, because for all it knows you are about to try some hot laps on a track.

As much as I like track mode, I'd still like to see the Performance M3 allow you to totally turn off traction and stability systems...

If someone comes out with an LSD kit for the car I'll buy it! (but that won't happen).


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## SalisburySam (Jun 6, 2018)

Very interesting slip test @JML, thanks for sharing this. The dual motor car acquitted itself very well indeed. I watched the first 10 minutes up until the "Next Day" banner...couldn't go further. The pace of this video is painfully slow...good demonstration for the value of a competent film editor though.


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## Dr. J (Sep 1, 2017)

SalisburySam said:


> Very interesting slip test @JML, thanks for sharing this. The dual motor car acquitted itself very well indeed. I watched the first 10 minutes up until the "Next Day" banner...couldn't go further. The pace of this video is painfully slow...good demonstration for the value of a competent film editor though.


You should watch (part of) the second half--he crashes the car, then has an adventure with the body shop. At around 20:00, a service technician comes out and gives his unbiased opinion of Tesla cars. It's quite informative. But unfortunately off-topic for this thread.


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## JML (Jul 26, 2018)

BostonPilot said:


> Coming from a Subaru STi (which has 3 limited slip differentials) I was a little surprised at how rear biased the torque was on the test. Made me wonder in normal snow driving how awd the car really is, vs just for getting unstuck... The STi with it's LSDs is fantastic for bombing around in the snow... Newer awd cars aren't always that great - you can see in these videos that it takes the computer a while to decide to limit the spin... When that happens and you're driving (as opposed to trying to get unstuck) you feel that as a sudden change... almost like hitting a dry spot on the pavement... And sure enough I've felt that when drifting the car on a highway offramp.


I think with the dual motor it is more that when going gentle on the throttle only the rear motor is activated. Once the computer figures out the rear wheels are slipping, then it powers on the front motor. I think going deep enough into the throttle to activate both motors immediately might generate enough torque to overcome the rollors' static friction, and just launch the car. Probably not a good idea for starting in real world slippery conditions. 4WD modes where power was equally generated by both motors would be good for offroading.

The other cars they've tested with electronically controlled all wheel drive systems behave similarly, in that the primary drive wheels initially spin, and then it takes some time for power to go to the other wheels. Some of them never quite figure it out, and just aren't able to get much torque to the wheel with traction in the most difficult tests.



BostonPilot said:


> We didn't have a lot of snow this year, but enough that I got to play a little. The M3 wasn't great when reaching the limits of traction until I turned on track mode. Then it got a lot better. It seemed kind of weird to hear the chiller running when I turned track mode on, when the outside temperature was below freezing! I guess it still tries to get it to a lower set point, because for all it knows you are about to try some hot laps on a track.


In real world snow performance my dual motor did pretty well. Just using the base all season tires, starting from an icy intersection was always drama free. Driving on snow was also fine. I bit of throttle on a turn could break the rear loose, but then the computer would bring things under control quickly. So, for somebody behaving sanely in real world winter conditions the stock setup is pretty good. Lapping an ice track, it would probably be awfully boring.


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