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Tesla in Park suddenly moved forward [air temp 33, snow on ground, parked on incline, OEM tires]

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It's relatively easy to put the Model 3 in Park and neglect to press the stalk button long enough to engage the parking brake. I'm not saying this is what happened...but a failure to engage the parking brake could contribute to the observed behavior.
Parking brake was definitely engaged - as noted above, when I saw it slide the second time I visually confirmed that the rear tire was not turning and that front tire was, which is consistent with a parking brake on the rear wheels only. I also posted a picture showing the slide pattern left by the rear tire.
 
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Parking brake was definitely engaged - as noted above, when I saw it slide the second time I visually confirmed that the rear tire was not turning and that front tire was, which is consistent with a parking brake on the rear wheels only. I also posted a picture showing the slide pattern left by the rear tire.
Good to know.

There are a few others in the thread with similar observations.

Someone "disliked" my post, yet did not respond to the discussion. Is something wrong with emphasizing a feature that can be easily overlooked?
 
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In my situation, waiting in the car would have done nothing - the car began to slide 4 minutes after everybody exited the vehicle and we were all well in the house by that time. I have exact times because I caught the whole thing on my Nest camera (I went back and checked to see if I had done something wrong). And my driveway is not particularly steep - I would estimate a 3% grade at most. It's in a suburb of a very flat portion of Ontario (Toronto), it's not hill country.

I'm not trying to suggest you did anything wrong, but it sounds like the car was working fine and the tires just suck. After driving the car through snow a few times on the stock tires, I picked up some studded snows. The stock tires are oddly quite good in cold weather, but snow/ice traction is terrible even for all seasons. I wouldn't be opposed to having all 4 tires lock, but I'm not convinced that's a solution in this case.
 
I'm not trying to suggest you did anything wrong, but it sounds like the car was working fine and the tires just suck. After driving the car through snow a few times on the stock tires, I picked up some studded snows. The stock tires are oddly quite good in cold weather, but snow/ice traction is terrible even for all seasons. I wouldn't be opposed to having all 4 tires lock, but I'm not convinced that's a solution in this case.
No worries, I didn't think you were suggesting that - I was just giving more background on the entire situation because it was quite a surprising one for me (which is why I went searching for forum posts about this topic and found this one). I don't think there are any faulty components in my car based on what I've read - but I do think there is space for improvement and given Tesla's track record of software improvements I believe this is fixable in some manner.
 
Model 3 engages the parking brake when you go into Park. You can hear it engaging. That's all park really is (there is no transmission interlock like on an ICE). If you press and hold the park button again, you can hear the parking brake tighten down even further. I'm not sure why it has that feature. Maybe if you are on a very steep slope on dry pavement the second application might prevent the rear wheels from rolling due to insufficient brake pad pressure. Regardless, I think this is all tangential to the original post which is related to the rear wheels sliding in the snow/ice.
 
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I picked up some studded snows.

Personally, I would not trust the traction of two studded snow tires to keep the car parked on a steep hill in freezing rain or snow near the freezing point.

I saw my dad skate up our driveway in Oregon with tungsten carbide studs, leaving gouges in the concrete all the way up, too many times to trust them as a solution to every condition. If the conditions are bad enough, they have to get through to pavement to work.

FWIW:
Model 3 Owner's Manual Page 63 said:
Warning: In snowy or icy conditions the rear wheels may not have sufficient traction to prevent Model 3 from sliding down a slope, particularly if not using winter tires. Avoid parking on hills in snowy or icy conditions. You are always responsible for parking safely.

Warning: Your Model 3 may display an alert if the road is too steep to safely park on, or if the parking brakes are not properly engaged. These alerts are for guidance purposes only and are not a substitute for the driver’s judgment of safe parking conditions, including specific road or weather conditions. Do not depend on these alerts to determine whether or not it is safe to park at any location. You are always responsible for parking safely.
 
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No worries, I didn't think you were suggesting that - I was just giving more background on the entire situation because it was quite a surprising one for me (which is why I went searching for forum posts about this topic and found this one). I don't think there are any faulty components in my car based on what I've read - but I do think there is space for improvement and given Tesla's track record of software improvements I believe this is fixable in some manner.

It's possible.

But best I can tell based on what systems appear to need to be on for almost anything, it sounds unlikely. Most likely the cars electronics were designed around being massively integrated, so it appears they consolidated everything they could with the minimum number of power rails on the boards and no way to isolate power from sections or functions unless required. So basically, half the car needs to be on for the doors to open.
 
Personally, I would not trust the traction of two studded snow tires on a steep hill in freezing rain or snow near the freezing point.

I saw my dad skate up our driveway in Oregon with tungsten carbide studs, leaving gouges in the concrete all the way up, too many times to trust them as a solution to every condition. If the conditions are bad enough, they have to get through to pavement to work.

Oh no doubt. I get nervous parking on any real slope in bad weather. I always turn the wheel and consider where the vehicle may end up. So far my epoxy floor appears to be tolerating the studs better than expected.

The difference from the stock MXM4s to Hakka 9's is quite staggering.
 
If it's not an issue with tire traction, my suspicion is that a small layer of ice is freezing on the pads, so they stop the car when they initially actuate, but after however many seconds the ice melts after absorbing enough heat from the rotors and the car rolls downhill.

I think someone should (safely!) try spraying the brake pads with a bit of water and then parking their car on an incline to see if they can replicate this problem.They may also need to brake enough to warm up the rotors, and will need to continue to spray the pads only until they see them ice up. Checking the rotor/pads with an IR thermometer after a drive should help with finding a good temperature/time range to try the experiment.

Another way to check could be to switch creep on. If you can't replicate the problem at all with creep on that suggests it's associated with one-pedal driving using the brakes a lot less in specific cold/wet environments.
 
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Nokian Hakkapeliitta 9's are studded. Quite loud and trash in dry weather, but the trade-off is they're good in snow and about as good as (sometimes) street legal on ice. I'd vote Hakkapeliitta R3's for most people, although some complain they're loud.
 
If it's not an issue with tire traction, my suspicion is that a small layer of ice is freezing on the pads, so they stop the car when they initially actuate, but after however many seconds the ice melts after absorbing enough heat from the rotors and the car rolls downhill.

I think someone should (safely!) try spraying the brake pads with a bit of water and then parking their car on an incline to see if they can replicate this problem.They may also need to brake enough to warm up the rotors, and will need to continue to spray the pads only until they see them ice up. Checking the rotor/pads with an IR thermometer after a drive should help with finding a good temperature/time range to try the experiment.

Another way to check could be to switch creep on. If you can't replicate the problem at all with creep on that suggests it's associated with one-pedal driving using the brakes a lot less in specific cold/wet environments.
In my situation described above, tire traction was definitely an issue - the rear wheel was locked while the car slid down the driveway, so the brake was definitely engaged and there was nothing interfering with it and causing the rear wheels to rotate.
 
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I wonder whether parking nose-down rather than nose-up might be a reasonable preventative measure? In that video, it is somewhat apparent that the vehicle started sliding when the weight shifted back (i. e. downhill), and the weight of the car simply "pushed" it down the hill.

I hypothesize that if the far were nose-down, the weight shifting uphill to the braking wheels might be beneficial; I'm not sure if the weight of the rest of the vehicle "pulling" downhill would or would not be less likely to cause a slide than the weight of the car "pushing".
 
OK, I'm posting the videos I have of my Model 3 self-driving toboggan. Weather conditions were ~5cm of snow in the last 12h on a completely clean and snow/ice-free driveway, temperature -2C (just below freezing)

11:28am: arrived home with two kids and everybody exits the vehicle (in retrospect, a bit worrisome that the younger one walks behind the car)
https://photos.app.goo.gl/gLBQxPeSQVacjzxm6

11:33am: vehicle slides down the driveway first time. Notice that it's not moving very fast and it stops immediately once it hits the road because the incline is not very steep at all
https://photos.app.goo.gl/hHFcsZfH46cQTeMM6

11:36am: I realize the car is in the road and go out to move it
https://photos.app.goo.gl/hHFcsZfH46cQTeMM6

11:38am: I pull the car back into the driveway and go to the garage to grab the shovel - shortly after I park it and leave it slides again
https://photos.app.goo.gl/DuF3bmojJCkZa7fDA

And this is a repost of the picture that I took showing the slide pattern off the rear tire among the rotation patterns of a moving tire:
img_20191206_124509-jpg.485402
 
Gotta love Nest Cams.

What does Tesla have to say after seeing the video?

Why would they say anything? You park on a slick incline and this may happen. It is more likely to happen with any vehicle that only locks two wheels while parked, but it is nothing specific to a Tesla. As others have said, RWD vehicles in park with a parking brake on would do exactly the same thing. The people surprised have never had a car that only locks 2 wheels while stopped (since it is twice as hard for those to start sliding, it would be super rare for the conditions to change such that it would happen, though it still could).
 
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