# Regen on ice



## JMac (Sep 26, 2017)

Let's say I'm going down a modest grade in my RWD Model 3. The road has a thin layer of packed snow/ice on it with some additional snow. I'm travelling at a speed that seems reasonable but I decide to slow a bit so I lift my foot off the go-pedal. Regen causes the rear wheels to lose traction. I see at least these three scenaria:

A. The stability system doesn't react. I lose control of the car.

B. The stability system notices that the rear wheels are rotating slower than the front wheels so it applies the front brakes.

C. The stability system notices that the rear wheels are rotating slower than the front wheels and that I have not applied the brakes so it reduces the amount of regeneration.

Does anyone here know what the Model 3 will do here?


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## Topher (May 11, 2016)

I don't think A. can happen. The reason that RWD cars lose control when the rear wheels lose grip is when they are being driven, they no longer track reliably. This is not the case in your scenario. Neither can you lose steering control since that is on the front tires. At worst, you aren't going to slow as quickly as you might have expected, and I assume your (or autopilot's) response to that will be to put on the brakes. Should the rear wheels totally lock up, there is nothing causing them to not just follow the center of mass of the car. If you are cornering, and the lateral force exceeds the friction of the tires with the road, you aren't going to turn, but that has nothing to do with regen braking, or electric cars, or even just cars.

B seems an unlikely response.

C. Probably.

Thank you kindly.


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## Bokonon (Apr 13, 2017)

Jeff MacDonald said:


> Let's say I'm going down a modest grade in my RWD Model 3. The road has a thin layer of packed snow/ice on it with some additional snow. I'm travelling at a speed that seems reasonable but I decide to slow a bit so I lift my foot off the go-pedal. Regen causes the rear wheels to lose traction. I see at least these three scenaria:
> A. The stability system doesn't react. I lose control of the car.
> B. The stability system notices that the rear wheels are rotating slower than the front wheels so it applies the front brakes.
> C. The stability system notices that the rear wheels are rotating slower than the front wheels and that I have not applied the brakes so it reduces the amount of regeneration.
> Does anyone here know what the Model 3 will do here?


I can't speak to the Model 3 (or Teslas in general), but both of the EV models that I've driven in snow (first-generation Chevy Volt and VW e-Golf) use Option C, so that would be my guess for the Model S, X and 3 as well.

One thing that's worth pointing out is that both of those models activate blended regenerative/friction brakes through the brake pedal, so in theory, you can just use the brake pedal for regen in the snow, and the car will automatically apply more friction if it detects slippage from regen. (Note: in my experience, the first-generation Volt can be slow to respond to this scenario, and sometimes lets the car slip for about a second before engaging the friction brakes.)

Teslas, by contrast, do not blend regen into the brake pedal -- it only applies friction brakes -- so the above technique does not work. All the regen comes from lifting your foot off the go-pedal. From what I've read, most Tesla owners seem to be in agreement that using a lower regen setting in slippery road conditions is advisable. Most in snowier climes with RWD models run winter tires to mitigate the slippage problem.


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## garsh (Apr 4, 2016)

I can't say that I've been in that exact scenario in my Leaf - anytime I start losing traction on a hill like that, I'm immediately hitting the brakes, so I'm not sure what the car is doing right before I hit the brakes. But I can say that - at least for the Leaf - when the anti-lock brakes intervene, it turns off all regeneration. I would imagine that Tesla does something similar.


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## JMac (Sep 26, 2017)

Topher said:


> I don't think A. can happen. The reason that RWD cars lose control when the rear wheels lose grip is when they are being driven, they no longer track reliably. This is not the case in your scenario. Neither can you lose steering control since that is on the front tires. At worst, you aren't going to slow as quickly as you might have expected, and I assume your (or autopilot's) response to that will be to put on the brakes. Should the rear wheels totally lock up, there is nothing causing them to not just follow the center of mass of the car. If you are cornering, and the lateral force exceeds the friction of the tires with the road, you aren't going to turn, but that has nothing to do with regen braking, or electric cars, or even just cars.
> 
> B seems an unlikely response.
> 
> ...


Thank you for taking the time to reply. I can assure that, in the absence of some sort of stability system, scenario A will happen. You can prove this to yourself quite easily by yanking up on the handbrake of a car so equipped. (I do this all the time in my GTI because, well, I can and it freaks my daughters out.) The rear end will snap around faster than you can say, "What the... ?" The reason it comes around is because, once the rear tires have lost grip, the sliding friction under the rear wheels is much less than the (relative) static friction of the front wheels. If (and that's a big if) you can keep the rear wheels right behind the front wheels then the car will go straight, but you can't do that for very long under real-world conditions.


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## Topher (May 11, 2016)

Jeff MacDonald said:


> You can prove this to yourself quite easily by yanking up on the handbrake of a car so equipped.


Regenerative braking is not the same as hand braking. Proof would require you to do it in a car with regenerative braking.

Thank you kindly.


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## JMac (Sep 26, 2017)

Topher said:


> Regenerative braking is not the same as hand braking. Proof would require you to do it in a car with regenerative braking.
> 
> Thank you kindly.


It is if it's strong enough and there's no stability system to step in and save you.


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## rxlawdude (Sep 12, 2017)

Jeff MacDonald said:


> Thank you for taking the time to reply. I can assure that, in the absence of some sort of stability system, scenario A will happen. You can prove this to yourself quite easily by yanking up on the handbrake of a car so equipped. (I do this all the time in my GTI because, well, I can and it freaks my daughters out.) The rear end will snap around faster than you can say, "What the... ?" The reason it comes around is because, once the rear tires have lost grip, the sliding friction under the rear wheels is much less than the (relative) static friction of the front wheels. If (and that's a big if) you can keep the rear wheels right behind the front wheels then the car will go straight, but you can't do that for very long under real-world conditions.


Is your GTI rear-wheel drive? Of course what you describe will happen in a FWD vehicle. Not so in a RWD.

By the way, we have an AWD MS, and when we road-tripped into snow country, we turned regen to LOW, as recommended by others who live in the snow. We were in a scenario as described (downgrade, icy road, traveling around 50mph) and were going to make a left turn. Well, the car decided to continue forward. At no time was there a loss of control (other than stopping in the intersection to make the turn). We proceeded to the next intersection, made a U turn, and then a right on the intended road with no other similar issues on the 3,300 mile trip.


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## Guest (Dec 23, 2017)

Nissan Leaf acts like described in C. And, AFAIK, all other EV's as well.
Regen is disabled immediately and abruptly.
I've also noticed that regen will hesitate with aggressiveness for short period later on,
even though regen is available and heavy regen mode is activated.
In any ABS or stability control protocol interaction, regen is not available.


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## JMac (Sep 26, 2017)

arnis said:


> Nissan Leaf acts like described in C. And, AFAIK, all other EV's as well.
> Regen is disabled immediately and abruptly.
> I've also noticed that regen will hesitate with aggressiveness for short period later on,
> even though regen is available and heavy regen mode is activated.
> In any ABS or stability control protocol interaction, regen is not available.


Thanks for the input. That's certainly what makes the most sense; makes stability much easier to manage when you take that extra input out of the equation.

So far, every oddball, off in the weeds, thing I've thought up has been asked and answered, in the best possible way, by some Tesla engineer. Makes me smile.


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## roflwaffle (Sep 25, 2017)

Jeff MacDonald said:


> Let's say I'm going down a modest grade in my RWD Model 3. The road has a thin layer of packed snow/ice on it with some additional snow. I'm travelling at a speed that seems reasonable but I decide to slow a bit so I lift my foot off the go-pedal. Regen causes the rear wheels to lose traction. I see at least these three scenaria:
> 
> A. The stability system doesn't react. I lose control of the car.
> 
> ...


I think it's...

D. The stability systems notices that the rear wheels are rotating slower than the front wheels, and it selectively applies individual brakes as needed and modulates the regenerative braking and/or motor power to regain traction.

http://teslaliving.net/2014/08/24/how-does-model-s-traction-control-work/


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## Guest (Dec 24, 2017)

Article explains how Electronic LSD works. And it is not Tesla nor EV specific. Almost all cars do that as almost all of them use 3rd part ABS/ESP module. Bosch often.


To regain traction vehicle must do nothing. No regen. No braking. No accelerating.


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## roflwaffle (Sep 25, 2017)

arnis said:


> Article explains how Electronic LSD works. And it is not Tesla nor EV specific. Almost all cars do that as almost all of them use 3rd part ABS/ESP module. Bosch often.
> 
> To regain traction vehicle must do nothing. No regen. No braking. No accelerating.


Which other manufacturers only offer an open diff and rely on stability control to simulate an LSD?


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## Guest (Dec 25, 2017)

roflwaffle said:


> Which other manufacturers


Like I said, almost all current vehicles. This feature is 3rd party driven for most manufacturers (Bosch).
My first vehicle, BMW 5-series, 1997 (that was rare back then, FWD vehicles did not have that at that time). 
It didn't have stability control, it had slip control. Reduce power in case of slip and brake x tire if spinning too fast.
Or Nissan Leaf, 2014. Though it is much slower to react, it does the same job. Proprietary ESP system. 
It might not apply to US. I was informed, that ABS has not been a standard feature there for way longer.
Though I believe it is now on every vehicle... so likely any new car purchased lately has electronic LSD.


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## roflwaffle (Sep 25, 2017)

arnis said:


> Like I said, almost all current vehicles. This feature is 3rd party driven for most manufacturers (Bosch).
> My first vehicle, BMW 5-series, 1997 (that was rare back then, FWD vehicles did not have that at that time).
> It didn't have stability control, it had slip control. Reduce power in case of slip and brake x tire if spinning too fast.
> Or Nissan Leaf, 2014. Though it is much slower to react, it does the same job. Proprietary ESP system.
> ...


Traction control is common, but who else uses the rear or front brakes to emulate an LSD during hard acceleration from a stop?


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## Guest (Dec 26, 2017)

Please. I already said. Pretty much all of them. This function comes from ABS module.

For example. Bosch ABS system. This is used by many car manufacturers, BMW, Audi, Mercedes, VolksWagen, list is very long. Same module on hundreds of car models.
https://qualityservicemanual.com/sites/default/files/BMW 3-Series (E46) 1998-2005.pdf

_Automatic Differential Brake Intervention (ADB) The Automatic Brake Differential (ADB) is an automatic differential lock that improves traction. A slipping wheel is braked, which allows the drive torque to be transferred to the wheel with the greater traction. This function acts much like a limited slip differential. ADB intervention is applied to the slipping wheel by regulating the brake pressure in the following manner:

https://www1.snapon.com/Files/Diagn...leCommunicationSoftwareManual_EAZ0025B42B.pdf
page 29._


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## roflwaffle (Sep 25, 2017)

There's no need to be rude. If your contention is that pretty much all cars come with open differentials and something similar to ADB, that's fine, but you need more than just a link to a 3-series manual to support your contention. Not that I'm disagreeing with you either.


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## Brokedoc (May 28, 2017)

There is a proposed class action lawsuit in California looking for $2.3 BILLION from Tesla because some guy went off a mountain when regen kicked in on an icy road. Link here.

I think this is driver error and getting into an accident when slamming on the brakes on an icy road isn't the cars fault either. The driver should modulate the acceleration and the brakes for safety whether real brakes or regen. The question is how would FSD do with this and who would be responsible if FSD got in a weather related accident?


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## Michael Russo (Oct 15, 2016)

Driving in ice is ALWAYS dangerous. No active safety device or winter tires will be a guarantee against human error or even bad luck. And agree with @roflwaffle , let's please all exchanges and even debates as civil in tone as possible. Thank you.


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## Guest (Dec 26, 2017)

Brokedoc said:


> There is a proposed class action lawsuit in California looking for $2.3 BILLION from Tesla because some guy went off a mountain when regen kicked in on an icy road.


Oh how funny some people are
I will demonstrate how it could happen in EU:
What tires were fitted on that vehicle when it was travelling on ice?
All-Season tires. 
Class action lawsuit denied. Incorrect tires fitted.
Vehicle insurance is denied due to incorrect tires fitted.
If vehicle damaged property, owner might have to pay for that too.


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## MichelT3 (Nov 16, 2016)

Michael Russo said:


> Driving in ice is ALWAYS dangerous. No active safety device or winter tires will be a guarantee against human error or even bad luck. And agree with @roflwaffle , let's please all exchanges and even debates as civil in tone as possible. Thank you.


I agree totally. And also let's keep in mind that there are people here from all over the world. With different backgrounds, needs, desires, legalities, cultures, etc. So, what may be true/logical/desirable for you, may not be such for someone else. Respect each other please.


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## Sub-hundred (Dec 31, 2017)

From the owners manual: 
"Both acceleration and regenerative braking may be limited when the ambient temperature is either very warm or cool."
Regen may not be a factor in the original post scenario.

Regen can also be reduced by traction control:
"the system minimizes wheel spin by controlling brake pressure and motor power [including reducing regeneration torque]"


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## teslaliving (Apr 2, 2016)

I've had to deal with this on the S. Some people change the regen setting to low, I never do that. I've been through 4 winters now in New England in the S (RWD only). I just feather the accelerator pedal to moderate the regen desired. You need some power to keep it straight on a slippery slope as you dont have front wheels pulling you into alignment.


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