One pedal driving interaction with ABS on slippery slope

[csimeon]

Today I was on a street with steep downhill incline, I was using one pedal drivimg, but when the ABS kicked in the car just lunged forward (cancelling the hold of the car from the motor) until I pressed the brake pedal.
Scary! I recommend not using one pedal driving on slippery downhill situations

BTW, SNOW driving mode cancels one pedal driving, guess it's related!?

 

[Alb]

Didn't realise that ABS could be triggered other than directly through the brake pedal.

 

[ReintjeWA]

Didn't realise that ABS could be triggered other than directly through the brake pedal.

I would imagine the electronic stability control has control over ABS?

 

[DixieTGS]

I assume electronic stability control and ABS both control each brake independently and can change the braking force very rapidly, whereas forward power and regen control pairs of wheels and operate relatively slowly. So if either ESC or ABS is activated on a slippery hill, it may have to alter the motor power or regen before it can even start to control the brakes.

To make matters more difficult, if it is swapping regen for braking (so it can do ABS) and wants to maintain the current speed and acceleration it would need to set the input to the brakes, such that the extra braking force 'exactly' matched the loss of braking force from reducing regen at all points in time. Both of these control inputs are probably very non-linear (especially the discs if wet or hot) so I expect this is a very difficult problem to get right in all cases.

As the OP says, all a bit scary.

 

[csimeon]

ABS engaged with my foot off the brake pedal, then released in a fraction of a second, "realizing" I was not braking, without the regeneration holding the car, I lunged downhill.
I controlled the car with the brake pedal.
Luckily no close traffic, obstacles and was moving in a straight line.

Sensing this may be a known issue and not specific to MG4, i asked ChatGPT for a search summary and sources.
Apparently Polestar, Chevrolet, Goodyear have recommendations to switch regeneration to minimum in snow or slippery conditions.
Article in WSJ
I repeat MG4s do this in SNOW mode.

My concern though is will I remember?
Or what about unforeseen condition changes, like an ice patch at a bend on an otherwise dry road, or a turn from a flat into a downhill street on a rainy day?

 

[ReintjeWA]

Just thinking out loud here; it could have been an instance of the car emergency braking but then almost directly canceling the emergency brake because your foot was on the accelerator? This is the procedure to override emergency braking according to the manual. I don’t know how emergency braking would work with OPD but I could see how this then disables OPD.

The manual also talk about different braking systems, predominantly the regen braking and the friction braking, but mentions a bucket load of acronyms that all monitor the brakes.

I agree it is all a bit vague and seems to show a lack of system integration at the very least.

 

[AlC]

My previous car (Toyota Corolla hybrid) used blended braking, so a light application of the brake pedal would increase the regen without using the friction brakes.

If it detected any wheel slip while doing this (e.g. if a wheel went over a manhole cover in the wet) it'd immediately ditch the regen and engage the friction brakes, the reason being that regen can't control for wheel slip on individual wheels but ABS can. It'd take a fraction of a second after regen was removed before the friction brakes kicked in and it was a little unnerving because the car would feel like it was accelerating for a second.

I wouldn't be surprised if MG employs the same logic, and normally that'd be fine without OPD because if you're slowing down sufficiently hard to cause a wheel to slip, you're probably on the brake pedal anyway and the friction brakes will take over.

In OPD mode however I can imagine there being some absence of logic in the software (because MG) such that wheel spin is detected and so regen is removed, but the friction brakes are not applied (or applied briefly and then removed) because there's no request for braking being read from the brake pedal.

 

[DixieTGS]

Considering the above, not applying the previous level of regen after recovering from wheel slip may be a reasonable response:

Applying regen again risks losing control again.

Applying steady brakes (not ABS) in place of regen without your foot on the brakes would just be weird, and so confusing.

So making the driver take control may be the best of the options despite the downsides.

 

[Muffler]

My concern though is will I remember?
Or what about unforeseen condition changes, like an ice patch at a bend on an otherwise dry road, or a turn from a flat into a downhill street on a rainy day?

Wet conditions are more of an unknown in this context, I guess. However, as you must be selecting OPD every time you get in the vehicle, surely the answer is to just leave it to default (off) when temps are showing below say 3C?

 

[JonnyG]

Thanks for posting your experience ... I always wondered how OPD would work with slowing the car downhill in slippy/snow/ice conditions and now I know !! I did stick to SNOW mode over the last week (having had snow here in the UK) and everything seemed fine traction / grip wise, but braking downhill is always a worry on Summer tyres.

 

[csimeon]

but braking downhill is always a worry on Summer tyres

Here (Greece has little snow) the country in low areas has little if any snow and winter tyres are not a common practice, norm and obligation is to wear chains in snow. Wonder what the car computer will make of that.... I know that certainly I would need to turn off as many automatics as possible...
But this is a very particular case, slippery is general, be it rain, snow or whatever.

 

[JonnyG]

..............But this is a very particular case, slippery is general, be it rain, snow or whatever.

So what what was specific to your case out of interest ? was it just a steep downhill street ? or something more as well ?

 

[ReintjeWA]

The manual states one should disable TCS (traction control system) and SCS (stability control system) when using snow chains. But even more interestingly MG actually recommends winter tyres instead of snow chains on the MG4.

 

[DaevM]

Jeez, I know I'm getting old! When I see stuff like this my immediate reaction, rather than trying to figure out what's actually happening and how to adapt or circumvent is "why can't they just leave things be" - go pedal / stop pedal. Learn how each works in various conditions, drive accordingly. I mean I love my MG4 but find 'computer says no' very unnerving. They may have been archaic in comparison but my first cars ( triumph herald, vw beetles) were a lot less worrisome when it came to knowing what was going on

 

[csimeon]

Jeez, I know I'm getting old! When I see stuff like this my immediate reaction, rather than trying to figure out what's actually happening and how to adapt or circumvent is "why can't they just leave things be" - go pedal / stop pedal. Learn how each works in various conditions, drive accordingly. I mean I love my MG4 but find 'computer says no' very unnerving. They may have been archaic in comparison but my first cars ( triumph herald, vw beetles) were a lot less worrisome when it came to knowing what was going on

Plus I could always yank the handbrake and steer in the snow!! silly abs?

 

[decrep]

If I was younger, not having a handbrake to pull on, when I wanted to turn fast, would have been a minus, but I'm beyond such things now.
And yes DaevM, my first car was also a Herald

 

[Ian Key]

I also had a Herald as my first car. Great in the snow as the skinny tyres went through the snow as opposed to sitting on top as modern fat tyres do.

Mind you if you had the lights on and the heater on when you turned on the after market stick-on heated rear window it would stall as the dynamo couldn't produce enough electricity if stationary in traffic.

 

[DaevM]

I also had a Herald as my first car. Great in the snow as the skinny tyres went through the snow as opposed to sitting on top as modern fat tyres do.

Mind you if you had the lights on and the heater on when you turned on the after market stick-on heated rear window it would stall as the dynamo couldn't produce enough electricity if stationary in traffic.

My lasting memory was not having any antifreeze in the coolant - first winter, first car - lesson learnt. Blew a ( no idea what they are called) little round disc out of the engine block. Had to wait for it to thaw, then fixed it meself and IT WORKED!!!

 

[Kithmo]

My lasting memory was not having any antifreeze in the coolant - first winter, first car - lesson learnt. Blew a ( no idea what they are called) little round disc out of the engine block. Had to wait for it to thaw, then fixed it meself and IT WORKED!!!

Core plug.

 

[DaevM]

Core plug.

That's it but it was over 45 years ago now. Can't remember what I had for dinner yesterday now, so thanks for that