Snow and modern electronics...

[Benrad]

My wife scared herself a little in the recent snow and I think it might be due to advice I gave her about driving in low grip conditions. She's a very competent driver, adapting to conditions, spotting and reacting to hazards early and decisively. She has very little experience of what it feels like when traction is lost though.

When driving in the snow I drive as though I don't have any driver aids. I kart a lot (last weekend in the wet on slicks was good practise for threshold braking and opposite lock!) so I've practised it in an environment where getting it wrong isn't expensive or particularly dangerous. She hates karting and isn't go to go out for a drive in the snow just to learn.

I advised her (she's driving a 2015 Fiesta on summer tyres) to keep the wheel pointing where she wants to go, be smooth but, and this is the thing I think I got wrong, if she really needs to stop then push the brake pedal until ABS kicks in and let it handle it. Of course also leaving extra separation distances and keeping speeds lower too.

She ended up having a little slide under braking, I wasn't in the car at the time, but it's freaked her out.

I've never tried letting the ABS handle it, I trigger it very rarely and when I do my first reaction is to cadence brake, even in the dry. I know the traction control seems to take much longer to respond in the snow on both of our cars. Was my advice good or should I teach her cadence/threshold braking?

We change our cars too often to buy winter tyres and whilst there's often plenty of snow here in North Yorkshire we live in a very flat bit, so getting around isn't a problem on summer tyres.

TL;DR
Can modern ABS be trusted with my wife's safety when driving a modern FWD hatch in snow or should I teach her to cadence/threshold brake?

[TooMany2cvs]

Benrad said:

(she's driving a 2015 Fiesta on summer tyres)

<whistles tunelessly>

Benrad said:

to keep the wheel pointing where she wants to go, be smooth but, and this is the thing I think I got wrong, if she really needs to stop then push the brake pedal until ABS kicks in and let it handle it.

No, that's the right advice.

Benrad said:

She ended up having a little slide under braking

Well, yes... What probably happened was all four wheels locked, and because the ABS couldn't see the difference in wheel rotation speed, it didn't know they were locked. As soon as just one finds enough grip to start rotating again, it'll start the ol' foot massage and do its job. Why did all four lock as soon as she looked at the pedal? Because there's zero grip. What's the difference between that and cadence braking? Nowt, because there's no grip, so you'll just keep relocking it with no retardation. Unless, of course, she just panic planted hard on, rather than braking smoothly and gently. But if that's the case, trying to teach cadence isn't going to help much.

Benrad said:

I've never tried letting the ABS handle it, I trigger it very rarely and when I do my first reaction is to cadence brake

Which is just the same as ABS does. Have a dog and bark yourself?

Benrad said:

I know the traction control seems to take much longer to respond in the snow on both of our cars.

No, it doesn't "take longer to respond" - there just isn't the grip for it to respond to.

Both ABS and TC simply try to take best advantage of what grip is there. They can't make more grip appear when it simply isn't there.

ABS is just automated cadence braking. The wheels have locked? Then unlock 'em and rebrake. Oh, they've locked again? Repeat.
TC is just automated throttle smoothness. The wheels are spinning? Then ease back on the power and try again. Oh, they're spinning again? Repeat.

Benrad said:

...whilst there's often plenty of snow here in North Yorkshire we live in a very flat bit, so getting around isn't a problem on summer tyres.

Well, apart from the problem she's having now, y'mean?

Grip isn't just for getting moving in the first place. It's also - probably more importantly - for changing direction and stopping, too. You're making the same mistake as people who think 4wd is all the answer. It's probably better if you can't get moving, than if you can but can't then change direction or stop, isn't it?

[lyonspride]

ABS on modern cars is pretty much your only option in snow, most non-sporty models of most car makes are fitted with cheapo single pot callipers, which have no "feel" and are over assisted to give that "wow, the brakes are great" effect on the test drive.... When in truth they just snatch and lock with the smallest input.

[Jambo85]

TooMany2cvs said:

Benrad said:

She ended up having a little slide under braking

Well, yes... What probably happened was all four wheels locked, and because the ABS couldn't see the difference in wheel rotation speed, it didn't know they were locked. As soon as just one finds enough grip to start rotating again, it'll start the ol' foot massage and do its job. Why did all four lock as soon as she looked at the pedal? Because there's zero grip. What's the difference between that and cadence braking? Nowt, because there's no grip, so you'll just keep relocking it with no retardation. Unless, of course, she just panic planted hard on, rather than braking smoothly and gently. But if that's the case, trying to teach cadence isn't going to help much.

I had never considered that about ABS but it makes sense - although surely some ultra modern cars have a solution to this?

Anyway - taking this a bit further - assuming she didn't stall then for all 4 wheels to lock up the wheels needs to be disconnected from the drivetrain one way or another. Perhaps she is depressing the clutch too early when coming to a stop? Car brakes are so good these days that in good grip conditions this doesn't matter but I'd argue that in snow and ice it does. Even worse - she could be doing what my other half does and shifting to neutral approaching junctions

I'll resist saying anything about tyres!

[TooMany2cvs]

Jambo85 said:

TooMany2cvs said:

Benrad said:

She ended up having a little slide under braking

Well, yes... What probably happened was all four wheels locked, and because the ABS couldn't see the difference in wheel rotation speed, it didn't know they were locked. As soon as just one finds enough grip to start rotating again, it'll start the ol' foot massage and do its job. Why did all four lock as soon as she looked at the pedal? Because there's zero grip. What's the difference between that and cadence braking? Nowt, because there's no grip, so you'll just keep relocking it with no retardation. Unless, of course, she just panic planted hard on, rather than braking smoothly and gently. But if that's the case, trying to teach cadence isn't going to help much.

I had never considered that about ABS but it makes sense - although surely some ultra modern cars have a solution to this?

Travelling slowly, with all four wheels going round. Hit the brake pedal hard. All four wheels stop going round at the same time. Did the car just stop suddenly, or did they all lock? How does the car know? GPS? Not exactly infallible.

Jambo85 said:

Anyway - taking this a bit further - assuming she didn't stall then for all 4 wheels to lock up the wheels needs to be disconnected from the drivetrain one way or another. Perhaps she is depressing the clutch too early when coming to a stop?

How do FWD cars ever lock the front wheels...? It usually is a panic clutch-brake-everything-both-feet-reach-plant.

And that's assuming it's not an autobox of one flavour or another.

Edited by TooMany2cvs on Tuesday 20th March 13:16

[Jambo85]

TooMany2cvs said:

Travelling slowly, with all four wheels going round. Hit the brake pedal hard. All four wheels stop going round at the same time. Did the car just stop suddenly, or did they all lock? How does the car know? GPS? Not exactly infallible.

Accelerometer. If the car goes from u mph to 0 mph (as measured by speedometer and ABS sensors with wheels locked) in time t without the accelerometer reading an acceleration of -u/t then something went wrong, engage ABS if brake pedal still depressed.

Same principle as ESP comparing steering input to lateral accelerometer readings. If this doesn't already exist it will soon.

TooMany2cvs said:

How do FWD cars ever lock the front wheels...? It usually is a panic clutch-brake-everything-both-feet-reach-plant.

Indeed - but I'd say learning to overcome this reaction is part of being a good driver. I remember as a learner my driving instructor tried to drum this into me when practicing emergency stops.

[Zod]

TooMany2cvs said:

Benrad said:

I've never tried letting the ABS handle it, I trigger it very rarely and when I do my first reaction is to cadence brake

Which is just the same as ABS does (much better than you can). Have a dog and bark yourself?

[anonymous-user]

ABS doesn't control wheel speed, it controls wheel deceleration.

I suspect she got a fright because she'd braked, and nothing much happened in terms of decel because she was driving on a very low friction surface. IME, most drivers get used to modern cars 'just stopping' when they brake (wet or dry, thanks to modern tyres being excellent) and have simply never experienced the feeling of the car "sliding", and hence get to feel that 'thrill' of the car being on the edge of adhesion (the same thrill that once mastered is the reason so many rally drivers spend so much time going sideways ;-)