Can't find the topic now - but on ABS threshold brake snow

[Somewhatfoolish]

I cannot find the topic now but I said I would experiment with whether it's better to just smash the brake pedal with ABS or to threshold brake with it, or to go to various inbetween levels - and to my surprise, at least in the super snowy conditions I was testing it in today, I found the best stopping distances were consistently to be mashing the pedal as hard as possible.

So, er, there you go.

[GravelBen]

Must be a very different ABS system to the ones I've used on snow! I've had some that would actually accelerate down snowy hills when the ABS kicked in.

Just goes to show, it pays to know your own car from experience rather than doing what some bloke on the internet tells you.

[upsidedownmark]

Somewhatfoolish said:

I cannot find the topic now but I said I would experiment with whether it's better to just smash the brake pedal with ABS or to threshold brake with it, or to go to various inbetween levels - and to my surprise, at least in the super snowy conditions I was testing it in today, I found the best stopping distances were consistently to be mashing the pedal as hard as possible.

So, er, there you go.

Would be very interesting to see if you can disable the ABS entirely and try the same (not threshold). The 'understood' wisdom is that because there's naff all grip and snow is soft (snow, not ice), that locking the wheels is advantageous; they cut down into the surface, push up a pile of snow in front of the wheel, and possibly cut through to some proper grip, whereas when they're rolling, they have naff all grip. WRT my comments on the original thread, I didn't think to make clear that the advantage of NOT having ABS on snow/loose gravel (think ball bearings) was the ability to deliberately *lock* the wheels.

ABS might be bettering 'threshold' braking by getting some locking at least.. Personally I'd kinda expect the result you got with ABS vs threshold, as:
1) It's pretty hard to tell a wheel is locked on snow
2) There's naff all weight transfer, so the threshold part isn't so effective
2.5) because of 2, the F-R brake balance will be completely wrong
3) One of the overlooked points about ABS is it allows ALL wheels to provide maximum retardation. Modulating the pedal modulates all 4. Great if the brake balance is correct, less so if it's not.

NB: The F-R brake proportions will be set at a best compromise that the manufacturer came up with - they want plenty on the front so when you stand on it on the pedal you stop fast and don't lock the rears (unstable). However, in the dry you stop harder, there's more weight transferred onto the fronts, and they can do more / accept a more forward brake balance. On a slippery road less so. You can pretty much guarantee that in snow the fronts are going to lock way before the rears.

I think that (confusingly) the thread was the one about ambulances with blues but not sirens!

Edited to add: @GravelBen - that's entirely possible; it's also entirely possible you'd accelerate down the hill without the ABS - just plain physics! However I do agree, some (particularly early) ABS systems can be woeful

Edited by upsidedownmark on Tuesday 22 January 10:48

[GravelBen]

upsidedownmark said:

@GravelBen - that's entirely possible; it's also entirely possible you'd accelerate down the hill without the ABS - just plain physics! However I do agree, some (particularly early) ABS systems can be woeful

That was a '97 Legacy so oldish but but not as old as some. My current '02 Legacy ABS is certainly better, though I'd still prefer being able to turn it off on gravel or snow. I think often the 'official' advice given to the public is based on the best latest-gen systems without them realising just how inferior older ABS can be.

I repeated the exercise several times to confirm what was happening - gentle threshold braking caused gradual deceleration (it would have stopped but not quickly), as soon as pedal pressure increased the extra fraction to reach the ABS trigger point it started accelerating. Was interesting rather than concerning as I had plenty of space in front of me!

As you say loose or corrugated gravel is another surface that ABS systems often struggle with as they aren't programmed for it - they tend to pulse away on the surface marbles and never bite through to the hard grippy layer underneath. Plenty of times I've slowed/stopped(/backed into tight corners ) cars with the handbrake when ABS was giving me nothing.

Out of interest OP, what are you driving?

[bp1000]

If you run winter tyres its best to keep the tyre rotating if you loose traction. Snow will build up in the treads as it picks up more and you stand a better chance of driving out of the uncontrolled slide, snow sticks to snow. You still have to control your downward speed by feathering the brake.

If on compacted snow and ice the same applies, as more of the tyre contacts with the surface, the surface under pressure melts the ice into the tiny slits in the tread helping regain some traction.

If you are on summers I would still opt to keep the wheel turning slightly if you are on a downward slide after loosing traction, just aim for the curb/gully.

[MartinQ]

It really depends on the car. I've got winter tyres on my 330Ci and also on SWMBO's Citroen C1. I would have expected the C1 to be better due to the skinny tyres, but the ABS is so much more intrusive than on the BMW that the brakes feel far less effective.

[upsidedownmark]

I'm going to start by quoting (the abstract) from an SAE paper. I don't have access to the whole thing..

SAE940724 said:

Eddie R., "Ice, ABS, and Temperature," SAE Technical Paper 940724, 1994

To examine the effect of anti-lock brakes with summer, all-season, and winter type tires on ice and snow, tests were carried out on a flooded frozen track and on asphaltic pavement with newly fallen snow. The friction values obtained were compared with those reported by other investigators.

The most important factor governing the amount of traction on snow or ice was the temperature of the tire/snow/ice interface at the time of the test. The same tires on the same surface produced twice as much or half as much traction during tests done later after the temperature had changed. Surface roughness was also a significant factor.

Overall, decelerations ranged from 0.10 g to 0.30 g. This is consistent with the majority of earlier published data.

Directional control was often lost during tests with the anti-lock brake system inactive. Control was never lost during tests with the anti-lock brake system active. Under most circumstances the anti-lock brake system offered a slight advantage providing greater deceleration than was recorded in tests with the anti-lock system inactive. With fresh snow on pavement and a temperature just below freezing, somewhat better traction was obtained with the anti-lock brake system inactive.

On smooth surfaces of ice or hard packed snow the winter, all weather, and summer tires had very similar traction. Any possible differences in deceleration were masked by variations attributable to changes in surface temperature. Snow tires gave the best traction in loose snow.

Couple of interesting things there:

- While you (might) be able to stop faster in loose snow without ABS.. it's not necessarily that much advantage
- On ice, it seems 'winter' tyres don't make that much difference. Surprises me a little, but not a huge amount

The major differences between winter and 'summer/performance' tyres are rubber compound (more natural rubber, silica), and tread patterns. At snow temperatures a summer tyre will be very 'hard', and won't conform to surface roughness, so gives up grip. It also typically has large tread blocks and limited treading - that provides block stability for performance and feel, but very few edges to gain traction on soft/loose surfaces.

In contrast, a winter tyre will remain flexible and compliant to help with friction, and has a lot of tread cutting and 'sipes' which give more edges. Additionally, there's less rubber in the footprint, which increases the loading on what is there, and enables the edges to cut in and develop grip.

Of course, if it's a hard surface with no roughness (ice), you're in deep trouble, no matter what.

bp1000 said:

If you run winter tyres its best to keep the tyre rotating if you loose traction. Snow will build up in the treads as it picks up more and you stand a better chance of driving out of the uncontrolled slide, snow sticks to snow. You still have to control your downward speed by feathering the brake.

Kinda, and kinda not. Obviously at all times you need to maintain directional control - if you do lock, deliberately or otherwise you're going to need to unlock and steer at some point. As to the grip, on snow it's all about the edges (of the tread blocks) - the heavy treading creates more edges, and tries to *shed* snow; if the grooves become filled, you don't have edges. I still maintain that locking can help, but it seems that the circumstances are very specific, and unless you're pretty handy you might not finish up where you intended (reference above)

bp1000 said:

If on compacted snow and ice the same applies, as more of the tyre contacts with the surface, the surface under pressure melts the ice into the tiny slits in the tread helping regain some traction.

If you are on summers I would still opt to keep the wheel turning slightly if you are on a downward slide after loosing traction, just aim for the curb/gully.

As I understand, the pressure melting model is largely obsolete/discredited in that melting is understood to be caused by friction, not pressure: If there is a torque applied to the wheel, it will have some slip. The leading edge of the contact patch will be 'dry sliding', at some point that transitions to wet sliding due to friction heating / melting.

In any case, water is not AIDING traction - quite the opposite. Slide an eraser along a sheet of glass. Then add water and do it again - which is easier?

Agree about the side of the road tho - have personal experience of using the grass verge to avoid sliding down a hill. Less sure about the kerb - that might turn you around completely.

Edited by upsidedownmark on Wednesday 23 January 12:21

[wst]

When I was doing a skidpan training course at Millbrook the cradle car had a switch to turn ABS on and off. Ok, this was basically on dry tarmac which obfuscates things a bit, but when the ABS was "on" and the cradle was set to "ice", the car would turn into a toboggan and not slow down at all upon hitting the brakes and holding the pedal down. Cadence braking while the ABS was "on" allowed me to regain some sort of control (I think the ABS was switching too fast, so there wasn't enough time for the wheels to get rolling to actually do anything, when not cadence braking) and turn (definitely wasn't able to stop in time at the approach speed and such like, the idea was "kid ran out in front unexpectedly").

With ABS off and cadence braking, I had the most confidence in the brake pedal and I felt I could control things more easily. I can't judge how fast I was going or how far I was from the "kid" as turning the ABS off also disabled the speedo.

I tried some threshold braking today on tightly packed snow (it was silent under my wheels) with "winters" (same tread pattern as Blizzaks, but harder wearing tread "all seasons") in my own car (no ABS), and it was just locking up. Generally found that cadence braking was again the solution.

If I'd seen the point about weight transfer, I'd have tried cadence braking both "in gear" and "clutch dipped" to see if that would make a difference to stability (for my car's layout anyway). On my car (FWD) I was finding the rear was getting out of line before the front. Without being methodical I can't tell you if that was with or without dipping the clutch - but I remember when I was being taught to drive that when emergency braking in a car without ABS that I should leave the clutch alone until the engine is about to stall, while on an ABS car I could just stand on the left pair of pedals (albeit on grippier surfaces) and let the car work it all out.

[Hungrymc]

I don't think ABS helps with distances in the snow - the severe lack of traction means the wheels once locked, don't start to rotate again very quickly. This makes the modulation far more crude and far slower than in dry or wet conditions.

I think.

[Logbert]

Disconnected the ABS on mine ... Bloody scary in snow ... Better off without by a long way

[Somewhatfoolish]

Said snow experiments were on the Isle of Man. I've been driving in Scotland today and it was amazingly perfectly gritted until literally two miles before my destination. After this I was basically only able to brake at the top of a hill due to my super conservative policy of stopping before a hill and then testing brakes.

I think the amount I have been drinking tonite I will not drive tomorrow, but the day after tomorrow I will piss arround in the proper snow there is here and pull the ABS fuse as well to test various things. Also relevant is the winter/summer tyre thing - not just the thread but does the temperature of the tyre matter in snow? personally I actually doubt it but would be interested to hear otherwise....

[upsidedownmark]

Somewhatfoolish said:

Also relevant is the winter/summer tyre thing - not just the thread but does the temperature of the tyre matter in snow? personally I actually doubt it but would be interested to hear otherwise....

On snow, I suspect you're right - *IF* they had the same tread pattern...