ABS lengthening braking
Discussion
I was driving in the Lake district yesterday, when I was approaching a juction, which was actually located on an elevated piece of road which one could not see from the approach.
As I approached the incline, braking smoothly and steadily, I hit a section of road which was actually a metal plate of some kind, running the full width of the road (of which, there was no indication). As it was wet, it immediatley triggered my ABS, increasing my stopping distance to such an extent, that by the time I did halt the car, I was actually significantly over the line of the junction.
I then proceeded to watch the car behind me do the same in my rear-view mirror.
I did not realise just how much ABS can lengthen one's braking distances. I really do believe that in such conditions, a better ABS system or even, not having ABS equiped, could have avoided this situation.
Are there any figures to hand that can indicate by just how big a margin ABS can do this?
As I approached the incline, braking smoothly and steadily, I hit a section of road which was actually a metal plate of some kind, running the full width of the road (of which, there was no indication). As it was wet, it immediatley triggered my ABS, increasing my stopping distance to such an extent, that by the time I did halt the car, I was actually significantly over the line of the junction.
I then proceeded to watch the car behind me do the same in my rear-view mirror.
I did not realise just how much ABS can lengthen one's braking distances. I really do believe that in such conditions, a better ABS system or even, not having ABS equiped, could have avoided this situation.
Are there any figures to hand that can indicate by just how big a margin ABS can do this?
Good old fashioned cadence braking usually gets around the ABS when it's being sensitive like in the circumstances you mention, or when there's snow around. It's still difficult in snow to stop as well as in a non-ABS car, but it's such an improvement on other situations that I'd prefer to have it...
I think the few situations where ABS does go bonkers are generally quite few and far between, mostly on surfaces that you can't that fast on (metal plate example is obviously not one though). The situations where you need to quickly brake without locking up are far more common I'd say and often at speed, making the pros outweigh the cons IMO.
The thing is though *if* you hit the same surface without ABS, you'd be left with locked wheels, so for a given amount of time you'd be left with severely reduced braking decelleration until you lifted and re-applied the brakes.
I remember someone on PH once posting a distance vs time curve where the driver has no ABS, skids, and then (in your average driver speed) re-applies the brakes after lifting to regain grip, took longer to stop than just skidding to a half (on normal dry flat tarmac)...
I'm sure the same applies here. Had you lifted to reduce the current braking demand to a level where the tyres regained grip, would it have been slower overall than the ABS?
I wonder why the ABS didn't detect that the current braking input was not sufficient to be exceeding the current grip level (is that possible?? isn't it all done on wheel speeds?)
Hmmmmm.
Dave
I remember someone on PH once posting a distance vs time curve where the driver has no ABS, skids, and then (in your average driver speed) re-applies the brakes after lifting to regain grip, took longer to stop than just skidding to a half (on normal dry flat tarmac)...
I'm sure the same applies here. Had you lifted to reduce the current braking demand to a level where the tyres regained grip, would it have been slower overall than the ABS?
I wonder why the ABS didn't detect that the current braking input was not sufficient to be exceeding the current grip level (is that possible?? isn't it all done on wheel speeds?)
Hmmmmm.
Dave
I'm not sure. It just suprised me by how much further it made the car travel. It was unexpected.
I know that non-ABS equiped cars 'suffer' more through not having it, but i do beleive that in that one small instance, had the ABS been less intrusive or indeed, not there, I would have been better off.
For any 'ordinary' member of the public, this does conflict with their supposed overall view that ABS is infallible.
I know that non-ABS equiped cars 'suffer' more through not having it, but i do beleive that in that one small instance, had the ABS been less intrusive or indeed, not there, I would have been better off.
For any 'ordinary' member of the public, this does conflict with their supposed overall view that ABS is infallible.
I would hope that a properly designed and working ABS system would enable the driver to use most of the available grip under all conditions. Surely the only valid reason for it to come in like that is that the surface was actually very slippery. In that case you aren't going to slow down much no matter what, and all the ABS is doing is keeping the wheels turning so you have some remaining steering and a chance to regain grip as soon as possible on the other side. If that's the case then the reason you over-ran the stop line is not because of the ABS, it is simply because you misjudged the amount of grip available.
On the other hand you may be saying that the ABS held the brakes off even when you were past the slippery region, in which case I'd say that was wrong.
On the other hand you may be saying that the ABS held the brakes off even when you were past the slippery region, in which case I'd say that was wrong.
there may be cases where a different wheel locking management strategy might be better. SO for example, in snow, building up snow with the wheel locked for a while could be a positive effect.
For example, I had an accident in a citroen in the snow as the abs came on with the lightest of brake pressures and I went straight on in a bend and into a fence. This was all at very low speeds but embarrasing all the same. Doing the same a few hours later in a fiesta was all sweetness and light as I could easily manage by locking the wheels and using cadence braking to do the steering bit. The citroen wouldnt allow cadence braking as the abs came on with the lightest brake application.
Now I view that as a poor ABS system, but it just goes to show how different things work when grip is lost.
Graham
For example, I had an accident in a citroen in the snow as the abs came on with the lightest of brake pressures and I went straight on in a bend and into a fence. This was all at very low speeds but embarrasing all the same. Doing the same a few hours later in a fiesta was all sweetness and light as I could easily manage by locking the wheels and using cadence braking to do the steering bit. The citroen wouldnt allow cadence braking as the abs came on with the lightest brake application.
Now I view that as a poor ABS system, but it just goes to show how different things work when grip is lost.
Graham
vonhosen said:
ABS is primarily about retaining steering ability, not reducing stopping distances.
In the majority of circumstances it will probably provide a by product of a shorter stopping distance, but not in all so don't rely on it.
In the majority of circumstances it will probably provide a by product of a shorter stopping distance, but not in all so don't rely on it.
Edited by vonhosen on Monday 25th September 21:01
Damm, beaten to it by Von ...
Further, in the dry, a locked wheel stops pretty much the same as ABS. If you're skillful enough to hold the braking at max, just before ABS cuts in, then you will stop quicker than ABS. Most of us mere mortals can't do that though ...
Martin
mph999 said:
If you're skillful enough to hold the braking at max, just before ABS cuts in, then you will stop quicker than ABS. Most of us mere mortals can't do that though ...
I think most normal ABS systems are set to keep the percentage slip quite low to ensure there is enough grip available to provide lateral stability even in slippery conditions. In very grippy conditions you may find that you can get more grip by running more slip than the ABS will allow i.e. disable the ABS and get the threshold braking right yourself and you can out-brake a mediocre ABS setup. I really good multi channel setup will keep each wheel on the limit of grip and no driver can achieve that with a single pedal.
mph999 said:
vonhosen said:
ABS is primarily about retaining steering ability, not reducing stopping distances.
In the majority of circumstances it will probably provide a by product of a shorter stopping distance, but not in all so don't rely on it.
In the majority of circumstances it will probably provide a by product of a shorter stopping distance, but not in all so don't rely on it.
Edited by vonhosen on Monday 25th September 21:01
Damm, beaten to it by Von ...
Further, in the dry, a locked wheel stops pretty much the same as ABS. If you're skillful enough to hold the braking at max, just before ABS cuts in, then you will stop quicker than ABS. Most of us mere mortals can't do that though ...
Martin
But a human can only control the brakes on all four wheels as a set - ABS can control individually. So ABS only releases the brake on a wheel that is just about to lock. If a human driver senses a wheel is about to lock he can only release the brake on all 4 wheels. Is this an advantage for ABS?
Over the years of working with different vans i've noticed that if your on any loosse surface then you've got to be really carefull if abs is fitted, as mentioned deepish snow is a problem where being able to lock up will stop the vehicle sooner by building up a bank of snow. But the worst is gravel where the abs detects no grip at all and if your going faster then 5mph it will just remove all braking.
It could just be certain manufacturers set their abs at different levels as some are more sensitive than others.
I have found that some systems kick in then once you are on a solid surface they don't release and allow for normal braking unless you let go of the brake pedal and then reapply it.
The best option seems to be to brake and treat the vehicle as though it hasn't got abs and leave the abs opption for the heavy emergency foot through the bulk head type of braking that it was really designed for - you know the time when someone pulls out of a junction without stopping or looking!
It could just be certain manufacturers set their abs at different levels as some are more sensitive than others.
I have found that some systems kick in then once you are on a solid surface they don't release and allow for normal braking unless you let go of the brake pedal and then reapply it.
The best option seems to be to brake and treat the vehicle as though it hasn't got abs and leave the abs opption for the heavy emergency foot through the bulk head type of braking that it was really designed for - you know the time when someone pulls out of a junction without stopping or looking!
vrooooom said:
But the worst is gravel where the abs detects no grip at all and if your going faster then 5mph it will just remove all braking.
It could just be certain manufacturers set their abs at different levels as some are more sensitive than others.
It could just be certain manufacturers set their abs at different levels as some are more sensitive than others.
I had that issue last week, one wheel onto gravel when braking - car stopped slowing down until the wheel came off the gravel, most disconcerting, but hankfully only at walking pace.
tigger1 said:
vrooooom said:
But the worst is gravel where the abs detects no grip at all and if your going faster then 5mph it will just remove all braking.
It could just be certain manufacturers set their abs at different levels as some are more sensitive than others.
It could just be certain manufacturers set their abs at different levels as some are more sensitive than others.
I had that issue last week, one wheel onto gravel when braking - car stopped slowing down until the wheel came off the gravel, most disconcerting, but hankfully only at walking pace.
Have had similar encounters with ABS cars, EBD cars, and no ABS cars, and in all cases the non-ABS car could be modulated to slip on the slippy stuff, but I could use the perfectly fine opposing pair of wheels to stop the car quickly.
With old ABS you simply got nothing at all, and with EBD I managed to stop "well", but with quite a nasty yawing moment...
Either way, I've had ABS help me out, but I've also been caught out without ABS and managed to be quick enough to cadence brake (time seems to slow down in these situations I find)...
Still a very handy tool, but maybe it still has a long way to go? I'm not sure if it should sense if it's a straight line 100% braking efficiency job, or if you start to steer it then optimises for steering!?!?
Not an ABS engineer. Read a complex paper on it once wrt to simulating it accurately in a driving sim, which left me more baffled, and I vowed never to try understand it again
Dave
Philbes said:
But a human can only control the brakes on all four wheels as a set - ABS can control individually. So ABS only releases the brake on a wheel that is just about to lock. If a human driver senses a wheel is about to lock he can only release the brake on all 4 wheels. Is this an advantage for ABS?
Actually it depends on the ABS system. Some are only two channel systems that will only detect speed differentials between two "pairs" of wheels (ie. each channel detects a pair, usually acraoos an axle IIRC). These "simple" ABS systems are more prone to increasing braking distances as when they modulate, they will affect both wheels on that channel.
More complex systems use four channels, that are able to modulate braking on individual wheels, so being more effective overall.
renny said:
More complex systems use four channels, that are able to modulate braking on individual wheels, so being more effective overall.
Yep, the old two channels were bad when both left wheels had no grip but the right ones did, but you got as good braking as only what the worst tyre of the front or rear pair could provide.
EBD/4 channel on my car is much better, but now you get a yawing moment in some cases, where the grippier side now slows hard and the slippy side doesn't, but it's still hard to modulate vs the direct input non-abs where you KNOW what you will get as opposed to the yaw moment fluctuating as the left wheels loose and gain grip!
Dave
vrooooom said:
The best option seems to be to brake and treat the vehicle as though it hasn't got abs and leave the abs opption for the heavy emergency foot through the bulk head type of braking that it was really designed for - you know the time when someone pulls out of a junction without stopping or looking!
Wonder where I have heard this before...
gridgway said:
I dont actually know, but I suspect that a large number of people dont think about ABS at all. They never encounter its use. AFAIK my wife has never had it come on (for example).
Graham
Graham
That's probably no bad thing either - if we can have the majority of people not needing ABS to come into play then that is good news.
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