Real World Antilock Brakes

[kevman]

The purpose of ABS is to allow you to control your car's direction in the event of a skid , by letting you steer the car without losing control.

 

[91TTZ]

I've heard that ABS will lengthen the stopping distance on plowable surfaces, meaning surfaces that pile up in front of the wheels while you're skidding, like sand, gravel, snow, etc. That's because although your wheels are skidding, that built-up material is helping you stop. With ABS, you'd roll right over that stuff.

And ABS will definitely shorten your stopping distance on a slick surface like a wet road. A locked up tire has a poor coefficient of friction. It's better to continuously regain grip.

 

[Zenmervolt]

Originally posted by: Howard

Originally posted by: Zenmervolt
ABS causes longer stopping distances on slick surfaces than completely locked brakes.

Stopping distances decreased substantially with four-wheel ABS on wet surfaces, but decreased only slightly on dry pavement and increased considerably on gravel.

http://www.nhtsa.dot.gov/cars/rules/regrev/evaluate/808206.html

And that would be exactly what I said... ABS only decreases stopping distance on dry or merely wet pavement (and on hard-pack snow oddly enough). ABS causes longer stopping distances on ice ( on which it almost doubles), gravel, scarfed snow, and fresh snow.

From the R&T Article (For reasons of avoiding laziness, I have omitted some of the commentary):

Quoted from Road & Track, July 1997, pp 99-105

Since its widespread introduction in the early Eightes, ABS has garnered
its share of plaudits, certainly from the likes of us, for example, in our
instrumented measurement of dry-road straight-line panic-stop braking. Its
efficacy has also been demonstrated in situations displaying uneven grip
side-to-side, technically, on "split-u" or "split-coefficient" surfaces"

There have been plenty of stories as well concerning ABS tradeoffs,
typically incolcing gravel, sand, snow, or ice. However, to the best of
our knowledge, these have been largely undocumented until now. Our goal
here is to apply instrumentation and scientific methodology to what has
hitherto been folk legend.

By way of foreground, the idea of ABS is to prevent wheel lockup in
emergency braking,. The principal advantages of this are twofold: First,
an unlocked wheel offers steering control; a locked wheel gives next to
none. Second, a wheel that's almost, but not quite, locked (at "incipient
lock") typically offers a shorter braking distance than one that's
completely locked.

Our testing here focuses exclusively on this second measure, braking
distance, performed in a straight line. However, before we go on, we
cannot emphasize too stronghly the importnace of the of the first aspect,
so let's restate it here (and repeat it later on): An unocked wheel offers
steering control. It offers you, the driver, another tool to avoid
whatever emergency is ahead. A locked wheel, by contrast, implies that you
and the steering wheel are pretty much along for the ride.

In the briefest of overviews, ABS uses sensors to monitor wheel rotation,
then invokes a rapid cycling of hudraulic pressure to the wheel that's
exhibiting lockup.

In fact, though, it's a lot more complicated. Front wheels usually get
individual control. The rears are often paired reacting to the less-grippy
side ("select-low" strategy) or the one with more grip ("select-high").

How we did it.

For our test car we chose a Mercedes-Benz, one of the first marques to
offer ABS, with excellent systems today and at the forefront of ABS
research and development for tomorrow. Our 1997 S600 Coupe was modified so
its ABS could be turned on or off by driver control.

Home again in the dry

We returned to familiar ground for the dry evaluations, with out usual
initial test speed of 60 mph. It came as no surprise that ABS beat the
other strategies handily. Lockup was dramatically longer, its 213-ft
distance being 60 percent longer than the 133 ft. achieved by ABS.

Two other points were noteworthy as well. First, Kim found that this
particular brake system was rather less amenable to driver modulation than
the best he recalled from pre-ABS days. What's more, the data corroborate
this, with the Educated strategies being rather off the pace compared to
pure ABS use.

Second, we were awed by the sight of this 2 1/2 ton car sliding with all
four wheels locked, Kim striving to keep it imore or less straight but
essentially just along for the ride. This lurid display of Physics was
quite enough to reinforce our faith in the ABS concept, its apparent
tradeoffs notwithstanding.

DRY BRAKING DISTANCES FROM 60mph

ABS: 133ft
Educated ABS: 167ft
Aducated Lock: 171ft
Lock: 213ft
(confidence window = 6ft)

Clearly, the car with ABS stopped shorter. The "Educated ABS" attempts
were when Kim Reynolds tried to modulate the brakes himself with ABS
switched on. "Educated Lock" was when Kim attempted threshold braking.
"Lock" is when he mashed the pedal and let all 4 wheels lock. For the
"ABS" attempts, he mashed the pedal with ABS on. The confidence window is
the result of statistical tests to determine the stasticically significant
difference.

Other Surfaces (* denotes "winner", for ties, note the confidence window):

ICE: (from 20mph)
ABS: 220ft
Educated ABS: 222ft
Educated Lock: 134ft*
Lock: 136ft*
Confidence Window: 8ft

HARD PACK SNOW (from 30mph)
ABS: 100ft*
Educated ABS: 120ft
Educated Lock: 135ft
Lock: 119ft
Confidence Window: 12ft

SCARFED SNOW (from 30mph)
ABS: 94ft
Educated ABS: 102ft
Educated Lock: 108ft
Lock: 84ft*
Confidence Window: 4ft

FRESH SNOW (from 30mph)
ABS: 97ft
Lock: 85ft*
Confidence Window: 2ft

GRAVEL (from 30mph)
ABS: 65ft
Educated ABS: 70ft
Educated Lock: 63ft
Lock: 52ft*
Confidence Window: 2ft

SAND (from 30mph)
ABS: 53ft*
Educated ABS: 59ft
Educated Lock: 60ft
Lock: 54ft*
Confidence Window: 4ft

WET PAVEMENT (from 30mph)
ABS: 37ft*
Educated ABS: 42ft
Educated Lock: 46ft
Lock: 45ft
Confidence Window: 1ft

 

[radioouman]

Originally posted by: slikmunks

Originally posted by: Zenmervolt
ABS causes longer stopping distances on slick surfaces than completely locked brakes. It always will. What it provides, however, is increased car control during the stop. You should NEVER rely on ABS, and if you're braking hard enough on snow to trigger it (outside of emergency, panic-stop, situations), you're doing something wrong.

Your car does not suck. Your driving skills do.

ZV

i've never heard this...

not calling bs on you, just never heard it... anywhere...

physics doesn't support this either, as the static coefficient of friction is higher than the kinetic coefficient of friction... and if the wheel is skidding, you're working with kinetic friction...

and if it were a wall of snow building up, it'd have to be a HUGE wall i would think... and i would also think that stopping on a layer of snow (rotating wheel) would be better than skidding on the ice (below the layer of snow) to build up a wall of snow that will slow down a xxxx lb car...

huge wall of snow? Ever heard of a wheel chock?

 

[slikmunks]

Originally posted by: Zenmervolt

Originally posted by: slikmunks

Originally posted by: Zenmervolt

Originally posted by: slikmunks
ABS should be just past threshold braking, and any harder you'll skid and you'll slow down more slowly... which is why the ABS comes on, to keep you from just sliding forward.

Yes and no. Tests have shown that on anything other than either dry or merely wet pavement (e.g. sand, gravel, snow, ice, etc), locked wheels will show the car down faster than ABS will. However, locked wheels give zero control, and it's better to have control.

The reason that ABS does not work well with gravel/sand/snow/ice is because the coefficient of friction is so small that the wheels don't pick up speed fast enough once the ABS lets off the calipers and therefore it cannot pulse optimally.

I'd still rather have control in the emergency situation though, so I think the tradeoff is a good one.

ZV

good to know, and yes, i concur, i'd rather have control... 🙂

so... what kind of car is this?

The car used in the R&T test was a '97 Mercedes S600.

ZV

i originally just meant to ask the OP what kind of car he has, not what kind of car was in the test, but that works too, haha

 

[slikmunks]

Originally posted by: radioouman

Originally posted by: slikmunks

Originally posted by: Zenmervolt
ABS causes longer stopping distances on slick surfaces than completely locked brakes. It always will. What it provides, however, is increased car control during the stop. You should NEVER rely on ABS, and if you're braking hard enough on snow to trigger it (outside of emergency, panic-stop, situations), you're doing something wrong.

Your car does not suck. Your driving skills do.

ZV

i've never heard this...

not calling bs on you, just never heard it... anywhere...

physics doesn't support this either, as the static coefficient of friction is higher than the kinetic coefficient of friction... and if the wheel is skidding, you're working with kinetic friction...

and if it were a wall of snow building up, it'd have to be a HUGE wall i would think... and i would also think that stopping on a layer of snow (rotating wheel) would be better than skidding on the ice (below the layer of snow) to build up a wall of snow that will slow down a xxxx lb car...

huge wall of snow? Ever heard of a wheel chock?

yes i've heard of a wheel chock. but out of snow that you're just pushing up?? it'd have to be a LOT, that's all i'm saying. sand? ok, that might be a little different as you're sinking in as you're building up your 'chock', but half of what makes the chock work so well is that all it has to do is keep the car from rolling, not to stop the car...

ever heard of a speed bump?

 

[Zenmervolt]

Originally posted by: radioouman
huge wall of snow? Ever heard of a wheel chock?

Actually, in the R&T testing they found no evidence of this. And it wouldn't have any affect on ice, where there's nothing to build up and where fully locked brakes had the greatest advantage.

What happens is that, on surfaces with very little friction, the wheels do not pick up speed again fast enough for the ABS to react properly. Remember that ABS works by sensing lock and then releasing the brake until the wheel comes back up to the proper speed. On very slick surfaces, this doesn't happen fast enough for ABS to be fully efficient.

ZV

 

[BD2003]

Originally posted by: Zenmervolt

Originally posted by: radioouman
huge wall of snow? Ever heard of a wheel chock?

Actually, in the R&T testing they found no evidence of this. And it wouldn't have any affect on ice, where there's nothing to build up and where fully locked brakes had the greatest advantage.

What happens is that, on surfaces with very little friction, the wheels do not pick up speed again fast enough for the ABS to react properly. Remember that ABS works by sensing lock and then releasing the brake until the wheel comes back up to the proper speed. On very slick surfaces, this doesn't happen fast enough for ABS to be fully efficient.

ZV

Unless you're driving on a frozen lake, which you absolutely shouldnt be, you're fairly unlikely to find a full 200ft stretch of ice in front of you, so ABS FTW.

But if you do regularly encounter such things, and still attempt to drive on an essentially undrivable surface, then ABS or not, you're still pretty screwed.

 

[Zenmervolt]

Originally posted by: BD2003

Originally posted by: Zenmervolt

Originally posted by: radioouman
huge wall of snow? Ever heard of a wheel chock?

Actually, in the R&T testing they found no evidence of this. And it wouldn't have any affect on ice, where there's nothing to build up and where fully locked brakes had the greatest advantage.

What happens is that, on surfaces with very little friction, the wheels do not pick up speed again fast enough for the ABS to react properly. Remember that ABS works by sensing lock and then releasing the brake until the wheel comes back up to the proper speed. On very slick surfaces, this doesn't happen fast enough for ABS to be fully efficient.

ZV

Unless you're driving on a frozen lake, which you absolutely shouldnt be, you're fairly unlikely to find a full 200ft stretch of ice in front of you, so ABS FTW.

But if you do regularly encounter such things, and still attempt to drive on an essentially undrivable surface, then ABS or not, you're still pretty screwed.

First of all, if you're driving within your limits, ABS shouldn't ever be invoked. If you cause ABS to kick in, you've already screwed up. Kinda like airbags. 😛

Second, ABS lengthens distances on fresh snow, scarfed snow, and gravel. All of which are pretty commonly encountered in normal, everyday driving and are not terribly unsafe surfaces assuming a responsible driver.

Third, I never said that ABS was bad. In fact, I said in several places that I would prefer to have the control at least.

ZV

 

[BD2003]

Originally posted by: Zenmervolt

Originally posted by: BD2003

Originally posted by: Zenmervolt

Originally posted by: radioouman
huge wall of snow? Ever heard of a wheel chock?

Actually, in the R&T testing they found no evidence of this. And it wouldn't have any affect on ice, where there's nothing to build up and where fully locked brakes had the greatest advantage.

What happens is that, on surfaces with very little friction, the wheels do not pick up speed again fast enough for the ABS to react properly. Remember that ABS works by sensing lock and then releasing the brake until the wheel comes back up to the proper speed. On very slick surfaces, this doesn't happen fast enough for ABS to be fully efficient.

ZV

Unless you're driving on a frozen lake, which you absolutely shouldnt be, you're fairly unlikely to find a full 200ft stretch of ice in front of you, so ABS FTW.

But if you do regularly encounter such things, and still attempt to drive on an essentially undrivable surface, then ABS or not, you're still pretty screwed.

First of all, if you're driving within your limits, ABS shouldn't ever be invoked. If you cause ABS to kick in, you've already screwed up. Kinda like airbags. 😛

Second, ABS lengthens distances on fresh snow, scarfed snow, and gravel. All of which are pretty commonly encountered in normal, everyday driving and are not terribly unsafe surfaces assuming a responsible driver.

Third, I never said that ABS was bad. In fact, I said in several places that I would prefer to have the control at least.

ZV

But I'm not disagreeing with you. 😛

Mostly.

If *you* drive within your limits, you shouldnt need ABS. I have it for times when other people dont drive within their limits, and do stupid things, ie. slamming on their brakes to catch a yellow light they could have driven through 5 times.

Those surfaces you mention are encountered fairly often, and no driver should expect ABS to save their ass, but the previously shown tests show mostly negligible changes changes in braking distance. If a 20% increase in distance is going to make a difference, then you're following too damn close for the conditions.

But ice is another story. On ice, braking distance is secondary, it's the near complete and utter lack of traction you should really be worried about. Unless you have tracks instead of tires, don't drive on ice. 😛

What they didnt test is traction while braking under those conditions...that'd be quite a bit more interesting.

But indeed, ABS FTW. The advantages far outweigh the disadvantages.

 

[DivideBYZero]

The 'plough effect' is more effective, than a car with ABS, in soft snow. Manufacturers are not brave enough legally to allow this to be switchable. Often you can yank a fuse, but will you remember to un-yank it?

 

[Vic]

The purpose of ABS is to help the driver keep control under panic braking NOT decrease stopping distances. The laws of physics dictate that stopping distances will always be limited by friction.

 

[Zenmervolt]

Originally posted by: BD2003

Originally posted by: Zenmervolt

Originally posted by: BD2003

Originally posted by: Zenmervolt

Originally posted by: radioouman
huge wall of snow? Ever heard of a wheel chock?

Actually, in the R&T testing they found no evidence of this. And it wouldn't have any affect on ice, where there's nothing to build up and where fully locked brakes had the greatest advantage.

What happens is that, on surfaces with very little friction, the wheels do not pick up speed again fast enough for the ABS to react properly. Remember that ABS works by sensing lock and then releasing the brake until the wheel comes back up to the proper speed. On very slick surfaces, this doesn't happen fast enough for ABS to be fully efficient.

ZV

Unless you're driving on a frozen lake, which you absolutely shouldnt be, you're fairly unlikely to find a full 200ft stretch of ice in front of you, so ABS FTW.

But if you do regularly encounter such things, and still attempt to drive on an essentially undrivable surface, then ABS or not, you're still pretty screwed.

First of all, if you're driving within your limits, ABS shouldn't ever be invoked. If you cause ABS to kick in, you've already screwed up. Kinda like airbags. 😛

Second, ABS lengthens distances on fresh snow, scarfed snow, and gravel. All of which are pretty commonly encountered in normal, everyday driving and are not terribly unsafe surfaces assuming a responsible driver.

Third, I never said that ABS was bad. In fact, I said in several places that I would prefer to have the control at least.

ZV

But I'm not disagreeing with you. 😛

Mostly.

If *you* drive within your limits, you shouldnt need ABS. I have it for times when other people dont drive within their limits, and do stupid things, ie. slamming on their brakes to catch a yellow light they could have driven through 5 times.

Those surfaces you mention are encountered fairly often, and no driver should expect ABS to save their ass, but the previously shown tests show mostly negligible changes changes in braking distance. If a 20% increase in distance is going to make a difference, then you're following too damn close for the conditions.

But ice is another story. On ice, braking distance is secondary, it's the near complete and utter lack of traction you should really be worried about. Unless you have tracks instead of tires, don't drive on ice. 😛

What they didnt test is traction while braking under those conditions...that'd be quite a bit more interesting.

But indeed, ABS FTW. The advantages far outweigh the disadvantages.

:beer:

ZV