Why do some people say ABS *DOESN'T* shorten the braking distance?

[Zenmervolt]

Originally posted by: WinkOsmosis

Originally posted by: Spencer278

Originally posted by: Zenmervolt

Originally posted by: Spencer278

Originally posted by: Zenmervolt

Originally posted by: Spencer278
In the simple ABS with the slame on the brakes release method in theory there should never be a time with less friction then when the car is skidding ie the black patch. The problem comes from not detecting when the wheel first start to spin and when the wheel first start to stop.

You're looking at it wrong. Friction between the tire and the ground is not the same as braking force. You're right that there is least friction between the tire and the ground when the tire is skidding, but there is less _stopping force_ (i.e. force being applied to the mass of the car for the purpose of deceleration) when the brake is released than there is when the brake is fully locked. There is minimal stopping force with a locked wheel. There is _zero_ stopping force when the caliper is released to allow the wheel to start spinning again.


ZV

Where does the wheel get the energy to spin when the caliper is released? From the moving car assuming your not on the gas at the same time the only way to turn the wheel is from friction on the ground decelerating the car. It takes the same amount of energy to stop a spinning wheel as it does to make a wheel spin.

I swear, it's like taking to a wall.

We aren't talking about the individual wheel. We're talking about the force imparted on the overall car. Two different things. One is rotational, the other is linear. The same amount of force is applied to the wheel, yes, but the same amount of force _is not_ applied to the _car_.

According to your claim, if I locked all four tires and immediately took my foot off the brake and coasted for the remainder of the stop I would stop in the same amount of time as if the brakes stayed locked. Not true.

ZV

Where does the energy come from to turn the locked up wheel. Or are you saying it never starts turning again. Only two things are in contact with the wheel the road and the car and I have yet to see a road that turns my wheels for me.

A little wheel doesn't dake much energy to spin...

Thank you. I'm glad that there are still people in here with functioning brains.

ZV

 

[C'DaleRider]

Originally posted by: WinkOsmosis

Originally posted by: Zenmervolt

Originally posted by: Spencer278
In the simple ABS with the slame on the brakes release method in theory there should never be a time with less friction then when the car is skidding ie the black patch. The problem comes from not detecting when the wheel first start to spin and when the wheel first start to stop.

You're looking at it wrong. Friction between the tire and the ground is not the same as braking force. You're right that there is least friction between the tire and the ground when the tire is skidding, but there is less _stopping force_ (i.e. force being applied to the mass of the car for the purpose of deceleration) when the brake is released than there is when the brake is fully locked. There is minimal stopping force with a locked wheel. There is _zero_ stopping force when the caliper is released to allow the wheel to start spinning again.ZV

As far as I know, it doesn't release the brake fully.

You are correct.....the brake doesn't fully disengage and allow the wheel to start spinning freely and then reapply the brake to slow the wheel back down.

There are many different variations and control algorithms for ABS systems. This is how one of the simpler systems works.
The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.

When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second. (I doubt a brake/release sequence of 15 times a second will allow any tire to spin up at all......)

Anti-lock braking systems use different schemes depending on the type of brakes in use. They are referred to by the number of channels -- that is, how many valves that are individually controlled -- and the number of speed sensors.

Four-channel, four-sensor ABS - This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.

Three-channel, three-sensor ABS - This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle.
This system provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness.


One-channel, one-sensor ABS - This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle.
So, the wheel is kept at its maximum point of deceleration until it either stops completely with the car or the brakes are released.

From NHTSA:

Do cars with ABS stop more quickly than cars without? ABS is designed to help the driver maintain control of the vehicle during emergency braking situations, not make the car stop more quickly. ABS may shorten stopping distances on wet or slippery roads and many systems will shorten stopping distances on dry roads. On very soft surfaces, such as loose gravel or unpacked snow, an ABS system may actually lengthen stopping distances. In wet or slippery conditions, you should still make sure you drive carefully, always keep a safe distance behind the vehicle in front of you, and maintain a speed consistent with the road conditions.

 

[Kwatt]

Originally posted by: LAUST
To tell you the truth Ndee The best thing to do is go to a snow covered parking lot late at night, and try braking and steering, then pull your ABS fuse and do it some more. There is NOTHING better then getting comfortable with it first hand. No computer is going to just save your azz, and all the info you get from others is a great tool but you still should get some experience in it yourself.

If I ran the Department of Transportation Drivers License Testing each facility would have a skating rink available so they could offer classes and the experience on the ice.

And the newspapers headlines would be : DOT Has BUMPER CAR Training



Kwatt

 

[Vic]

Originally posted by: ndee
I read that this "build up a wedge of gravel or snow" was a myth.

It's no myth. Last week, we had snow here that was so wet that just letting off the gas caused the car to slow down like a boat in water. It just depends on the conditions. But then again, everything depends on the conditions. Which is why carmakers put ABS on their cars, because the majority of humans are inflexible, won't think, and don't get that simple reality. They don't realize that it is always possible to apply too much braking force. They just want to stop NOW, so they panic brake with maximum brake pressure, lock it all up, and wreck. ABS is an idiot system designed to prevent that.

ABS always lengthens stopping distances. Its key advantage is that it enables more control of the vehicle under hard braking. Also, if you manually lock up the brakes and they don't unlock when you release them, then there is something wrong with your braking system -- my first guess would be bad rotors or wheel cylinders.

And IMO, the ideal braking system would be one that always applies the maximum consistent braking force just short of locking up.

 

[thomsbrain]

Originally posted by: C'DaleRider

The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.

When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per second. (I doubt a brake/release sequence of 15 times a second will allow any tire to spin up at all......)

bingo. as someone without ABS, but who has driven the exact same car with ABS installed, i'll tell you that ABS is better in 9 out of 10 dry road situations. locked brakes are like sliding on ice, the car doesn't even feel like it's slowing down, and that's assuming you're not trying to turn. ABS is pretty close to as good as a good driver can do with adequate warning (i.e. a race situation, not a panic stop on the road). i lock my brakes on dry pavement about once every 6 months or so, and only after having several near-misses and one (very minor, luckily) accident have i learned to modulate brake lockup in a panic situation, and even now i would still prefer ABS. obviously i could modulate just fine in a parking lot or if i was purposely driving very quickly on a backroad, but when you're on the freeway and the BMW in front of you slams on the brakes with no warning, it's completely different ballgame.

zemmervolt, your arguement is valid, but only applied to real human beings. the fact is most people can't ride the cusp of lockup reliably, the best they can do is pump wildly, which what you're assuming ABS does. for a lot of drivers, they will stop fastest by simply pushing the pedal to the floor and hanging on, because they'll only make things worse by trying to pump. but that's the beauty of ABS, it does a vastly better job of pumping than we ever could, almost as good as someone doing a good job of riding the cusp without pumping. in some cars (such as mine), a very poor front/rear brake balance means that the rears lock up way too early, when the fronts still have plenty of extra traction. ABS would allow cars like mine to maximize braking potential at ALL wheels, not just the rears.

 

[ndee]

ok, I'm drunk now but I gonna read up on it tomorrow. Seems like there is tons of information here. Thanks ATOT :beer:

 

[Ladies Man]

Originally posted by: ndee
ok, I'm drunk now but I gonna read up on it tomorrow. Seems like there is tons of information here. Thanks ATOT :beer:

i hope you were drunk the whole time in this thread... and most others in here too... gezz people either don't read or are sitting on 1/2 their head nowadays

 

[ndee]

Originally posted by: Ladies Man

Originally posted by: ndee
ok, I'm drunk now but I gonna read up on it tomorrow. Seems like there is tons of information here. Thanks ATOT :beer:

i hope you were drunk the whole time in this thread... and most others in here too... gezz people either don't read or are sitting on 1/2 their head nowadays

sorry that 1/2 of this thread doesn't haver your level of smartness. Hail to the Smarties.

 

[DrPizza]

Originally posted by: ZeroNine8
Actually you can stop a car faster if you don't lock up the brakes because tires, like most things, have higher static friction than kinetic friction. By keeping the tires just on the verge of breaking loose from pavement you will have greater stopping force than just locking them up. ABS works better than non ABS for the average driver, however it is possible to stop faster without ABS for skilled drivers who know how to keep the tires closer to that break point. The other big thing, as was already said, locked up tires don't allow any real steering, so you essentially have no control while skidding.

What he said.

 

[TechnoKid]

From what I understand, your brakes will lock up if the brakes have a higher "friction" than the tires do.

Stickier tires will help you stop shorter because they would tend to not lock-up like a less sticky tire in the same situation would.

I remember reading an article (I believe it was linked here on ATOT) that the tire is the thing that really stops you when you hit the brakes in the car. Tires are perhaps one of the most important [safety] things on a car.

 

[WinkOsmosis]

Originally posted by: TechnoKid
From what I understand, your brakes will lock up if the brakes have a higher "friction" than the tires do.

Stickier tires will help you stop shorter because they would tend to not lock-up like a less sticky tire in the same situation would.

I remember reading an article (I believe it was linked here on ATOT) that the tire is the thing that really stops you when you hit the brakes in the car. Tires are perhaps one of the most important [safety] things on a car.

I'm trying to figure out why people modding their cars get huge discs and calipers, often without getting better tires... Just increases tendency for locking. I lock sometimes with my stock calipers and discs.

 

[Vic]

Originally posted by: WinkOsmosis
I'm trying to figure out why people modding their cars get huge discs and calipers, often without getting better tires... Just increases tendency for locking. I lock sometimes with my stock calipers and discs.

Big brakes don't necessarily increase the tendency for locking. The increased surface area enables better brake cooling (which reduces fade), and enables more control so that a skilled driver can more easily apply the precise maximum amount of brake pressure that is possible given the conditions without locking up.

While I agree with you that big brakes with poor tires is stupid, it is still smarter than poor brakes with poor tires. Ideally, you would want big brakes with excellent tires.