Do our anti-lock brakes work in reverse?
12-16-10, 01:59 PM
#1
Racer
Thread Starter
Do our anti-lock brakes work in reverse?
I don't make it up a snow/ice hill and was backing back down. When attempting to slow, the brakes lock up.
I didn't really take the time to determine whether anti-lock works in reverse (I needed to avoid a cement wall), but it didn't appear like the anti-lock wanted to kick in.
Anyone have a confirmation on that?
I didn't really take the time to determine whether anti-lock works in reverse (I needed to avoid a cement wall), but it didn't appear like the anti-lock wanted to kick in.
Anyone have a confirmation on that?
12-16-10, 02:21 PM
#2
Moderator
I don't believe so and here's why...
Several "systems" and "states" are necessary for anti-lock brakes to function. And remember, antilock doesn't necessarily mean faster stopping, it means you have control over stopping. But on snow or ice the lack of substantial friction between the two surfaces usually means you'll slide on your last vector until, either gravity overcomes inertia or you hit something that alters your last vector.
But back to systems:
The ABS controller monitors the "speed sensors" and is looking for deceleration that is out of the ordinary. I don't believe going in reverse is monitored but I cold be wrong about that. Nevertheless, can you reach a speed going backwards, where there is "extraordinary deceleration" where a wheel would lock up? Or, could you experience a rapid deceleration in reverse? Since a wheel can stop turning much faster than a car stop, the controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration of the wheel, then it increases the pressure until it sees the deceleration again. Thus the rapid pulsing you feel when engaging the antilock brake function. The result is the system gets maximum braking power, and control over direction due to the coefficient of friction between the wheel and pavement.
Several "systems" and "states" are necessary for anti-lock brakes to function. And remember, antilock doesn't necessarily mean faster stopping, it means you have control over stopping. But on snow or ice the lack of substantial friction between the two surfaces usually means you'll slide on your last vector until, either gravity overcomes inertia or you hit something that alters your last vector.
But back to systems:
The ABS controller monitors the "speed sensors" and is looking for deceleration that is out of the ordinary. I don't believe going in reverse is monitored but I cold be wrong about that. Nevertheless, can you reach a speed going backwards, where there is "extraordinary deceleration" where a wheel would lock up? Or, could you experience a rapid deceleration in reverse? Since a wheel can stop turning much faster than a car stop, the controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration of the wheel, then it increases the pressure until it sees the deceleration again. Thus the rapid pulsing you feel when engaging the antilock brake function. The result is the system gets maximum braking power, and control over direction due to the coefficient of friction between the wheel and pavement.
12-16-10, 02:48 PM
#3
Racer
Thread Starter
antilock doesn't necessarily mean faster stopping, it means you have control over stopping.
reduces the pressure to that brake until it sees an acceleration of the wheel, then it increases the pressure until it sees the deceleration again. Thus the rapid pulsing you feel when engaging the antilock brake function. The result is the system gets maximum braking power, and control over direction due to the coefficient of friction between the wheel and pavement.
reduces the pressure to that brake until it sees an acceleration of the wheel, then it increases the pressure until it sees the deceleration again. Thus the rapid pulsing you feel when engaging the antilock brake function. The result is the system gets maximum braking power, and control over direction due to the coefficient of friction between the wheel and pavement.
I thought that antilock DOES mean faster stopping because the computer can monitor the maximum braking power before lockup and achieve that point better (more often) than any human can.
And I thought maximum braking power occurs just before lockup because at maximum braking power the stopping friction is achived through the entire brake pad, whereas at lockup, the only friction point is the relatively tiny rubber patch at the road.
Me wrong? (wouldn't be the first time)
12-16-10, 04:03 PM
#4
Moderator
Well I have a fair amount of "reconstruction" work in my background, and have qualified as an expert a few time in court.
While I want to make it clear I an NOT an engineer, I would argue, the road surface also is a big factor. That is, once your sliding on ice for example, you can turn the wheel all you want and you'll still be sliding toward the wall with or without anti lock brakes. But once gravity overcomes inertia you'll get some control back. But that's when most people steer off the road and crash anyway. My experience with skidding or sliding cars, is the wall is usually too close no matter what was done, or claimed to have been done.
Regarding faster stopping or shorter distances, ... semantics maybe... shorter distances, yes, again depending on the surface and friction. And you are certainly correct the "systems" do alter the state of wheels much faster than any human could hope to accomplish. But even with anti-lock brakes many other factors affect the outcome. Most of the time its error in judgement, i.e. turning into a collision, or in trying to avoid, turning into where object will be rather than where it was, relative to time/space.
Hmmm brake pad larger or smaller than the tire surface contact patch... well in the dynamics of a crash, the center of gravity of the car is altered slightly, (depending on the car's suspension) with weight shifting forward and down in the front thus slightly increasing the contact patch with pavement. Of course the rear end is lifted up slightly reducing the contact patch on the ground. An interesting thought and would depend on the disc surfaces vs tire size, inflation etc etc.
While I want to make it clear I an NOT an engineer, I would argue, the road surface also is a big factor. That is, once your sliding on ice for example, you can turn the wheel all you want and you'll still be sliding toward the wall with or without anti lock brakes. But once gravity overcomes inertia you'll get some control back. But that's when most people steer off the road and crash anyway. My experience with skidding or sliding cars, is the wall is usually too close no matter what was done, or claimed to have been done.
Regarding faster stopping or shorter distances, ... semantics maybe... shorter distances, yes, again depending on the surface and friction. And you are certainly correct the "systems" do alter the state of wheels much faster than any human could hope to accomplish. But even with anti-lock brakes many other factors affect the outcome. Most of the time its error in judgement, i.e. turning into a collision, or in trying to avoid, turning into where object will be rather than where it was, relative to time/space.
Hmmm brake pad larger or smaller than the tire surface contact patch... well in the dynamics of a crash, the center of gravity of the car is altered slightly, (depending on the car's suspension) with weight shifting forward and down in the front thus slightly increasing the contact patch with pavement. Of course the rear end is lifted up slightly reducing the contact patch on the ground. An interesting thought and would depend on the disc surfaces vs tire size, inflation etc etc.
Last edited by VVTiBob; 12-17-10 at 08:06 AM.
He should try driving the car forward at hi speeds, not reverse.
12-17-10, 02:49 PM
#7
Lead Lap
Antilock brakes do not ensure that you stop faster, in fact in certain situations they actually extend the stopping distance, especially for an experienced driver. In a grossly simplified explanation, all they do is monitor the wheel speed vs the car speed (taking into account acceleration/deacceleration) and if there is a mismatch, the pressure is released on the brakes to regain rolling traction. The friction on rolling traction is much higher than that of sliding friction. On gravel (and other situations), antilock brakes CAN actually make matters worse. For most people they help. The purpose of antilock brakes is to prevent slippage (tires on pavement), in other words, prevent the wheels/tires from locking up and sliding.
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12-17-10, 06:59 PM
#8
Lead Lap
BTW JohnnyCake, you brought up a very good question. Unfortunately, I don't have an answer for it.
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