How does ABS actually work

thexav

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I know what the end effect is but how does ABS actually work?

How does it know if a wheel is losing grip whilst braking? Are they mechanical or electronic systems? Do all four wheels benefit from ABS or just the front axle?
 

TCJBOLDIE

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I had some concerns with what a high school course that I was contemplating joining was teaching when they said "ABS prevents the brakes from locking " without explaining that the brakes have to be applied hard enough for a wheel to lock before the system works.
 

herb

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I had some concerns with what a high school course that I was contemplating joining was teaching when they said "ABS prevents the brakes from locking " without explaining that the brakes have to be applied hard enough for a wheel to lock before the system works.
That's not quite right abs will kick in even under soft braking!
Admittedly when this has happened to me it's been on a wet surface or ice/snow it's the sensors that trigger the abs not the force of the pedal;)
 

HDi fun

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I had some concerns with what a high school course that I was contemplating joining was teaching when they said "ABS prevents the brakes from locking " without explaining that the brakes have to be applied hard enough for a wheel to lock before the system works.
It is wheel by wheel. The latest systems attempt to look at wheel deceleration and if it appears that it's impossibly rapid they will intervene before lock-up occurs.

Whether this is a good thing is debatable. Cadence braking is the driver's own ABS substitute. As to how we get ourselves in a situation requiring cadence braking is secondary to this thread Threshold braking too. The problem is that we all need four feet to control braking wheel by wheel.

I have read somewhere that ABS was initially developed for commercial aircraft applications.
 
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TCJBOLDIE

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That's not quite right abs will kick in even under soft braking!
Admittedly when this has happened to me it's been on a wet surface or ice/snow it's the sensors that trigger the abs not the force of the pedal;)
Herb sorry but I have to disagree with you as your "soft braking" force on the pedal was HARD ENOUGH to lock a wheel for the ABS to kick in on the surfaces you mentioned.:)

The force/pressure on the pedal required to lock a wheel and activate the ABS varies with the tyre grip available on different surfaces.
 
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HDi fun

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Current ABS systems are much quicker [than earlier generation ones] to react to potential lock up. The problem is that they can be overly reactive and come into operation totally needlessly - to the annoyance of the driver. eg: Braking very very gently on a wet surface and one wheel rolling over a slightly sunken metal cover for 1/10 of a second was enough for the system in my Primera to get into action.
 

old-git

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Agreed, I thought there was something wrong with the brakes on the Fox before I realised that it was the ABS kicking in when not required.
 

TCJBOLDIE

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It is wheel by wheel. The latest systems attempt to look at wheel deceleration and if it appears that it's impossibly rapid they will intervene before lock-up occurs.
I have read somewhere that ABS was initially developed for commercial aircraft applications.
Unfortunately I have no experience with the latest systems you mention BUT I do know that the system only comes into action when any wheel is locked and only releases enough pressure on that wheel to allow it to rotate.

We explained it this way to course participants.
"when the car is standing there is zero pressure 0/10 on the brake pedal and when you lock the brakes that we call that 10 out of 10 pressure so the ABS kicks in and releases pressure on the locked brake to say 8/10 which will allow the wheel to rotate momentarily and if you are still hard on the brake the wheel will lock again and release so we have a 10,8,10,8,10,8 pressure on the on the brake as long as you continue to push the pedal and lock the wheel.You will hear as well as feel the pulsing of the ABS on the pedal.We also used our hands to demonstrate.

T9'S dog pic is an excellent example of the lock and release action of ABS
 
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HDi fun

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Nope. Some systems do [claim to] detect impossibly rapid wheel deceleration and thus intervene before that wheel actually locks fully. I know how the system works once deployed, and that is much the same as always except for the fact that later systems cycle more rapidly.
 

TCJBOLDIE

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Nope ?? to which part of my post are you referring ? Please explain :confused:
"impossibly rapid deceleration" without locking a wheel ? I would tend to describe this phenomenon as "threshold braking" so IF as you say it reportedly "intervenes" does that mean that the system will keep it from locking and therefore no ABS pedal pulsing to be felt??.

Just looking for enlightenment as I have an open mind and hopefully can still learn more at my age ;):)
 
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HDi fun

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Nope ?? to which part of my post are you referring ? Please explain :confused:
"impossibly rapid deceleration" without locking a wheel ? I would tend to describe this phenomenon as "threshold braking" so IF as you say it reportedly "intervenes" does that mean that the system will keep it from locking and therefore no ABS pedal pulsing to be felt??.

Just looking for enlightenment as I have an open mind and hopefully can still learn more at my age ;):)
I am referring to your point that ABS only intervenes when a wheel is fully locked as a result of too much pedal pressure for whatever the conditions are.

I am noting that ABS has evolved over the years since its inception.

I am not suggesting that it intervenes as a result of impossible car deceleration. Just that one or more wheels exhibit physically impossible deceleration.

It's a very difficult concept to grasp.
 

TCJBOLDIE

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Progress,refinement etc undoubtedly has happened since ABS was introduced as the original was hopeless/useless on ice and heavy loose surfaces and am sure that a lot of reprogramming has been done over the years since it became available to improve performance on less than ideal surfaces.
 
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