Torque Cars

How does ABS actually work

Discussion in 'Brakes' started by thexav, 28 September 2015.

  1. thexav

    thexav Pro Tuner Staff Member

    Messages:
    2,044
    Car:
    2002 Clio 172
    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?
     
  2. HDi fun

    HDi fun TC ModFather

    Messages:
    15,384
    From:
    Buckinghamshire UK
    Car:
    Passat 2.0 TDi
    obi_waynne likes this.
  3. TCJBOLDIE

    TCJBOLDIE Torque King

    Messages:
    4,863
    From:
    Brisbane
    Car:
    JB Starion
    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.
     
  4. herb

    herb modherbrator Moderator

    Messages:
    4,057
    From:
    west midlands
    Car:
    Seat Leon Cupra
    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;)
     
  5. HDi fun

    HDi fun TC ModFather

    Messages:
    15,384
    From:
    Buckinghamshire UK
    Car:
    Passat 2.0 TDi
    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.
     
    Last edited: 28 September 2015
  6. TCJBOLDIE

    TCJBOLDIE Torque King

    Messages:
    4,863
    From:
    Brisbane
    Car:
    JB Starion
    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.
     
    Last edited: 29 September 2015
  7. HDi fun

    HDi fun TC ModFather

    Messages:
    15,384
    From:
    Buckinghamshire UK
    Car:
    Passat 2.0 TDi
    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.
     
  8. old-git

    old-git Moderator

    Messages:
    9,182
    From:
    Essex
    Car:
    Elan & Robin Hood
    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.
     
  9. T9 man

    T9 man TC Pro Founder Moderator

    Messages:
    20,369
    From:
    London, UK
    Car:
    Saab 9-3SS T9
    An easy guide ;)

    [​IMG]
     
    obi_waynne and TCJBOLDIE like this.
  10. TCJBOLDIE

    TCJBOLDIE Torque King

    Messages:
    4,863
    From:
    Brisbane
    Car:
    JB Starion
    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
     
    Last edited: 29 September 2015
  11. HDi fun

    HDi fun TC ModFather

    Messages:
    15,384
    From:
    Buckinghamshire UK
    Car:
    Passat 2.0 TDi
    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.
     
  12. TCJBOLDIE

    TCJBOLDIE Torque King

    Messages:
    4,863
    From:
    Brisbane
    Car:
    JB Starion
    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 ;):)
     
    Last edited: 30 September 2015
  13. HDi fun

    HDi fun TC ModFather

    Messages:
    15,384
    From:
    Buckinghamshire UK
    Car:
    Passat 2.0 TDi
    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.
     
  14. TCJBOLDIE

    TCJBOLDIE Torque King

    Messages:
    4,863
    From:
    Brisbane
    Car:
    JB Starion
    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.
     
  15. HDi fun

    HDi fun TC ModFather

    Messages:
    15,384
    From:
    Buckinghamshire UK
    Car:
    Passat 2.0 TDi
    Definitely.
     

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