Conference terri::cars_uk

    RE: .6
    >Theoretically, a car with locked brakes may stop more quickly than
    >an ABS-equipped car on dry surface - where the coefficients of
    >sliding & rolling friction are similar, and the ABS equipped car
    >won't be braking as effectively while the ABS releases the braking
    >pressure.

    Ah but how many roads have similar "coefficients of sliding & rolling
    friction" ?
    
    On a different but related subject.  How does ABS know when the
    car is stationary ?  The paradox that brought this question about
    goes like this.  The brakes release the wheels when the wheels are
    stationary, but when the car is stationary so are the wheels, so
    if you brake when the car is stationary the brakes shouldn't work.
    
    TP

    
    Why would you want to brake when the car is stationary?
    
    Baffled.

>>    On a different but related subject.  How does ABS know when the
>>    car is stationary ?  The paradox that brought this question about
 

how does the speedo know that the car is doing 0 mph?

ABS on Audi 200t's is automatically switched off at speeds under 5mph;
this is probably the case with most ABS systems


...art

    I though that abs had some sensor detecting wheel rotation, otherwise
    how do you know that it has stopped. That being the case it don't work 
    stationary. The abs fitted to my bmw bike does not work at speeds less
    that 5 kph and will not calibrate less than that ie: don't work. The
    only circumstatnce in which abs does not work better that normal brakes
    is in snow.
    

    If I took a car out for a spin on a frozen lake ( With ABS ) and
    slammed my foot on the break peddle , with ABS does this mean the car
    will come to a slow stop and not skid ?
    
    
                            KR

    Yes.
    
    I found it great sport in my old Grandad on a really icy day to
    accellerate to about 35, and then stand on the brakes. No close
    encounters with local trees - just stop in a straight line.
    
    Steve

    > Why would you want to brake when the car is stationary?
    
    Well, if the car was standing on a slope, for instance, I might.
    
    Jeff.
    

    
    Sure,
    
    But in that case if you have your foot on the pedal and the car start
    to move then the brake will activate stopping the car.
    
    Probably, as others have said, the ABS cuts out below a certain speed.
    
    Mark
    
    PS ABS is for wimps :^) :^) :^)

    I believe ABS differentiates by looking for one or more wheel stopped,
    with the others still turning. Even in a 4 wheel skid, it must start
    out with one wheel locking first. Based on that it momentarilly
    releases the brakes and if it starts turning it continues to pulse the
    brakes on the affected wheel(s).
    
    Richard

From ABS's point of view (and also your speedometer), there is no difference
between four wheels locked and being stationary.  It can only detect that a
wheel has locked if another wheel is still moving.

Scott

    <<< Note 587.12 by COMICS::COOMBER "We come in peace, shoot to kill" >>>
                            -< no good in the snow >-
    
    stationary. The abs fitted to my bmw bike does not work at speeds less
    				     ^^^^^^^^
    
    Not one of those yummy K series is it? (Maybe the Red K100 I saw in DP
    carpark the other day?).
    
    James

    Description right 'Red K100LT' but it was parked in viables yesterday
    and not dec park.
    
    
    Garry
    

    Hmmm,  I'm immensely jealous now!!!
    Top of the range (LT) with ABS = Big bucks!
    
    If only I could afford the insurance/machine/ABS...
    
    Cheers,
    James.

    Havn't been to dec park for years. The insurance is dearer that the abs
    but the pioneer stereo is cheaper that both.
    
    Garry

	Well, 15000 miles old, and the ABS warning light has come on and
	stayed on. The garage says its nothing to worry about, the manual
	says it merely means that one or more wheels are no longer under
	the control of ABS, it should be covered by the unlimited warranty,
	but I have this slight sensation of fear of the unknown every
	time I push the brake pedal, and I wish I had normal brakes!

	Anyone else had ABS pack up?

	Jonathan.

    yes, mine went west after about 2 or 3 months. The abs brain went down,
    but BMW put a new one in no trouble.
    
    
    Garry

    In all the cars I have had with ABS (two Grandads, one BX GTi and a
    Carlton) the light comes on if the ABS goes down, but fails safe to
    normal brakes. Thus, you now have ordinary, non-ABS brakes until you
    get it fixed. 
    
    Steve

	Presumably I've still got dual circuit braking?

	Jonathan.

    Should do - its only the automatic cadence braking that has gone,
    assuming I am right and it is only the ABS that is down. If it is
    stopping straight and no leaks etc, you should have all your other
    braking faculties.
    
    Steve

    .18  states that if all four wheels are locked on a car fitted
    with ABS, ABS believes the car is stopped. I don't believe
    this to be true. 
    
    Since ABS "releases" the brakes when it detects a wheel is locked 
    - there is at least one sensor per wheel, not one for all four - 
    if the car is moving the wheel that is released will turn. As soon
    as the wheel turns, ABS will "know" the car is not stopped.
                         
    

    Yes, I agree that there are sensors on each wheel, but your view is a
    little simplistic. A car's wheels do not stop instantaneously, nor
    simultaneously. Thus, it is very improbable that they will all lock up
    quick enough (milliseconds if not microseconds) to fool the computer.
    Also it ( the computer) should be able to track the rate of stopping,
    thus also giving it a basis for deciding if it's normal braking or a
    lock-up.
    
    Thus I don't believe ABS ever needs to release a wheel to check if it's
    got a lock-up it's already determined it!.
    
    Richard

I believe that ABS is a little cleverer that just waiting for a wheel to stop
and then releasing it - what I would expect it to do is to look at the rate of
change of wheel velocity in addition to the instantaneous velocity. It can 
then predict when and if the wheel is going to stop before the next pulse from 
the sensor, and release the brakes to prevent this - the pulse rates are 
likely to be something like 32 per wheel rev I would guess.

So - if the instantaneous velocity is zero  AND the rate of change of velocity 
is zero, the car is stationary and no action needs to be taken!

    I heard of a "woman driver" who had an accident, hitting another car
    with her ABS fitted car.  The Police who attended the accident wouldn't
    believe her that her brakes had failed - They always "fail safe on ABS
    equiped cars madam" they said and proceeded to "do" her.  Before the
    Police went away, she finally managed to persuade one of the Policemen
    to try her brake pedal, whereupon it went straight to the floor!
    
    	They "undid" her!
    
    	Like everything else, ABS can fail failed as you might say, leaving
    you with nothing.  Whilst ABS has certain major advantages, it is not a
    substitute for safe driving - says one who hit the back of someone else
    last year at the top of the slip road coming off the M4 into DECpark
    last year.  It is, however, one more thig to go wrong with a car.
    
    				Malcolm.

    re .31
    
    Was it the ABS that failed, or some other part of the braking system ?
    If you lose all your brake fluid then no ABS system will keep your
    brakes working.
    
    Andrew

    Does anyone know what the orange ABS warning light actually means when
    it comes on while breaking (I know it always comes on when starting)?
    
    Mine came on once today while the ABS was "abs-ing" and only went off
    when switching the engine off.  It has not come back on since.
                                              
    The manual only names the light - it doesn't say WHY it comes on!
    
    Jc
    
    

    It usually means that the ABS is no-longer functioning. As ABS systems
    are designed to failsafe, it is telling you that your brakes are now
    the same as Mr Average's.
    
    This happend to me when one of the wheel sensors gave up the ghost on
    my Calibra.
    
    Clive

    It probably depends on the car, but on mine (a Mondeo) it appears to
    mean that the ABS is non-operational.
    
    I know this from an 'experience' when I was going up a hill in the
    snow, I was skidding all over the place, wheels spinning etc. The ABS
    light came on and stayed on. I found when I wanted to use the brakes
    going down the other side that the ABS was not operational. The car
    stalled as I ran into the ditch. When I started the engine again, the
    ABS worked again.
    
    It could be that the skidding about got the ABS confused and decided to
    switch off. I would be concerned if the same thing happened during
    'normal' operation.
    
    Graham