ABS

[John Hess]

Do you drive on ice?


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From: 
 RABBITT_Andrew at mail.orbeng.com.au[SMTP:RABBITT_Andrew at mail.orbeng.com.  
au]
Sent:  Thursday, January 30, 1997 8:06 AM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  re: ABS


>Unfortunately, on very slick surfaces such as ice, all four wheels
>lock up at once.  Generally the ABS systems work on a speed
>differential algorithm.  If a wheel is going appreciably slower than 
>another (the others), then that wheel is considered locked and the
>brake system is modulated.  If all four are locked at the same time
>(at least at very slow vehicle speeds), the system sees the vehicle
>as stopped and is not activated.  This is why ABS is generally found 
>to be ineffective on ice.

Some ABS systems have chassis mounted G-sensors which I presume are
used to compare wheel deceleration with vehicle deceleration.  If all 
four wheels lock instantaneously then you would be able to detect this 
by such a comparison.  In any case there would be a limiting value of 
wheel deceleration, beyond which a lock-up condition could be
inferred.

Andrew Rabbitt