OK time for ABS 101 class
In a message dated 01/21/2000 5:30:09 AM Pacific Standard Time,
ckotting@iwaynet.net writes:
> Now for the next question: How can an ABS system tell the difference
> between these two states (a) all four wheels locked and sliding, and (b)
> stopped?
>
> Answer? It can't. And, until they come up with a sensor to pick up on
> the abruptly rising adrenaline level in the driver's bloodstream, no
> amount of software tweaking will help. (Okay, a fuzzy logic system that
> compared pre-braking wheel rotation speeds to post-braking wheel
> rotation speeds, AND from that figure out that the indicated rate of
> deceration was impossible, AND conclude correctly that you were indeed
> sliding, COULD do it, but don't count on that getting implemented for a
> while.)
Wrong, the answer is it can and does know the difference between a stopped
car and one that is moving. How does it do this you ask. Simple the system
calculates the G force (both positive and negative) for each wheel
independently as well as an overall G force for the entire car. The system
knows that it is impossible to go from 60 MPH to zero in oh say 0.01 seconds
therefore the car is still moving. (yah I know about hitting a brick wall,
but cut me some slack here) Also a good ABS system will not allow the brake
to lock above a very low speed (2-3MPH) The scan tools that I use in my
training center will display these G force figures, and can be knida fun to
watch
> Herein lies the problem: ABS (current technology) helps a bunch when
> you're in a slightly tricky situation (one or more wheels sliding),
> particularly if its the front wheels sliding and you need to steer. BUT
> it can engender a false sense of confidence in the machine's ability to
> take care of you, which will fail you totally when you are in a
> situation where you really need all the help you can get (all 4 wheels
> sliding).
OK, go back to basic drivers training. The shortest stopping distance is
achieved by NOT locking up the wheels, but keeping the wheels just on the
verge of lockup. Locked up wheels (skid) increases the stoping distance. A
good ABS system will regualte the pressure on each wheel to maintain optimum
braking.
Now as a side note, I took my company car in for service and rented a car
today. On purpose I did not rent another Volvo but got a different car just
so I could see what the ABS was like. Whe I got out of the rental yard I
found an area where it was safe and proceeded to to some bake tests. During
normal the driving the brakes felt OK, when I went to try the ABS they were
not the best and the ABS was slow to react. I wold have to agree that on
this car I probably could out brake it on dry pavement if I was prepared and
paying attention. This is not something I can say about the system on my
company car. It reacts much faster and the brakes are way stronger.
Rick Ewald
|