> If I have a car with ABS that cannot be disabled is it
> reasonable for me to assume that the car will stop in
> the shortest distance possible if I simply slam on the
> brakes (thereby engaging ABS)?
No.
- ABS is not necessarily superior to manual threshold braking, though it
should be close for most conditions. This is the lesser of the reasons.
- Slamming on the brakes will cause the front brakes to lock (engaging
ABS)before significant weight transfer occurs. Many/most ABS systems do
not deal correctly with increased traction during ABS cycling.
> If this is not so then what sort of penalties are likely
> in this scenario? In other words does a good ABS system
> get within 10-20% of ideal non-assisted braking? How
I think you're asking the wrong question. Look at it this way: ABS
determines what happens to your braking on a per channel basis once the
wheel is locked up. The better question is: why are your wheels locking
up?
> about in the wet (where yesterday I had abysmal braking
> most likely due to driver errors)?
ABS is probably most effective in the rain, next in dry conditions, and
can be a significant detriment in loose traction conditions such as sand,
snow, or Golden Gate Fields. The great value in the rain is that if one
side of the car hits a puddle and experiences significantly less traction
than the other side, you don't lock that wheel and throw the car into a
very unstable situation - the wheel keeps turning while providing what
braking capacity it has.
A similar effect is of value in the dry. If you are threshold braking
while the car is undergoing ANY lateral acceleration; you have two choices
- you can brake to the traction limit of the outside wheel, which will
cause the inside wheel to lock/enter ABS, or you can brake to the traction
limit of the inside wheel, which will cause the car to slow down less
rapidly. * Without ABS, most people choose option B, because flatspotting
tires gets expensive, and because locking the inside wheel upsets the car
even on dry pavement. With ABS, most people choose option A, braking to
the threshold of the outside wheel and allowing ABS to handle the minor
locking on the inside. However, some ABS systems are crude enough that
even this amount of locking is counterproductive.
> Also if engaging the ABS is evil what techniques can be
> used to avoid the dreaded HAL9000 intervention?
In general you want to avoid getting into ABS on more than one occasional
wheel at a time, possibly a bit more than that in the rain. Earlier
braking helps, more importantly understanding the weight transfer and how
to increase braking as more weight improves traction helps.
My general rule is that braking too late is the most costly mistake you
can make on an autocross course, while braking too soon is the least
costly mistake you can make.
KeS
* I am ignoring the rear axle for the purposes of this discussion. My
experience is that in any modern stock car, the brake bias is adjusted so
far towards the front wheels that it is impossible to lock the rear wheels
during normal maneuvering even with race tires installed. There are ways
to compensate for this, but its really a separate topic.
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