From: johnl@pcx.ncd.com (John Light)
Newsgroups: rec.autos
Subject: Re: Anti-Lock Brakes makes stopping SAFER?
Date: 29 Dec 92 21:10:10 GMT
Article-I.D.: pcx.1992Dec29.211010.18287
Organization: Graphic Software Systems, Beaverton, OR

In article <1haiiaINNanq@usenet.INS.CWRU.Edu>, ce423@cleveland.Freenet.Edu (Josh Lackey III) writes:
> 
> 
> I've yet to drive a car with ABS. I've lost control of my
> car once, and have nearly lost control several times since. 99% of my
> close calls were on wet or drenched pavement. In every near loss of control
> incident I can remember, the braking system was not a factor.
> All of my close calls were at highway speeds and resulted from
> hydroplaning. Dont think ABS can be effective during 
> hydroplaning conditions?

I'm a strong advocate of ABS brakes because they increase braking quality.
That is, they reduce the variations of braking results from expectations.

Yes, I believe they can help during hydroplaning.  Keep in mind that if
there is no traction available, e.g., sheet ice, ABS can't help.  But
frictionless conditions are rare.  When you hydroplane on a highway, you
actually travel over a large difference, with small changes in water
depth and surface texture.  With normal brakes, once hydroplaning starts,
it tends to continue indefinitely.  Consider your dilemma:  should you
step on the brakes or not?  The answer depends on exactly what is
underneath your tires at each instance.  When you are actually hydroplaning,
you want to take your foot off the brakes.  When you stop hydroplaning,
even for an instant, you want to apply the brakes so you stop moving in
the new direction/attitude you have aquired while sliding.  Most people
are unable to make the minute adjustments to braking that are needed when
hydroplaning.

With ABS, you hold the brakes on.  If the road rises enough that
the water is shallow enough to allow traction, ABS will sense the change
within milliseconds and grab whatever traction is available, even if the
road drops, and you start hydroplaning again.  Most importantly, when
you finally leave the water, you are stopping ASAP.

The same scenario applies to ice.  What you may see as sheet ice really
consists of several sheets of ice connected with rough ice.  ABS can
use the small amounts of traction at the junctions to slow your vehicle.
I defy anybody "pumping" the brakes to cycle the brakes fast enough to
outbreak my Previa on ice.

The one scenario where ABS may perform less effectively than normal brakes
is when a soft surface overlies a harder, but frictionless surface.   Two
common examples are loose snow over ice and sand over gravel.  Locking
normal brakes will build up the softer material under the front of the
tires.  For these rare circumstances, ABS may not brake optimally.

> I'm wondering what the consensus
> is for ABS users as to the usefulness and added safety they feel
> the system provides? Please include specific examples where you
> feel the ABS provided you with an extra margin or safety!

I was cruising down I-5 in a rural area a couple years ago in the right
lane at 70 MPH with no traffic in sight.  An older American car zoomed
past me, pulled over into my lane and slowed to about 55.  I thought
nothing of it, so I continued my leisurely driving.  As I approached the
car, I pulled into the left lane and started to pass, at which time my
wife's scream helped me notice that the car was pulling left.  I slammed
on the brakes and pulled left.  I need to do both to avoid a collision.
The center of I-5 had an unforgiving solid barrier, and the narrow left
margin was covered with gravel.  My left wheels were braking on gravel
covered asphalt.  My right wheels were braking on solid asphalt.  And I
was inches away from a solid barrier.  All in a fraction of a second.
If my van had fish-tailed at all under heavy braking, it would have hit
the barrier while I was going 70.  The most likely outcome would have
been a spin at speed.  Instead, nothing hit anything.  The car then
sped away.  I feel that ABS saved my whole family.

The key word is "unexpected".  People talk about what a trained driver
on a track can do without ABS.  I prefer to talk about what most of can
do when the unexpected happens.  For most people ABS is a big win.
> 
>   Some ABS systems now incorporate traction control, but my
> perception is that ABS for the most part only provides added
> margins of safety during emergency braking conditions. 

If you think of braking as planned stepping on the brake on a straight-away
in daylight in dry kweather, I'll grant your description of "margin".
If you can imagine braking at night in the rain around a curve when you
least expect it, I think "margin" is too weak a word.

>    One last question:  Are all ABS systems designed to monitor 
> all four wheels on the car, or are some only active in front?

ABS has the following differences:

	How many wheels are sensed?
	How many wheels are controlled?
	How many independent controls are there?

The answer falls into three categories:

1) rear wheel ABS.  Only the output shaft of the transmission is sensed.
   The two rear wheels are controlled.  The rear wheels are controlled
   together (they are both either engaged or not).  (Trucks have this.)

2) 4 channel ABS.  Sensor on each wheel.  Each wheel is controlled
   independently.  (Luxury and performance cars have this.)

3) 3 channel ABS.  Sensor on each wheel.  The front wheels are controlled
   independently.  The rear wheels are controlled as one, but independently
   of the front wheels.  (My Previa has this.)

Has anyone seen another layout?

Hope this helps.

John Light
Newsgroups: rec.autos