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Re: The poop on ABS
At 12:01 AM 8/24/99 , Dan Brideau wrote:
>Friends and neighbors,
>
>ABS equipped cars see, on average, a 6.5% stopping distance benefit over
similar non-ABS cars.
>
>You non-believers in ABS got me worked up. Month after month I see
complaints that your cars are too slow, and what can I do to make it go
faster.
Thank you, Dan.
I, personally, am sick to death of defending the benefits of ABS to people
who never bother taking the time to learn how and why the system works, and
who blame it for causing accidents that would have happened with or without
ABS.
Why, just the other day, on this very list, I laughed to myself as someone
on this very list blamed ABS for making his wife's Honda rear-end someone,
and then went so far as to ludicrously accuse ABS for the brake dive which
caused the front of his car to hit the car in front below the bumper.
Buddy, no offense-- but get your facts straight: ABS doesn't cause Brake
Dive; PHYSICS does.
People, wake the fuck up. ABS will _NOT_ increase your stopping distances
on dry pavement. And until you can modulate all 4 wheels individually, 15
times a second, in a panic stop, get it through your heads that it will,
under just about every condition you'll encounter driving, help you stop
more quickly, and (most importantly) maintain steering control while
staying at the limits of the tires' adhesion.
You can NOT modulate each wheel individually, which means you can NOT
compensate for one wheel which will lock up before the others do. You can
NOT modulate your brakes quickly enough to keep the wheels at impending
lockup over the changing surface and surface texture of a road... or to
compensate for weight distribution changes from bumps. You can NOT
suddenly apply enough force to your brake pedal to have the wheels achieve
maximum braking instantly without locking your wheels. Put all these
together: YOU CAN NOT STOP YOUR CAR AS QUICKLY AS YOU CAN WITH ABS ON DRY
PAVEMENT IN THE REAL WORLD. END OF STORY.
Yes, if you spend umpteen thousands of dollars with sophisticated equiment,
you could probably apply a series of proportioning valves to your brake
lines so that each wheel brakes which exactly the correct force to keep it
at impending lockup. However, despite all the (rediculous) time and money
spent, the first time you have a 20 pound bag of fucking GROCERIES in the
trunk, all of that will be thrown off. Forget about what happens around a
turn, or over bumps.
An ABS system has control over each wheel individually. You cannot; you
have one brake pedal for all 4 wheels. Your right front tire, for
example, might be at impending lockup, giving fully 100% of its maximum
braking capacity. However, the front left wheel might be at 90%, and the
rear wheels at 60%. You could hit the pedal harder to bring the left front
up to 100%, but then you're locking up the front right, which diminishes
steering control significantly, and reduces that tire's braking
effectiveness to below 100% anyways.
Dan, and others, here's an exerpt from one of my famous rants about ABS on
the A2-16v list. We had a huge debate on that list months and months ago
on this very topic. Jason spoke, and everyone shut their mouths. You
can't argue with facts. So, non-believers, go to bed with your physics
book and learn the facts before you start accusing one of the Automotive
Industry's greatest technologies for causing accidents, when it was you in
the first place that caused the damn accident.
Jason has spoken.
Read on:
At 09:53 AM 5/27/99 , Williams, Chris (RSCH) wrote:
>Terry wrote:
>
> Only on the most advanced systems does ABS individually modulate the
>wheels
Okay... I found the Road and Track Article about the effectiveness of ABS.
My scanner died last night (argh), so I will type only the relevant parts
of the article. Please ignore any spelling mistakes, I'm at work and wanna
get this done as quickly as possible:
Quoted from Road & Track, July 1997, pp 99-105
Since its widespread introduction in the early Eightes, ABS has garnered
its share of plaudits, certainly from the likes of us, for example, in our
instrumented measurement of dry-road straight-line panic-stop braking. Its
efficacy has also been demonstrated in situations displaying uneven grip
side-to-side, technically, on "split-u" or "split-coefficient" surfaces"
(This is what I said yesterday fwiw).
There have been plenty of stories as well concerning ABS tradeoffs,
typically incolcing gravel, sand, snow, or ice. However, to the best of
our knowledge, these have been largely undocumented until now. Our goal
here is to apply instrumentation and scientific methodology to what has
hitherto been folk legend.
By way of foreground, the idea of ABS is to prevent wheel lockup in
emergency braking,. The principal advantages of this are twofold: First,
an unlocked wheel offers steering control; a locked wheel gives next to
none. Second, a wheel that's almost, but not quite, locked (at "incipient
lock") typically offers a shorter braking distance than one that's
completely locked.
Our testing here focuses exclusively on this second measure, braking
distance, performed in a straight line. However, before we go on, we
cannot emphasize too stronghly the importnace of the of the first aspect,
so let's restate it here (and repeat it later on): An unocked wheel offers
steering control. It offers you, the driver, another tool to avoid
whatever emergency is ahead. A locked wheel, by contrast, implies that you
and the steering wheel are pretty much along for the ride.
In the briefest of overviews, ABS uses sensors to monitor wheel rotation,
then invokes a rapid cycling of hudraulic pressure to the wheel that's
exhibiting lockup.
In fact, though, it's a lot more complicated. Front wheels usually get
individual control. The rears are often paired reacting to the less-grippy
side ("select-low" strategy) or the one with more grip ("select-high"). ...
How we did it.
For our test car we chose a Mercedes-Benz, one of the first marques to
offer ABS, with excellent systems today and at the forefront of ABS
research and development for tomorrow. Our 1997 S600 Coupe was modified so
its ABS could be turned on or off by driver control.
(The tests were conducted on a number of different surfaces: Ice, Hard
Pack Snow, Scarfed Snow, Fresh Snow, Gravel, Sand, Wet Pavement, and Dry
Pavement. Since I'm arguing :) with Inaki about stopping distance on _dry
pavement_, I'll omit the rest of the results).
Home again in the dry
We returned to familiar ground for the dry evaluations, with out usual
initial test speed of 60 mph. It came as no surprise that ABS beat the
other strategies handily. Lockup was dramatically longer, its 213-ft
distance being 60 percent longer than the 133 ft. achieved by ABS.
Two other points were noteworthy as well. First, Kim found that this
particular brake system was rather less amenable to driver modulation than
the best he recalled from pre-ABS days. What's more, the data corroborate
this, with the Educated strategies being rather off the pace compared to
pure ABS use.
Second, we were awed by the sight of this 2 1/2 ton car sliding with all
four wheels locked, Kim striving to keep it imore or less straight but
essentially just along for the ride. This lurid display of Physics was
quite enough to reinforce our faith in the ABS concept, its apparent
tradeoffs notwithstanding.
DRY BRAKING DISTANCES FROM 60mph
ABS ************* 133ft
Educated ABS ***************** 167ft
Aducated Lock ***************** 171ft
Lock ********************* 213ft
(confidence window = 6ft)
Clearly, the car with ABS stopped shorter. The "Educated ABS" attempts
were when Kim Reynolds tried to modulate the brakes himself with ABS
switched on. "Educated Lock" was when Kim attempted threshold braking.
"Lock" is when he mashed the pedal and let all 4 wheels lock. For the
"ABS" attempts, he mashed the pedal with ABS on. The confidence window is
the result of statistical tests to determine the stasticically significant
difference.
---------------
Inaki said "And again, on dry pavement, a car with ABS generally (I said
GENERALLY) takes longer distance to stop."
With the results above, I hereby declare war on his statement. :)
"Generally" refers to the situations under which most drivers would be
found... and that basically means that people will slam their foot on the
brake pedal with all their might when in a panic stop. The results speak
for themselves - 133ft vs 213ft. That is a _huuuuuuugggge_ difference!!
That is a life or death difference.. and it's very sobering.
However, to look at the data one step more closely-- despite the fact that
the driver felt that the brakes were more difficult to modulate than other
cars-- Kim Reynolds is the Road Test Editor of R&T! He does this every
single day of his life... and that means, as far as I'm concerned, without
a shadow of a doubt, that he was able to modulate those brakes better than
any one of us would have been able to.
The bottom line, though, is when put in a true panic situation, most of us
don't have the presence of mind to modulate our brakes. Yeah, okay, I've
done it a few times perfectly... but then there's that time when you least
expect anything to happen, and you're dancing around to some music, and all
of a sudden *BAM* someone pulls out in front of you and you mash the pedal.
Look at what happens-- 133ft vs 213!! Or even let's say that you had the
presence of mind to threshold brake... you would have still stopped almost
40 feet shorter if you had ABS... And 40 feet is a _long_ distance. Think
about it-- go outside and take a look at a 40-foot section of road. And
then imagine one car stops a the beginning, and one at the end. And then
imagine what those distances would be at 70mph, and 80. ****FROM ONLY
60MPH, with locked wheels, that S600 took 30 feet shorter to stop than it
would have from **80** with ABS!
Now, in all fairness, here are the results from the rest of the conditions,
under many of which ABS took longer to stop:
(*) denotes "winner"
ICE: (from 20mph)
ABS 220ft
Educated ABS 222ft
Educated Lock 134ft*
Lock 136ft Confidence Window: 8ft
HARD PACK SNOW (from 30mph)
ABS 100ft*
Educated ABS 120ft
Educated Lock 135ft
Lock 119ft Confidence Window: 12ft
SCARFED SNOW (from 30mph)
ABS 94ft
Educated ABS 102ft
Educated Lock 108ft
Lock 84ft* Confidence Window: 4ft
FRESH SNOW (from 30mph)
ABS 97ft
Lock 85ft* Confidence Window: 2ft
GRAVEL (from 30mph)
ABS 65ft
Educated ABS 70ft
Educated Lock 63ft
Lock 52ft* Confidence Window: 2ft
SAND (from 30mph)
ABS 53ft*
Educated ABS 59ft
Educated Lock 60ft
Lock 54ft* Confidence Window: 4ft
WET PAVEMENT (from 30mph)
ABS 37ft*
Educated ABS 42ft
Educated Lock 46ft
Lock 45ft Confidence Window: 1ft
Enjoy
Jason
----------
1987 Scirocco 16v
1989 Mercedes 190E Sport Euro
http://members.aol.com/rocco16v
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