Re: E30 ABS

[Scott Ruffner <jpr9c@xxxxxxxxxxxxxxx>]

On Mon, 6 Jan 2003, Rob Ellis wrote:
> But after reading your arguments for your braking method with
> ABS, I'm a bit confused.  If you have a minute to read and reply,
> I'd appreciate it.  I would definitely like to be sure I'm using
> my ultimate driving machine properly (as this is also my first
> car with ABS).
>
> My experience with ABS has been in situations when steering was
> not the issue, only a matter of stopping before I slide _forward_
> into something (like icy parking lots, or approaching a stop sign
> on slippery pavement) from about 10-15mph.

Yes, and in this case it's probably generally true (ie, for most of us)
that ABS will do better than we do at feathering the pedal on the edge of
lockup.

> I've found for these forward slides, jamming my foot down on the
> brake and holding it there stops the car almost immediately (from
> 10mph), whereas my attempts to pump the pedal always results in a
> longer stopping distance (enough that I might hit the car in
> front of me at a stop sign) and a heart rate increase.

Well, fortunately, I haven't had a "stoppie" in a while, at least not
while driving this car, so as I said before, I may not be able to beat the
ABS for straight-on, dry pavement stopping.  I would say in general that
we're all better off using ABS for this situation since we can't cycle the
pedal nearly as fast as the ABS pump can.

> Now, it may be a reasonable argument that I'm not a very skilled
> driver.  However, I got pretty good at pumping the pedal to stop
>   when I had a 5.0 Mustang.  And if any car slid in the
> snow/ice/rain/dry pavement/etc this Mustang did.

Well, I pretty well lobbed that insinuation out there, didn't I?  Let me
retract it immediately and apologize.  I don't have a copy of it handy,
but the bit about the troopers in their new Chevy Caprices was in a story
I read, and I do know that I have been able to get a couple of different
non-ABS cars to stop by letting up the pressure on the brakes and getting
some static (non-sliding) friction back on the tires, in situations where
I definitely would have hit someone had I continued with the wheels
locked.  The coefficient of static friction (what you have when the wheels
are rolling) is higher than the sliding coefficient.  I'm sure ABS, which
in theory gets you the maximum amount of friction by keeping you right on
the edge of static/sliding, would have done better.

> So I guess my question is: am I a bad driver, or are there
> differences in the details of our "parking lot tests"?

No, and Yes.  No=not a bad driver, Yes=different test/standard.  These
days I generally keep a bigger following distance than I used to...so I'm
mostly interested in skid/slide control.  Snow and Ice type situations.

My bet is that you would find that at 10MPH, your straight on stopping
power ABS and NON-ABS is very similar.  I think the differential between
sliding/static coefficient on ice is much less than the differential on
dry pavement.  My test was in part with turns involved too...I was able to
regain control of steering in the turn...this is very important to me.

I think (a gross generalization which will doubtless tar and slander many
underservedly) that most drivers do not have the presence of mind to
make the counter-intuitive reponses necessary to regain control of a car:
turn into the slide to get steering back, and let off the brakes to regain
static friction.  ABS is sometimes treated like an idiot-proofing
solution.  I think it's probably a great net gain, but does not mean that
drivers should stop thinking about how they're using their brakes.

> Well, thanks for your time.  I'll appreciate any additional info
> you can provide.

Thanks for your politeness after my insulting remarks.  I should've stuck
to my diagnostic information instead of a diatribe!  I'm just grouchy
about having less and less direct control of things.  I feel like next
they'll take away the clutch and stick and tell me I get better control
using an automatic.

Again thanks for your reply.

Scott
==================================================================
Scott Ruffner
Computer Systems Senior Engineer       Computer Science Department
ruffner@domain.elided                University of Virginia
(434)982-2219                    http://www.cs.virginia.edu/~jpr9c

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