Stopping distances on snow/ice: ABS or not? (longish)

[pjrose@servtech.com (Phil & Judy Rose)]

Last week I decided to reconsider an assumption I had been making since I
began using ABS (starting with my '89 Probe GT). I assumed that using ABS
gave better-or-equal stopping distances as well as improved control--even
in snow/ice conditions. Qlist responses in a recent thread almost
unanimously disagreed with my view.

I had to accept that a group of people who've been driving Audis with
switchable ABS ought to be in a position to know more about this question
than did I, an Audi newbie with nary a winter under my tires. But since I'm
not one who necessarily accepts a majority view (or the owner's manual), I
decided to start doing some controlled braking tests in my '91 Audi 200.
Well, semi-controlled tests anyway.

I've publishing lots of data over the span of a 30 yr career, so I'm
prepared to be laughed out of town for reporting the statistically dubious
(some might say "meaningless") results that follow. But real life ain't
always a chemistry lab, and sometimes it's useful to set forth preliminary
results--with the usual caveats. I repeated the test runs in rapid
succession over the same portion of road. Everything was held constant,
except my long-suffering spouse, whose doubts about my sanity grew as the
tests progressed.

******************
12/06/97
temperature:28 F
road: level, dirt/gravel surface with 1" to 3" ice, packed snow + approx.
1/2" fresh snow
vehicle: '91 200q (calibrated spouse in front passenger seat)
tires: Goodrich Comp T/A, new, 33 psi
speed: 32 mph

ABS ON  (full panic mode): 100ft (+/- 5 ft)
ABS OFF ("     "     "  ): 75-80 ft
ABS OFF (modulated)      : 110 ft
*******************

12/07/97
temperature:32 F
road: level, dirt/gravel surface with 1" to 4" ice, packed snow + approx.
1" fresh snow
vehicle: '91 200q (no passenger aboard)
tires: Goodrich Comp T/A, new, 33 psi
speed: 40 mph

ABS ON  (full panic mode): 170 ft (+/- 5 ft)
ABS ON  (full panic mode): 155 ft
ABS OFF ("     "     "  ): 125 ft
ABS OFF ("     "     "  ): 120 ft
ABS OFF (modulated)      : 145 ft
*******************

Tentative conclusions:
(1) Not nearly enough data here to establish the repeatability of the
measurements to anyone's satisfaction, but enough to shake my preconceived
notions.

(2) Although ABS clearly provided superior control (i.e., there was no
significant loss of directional stability), its use appears to _increase_
the stopping distances by as much as 25% in a panic stop. Modulated non-ABS
stopping distances are likely to be more variable than with panic-mode
non-ABS, although I'd expect now that even modulated non-ABS stops can
sometimes be shorter than with ABS.

So it appears I was certainly wrong about ABS stopping distance. When
steering control is not the crucial consideration, keeping ABS-off seems to
be the best choice for shortest stops.

I think  what originally made me object to the claim for non-ABS stopping
distances (being shorter than ABS) was the suggestion of a "snowplow
effect". I  have trouble--even now--accepting that this is the primary
factor (except in deep snow). An acceptable explanation, in IMHO, is simply
that on a slippery surface the ABS will usually cause very extensive brake
pressure modulation, so  the brake duty cycle becomes significantly less
than 100%. During the "pressure off" segments of the duty cycle, the only
forces slowing the vehicle (other than air resistance) are (1) tire rolling
resistance and (2) internal (bearing) frictional forces. But if frictional
forces from locked (non-ABS) tires--even on the slippery surface--are
greater than the sum of (1) and (2), then ABS will be the loser. Of course,
one might argue that locked-up tire friction on these slipperey surfaces is
so high only because of material plowed up at the tires' front edge. I
dunno. What about clean ice? Is ABS still the poorer performer here?

On dry, hard surfaces, without any possibility of a plowing effect, I would
think that ABS still ought to lose the panic stopping-distance contest,
based on the reasoning I offered above. Yet the statement in our Audi
manuals seems to imply that there is no advantage (in stopping distance)
obtained from switching off ABS under these conditions. Anyone know of data
that applies here?

Phil Rose