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

[Paul Holzworth <hph@fns.com>]

Bringing up some old college physics:  The idea is that the coefficient of friction (COF) of a locked up tire dragging across the surface is less than the COF of a non-locked tire (dynamic vs static COF).  The stopping force is maximally the weight x COF.  The static COF for materials is always (almost always?) greater than the dynamic COF.  For maximum braking ability, you want to brake to the point that the tire is just beginning to lock up and then let the pressure off for just enough time to make sure the tire doesn't lock and then apply it again.  The duty cycle is not really relevant because you have reached the maximum braking force at the instant the tires start to lock.  Any further braking pressure will put you into the realm of dynamic COF where the friction (braking action) will be less.  This is the theory.  Now of course the implementation may not be effective.  For instance, if you can't modulate the pressure fast enough, you may allow the brakes to lock temporarily or you may not reassert braking pressure quick enough.  I guess the conclusion is that ABS may or may not be more effective depending on the implementation.  In theory, it certainly can be more effective to use ABS.   Does anyone know more about Audi's ABS such as modulation rate, sensor mechanism, etc? 
Paul Holzworth

At 5:57 PM -0500 12/7/97, Phil & Judy Rose wrote:
>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