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

[DeWitt Harrison <de@aztek-eng.com>]

On Sun, 7 Dec 1997 17:57:15 -0500 (EST), Phil 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). [ ... ]
>[ ... ]

Kuddos to Phil for going out and taking data! We should all do a lot
more of that.

>[ ... ]
>I think  what originally made me object to the claim for non-ABS stopping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?

I agree with your logic. I believe that the reduction of braking during the
OFF part of the ABS braking cycle - I wonder how far the braking system
pressure is actually lowered - simply allows the car to roll forward
part of the time during the stop.

>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?

I like ABS on the track. Not having one or more wheels lock up coming into
the corners really simplifies my life.

DeWitt Harrison    de@aztek-eng.com
Boulder, CO
88 5kcstq