This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Mon, 13 Oct 1997 15:02:23 PDT
Although the tire wiping has mostly gone the way of the tubular tire,
some riders have remained believers in this practice, that never had
any validity in the first place. It is purportedly done to prevent
punctures by wiping off glass that may have "stuck" to the tire.
If one considers the rotation rate of a wheel in typical bicycling,
about 15-20mph, it comes to about 3.5 revolutions per second. When
observing a tire wiper, the time between noticing hazardous debris on
the road and the first wipe is more than a second. Hence, any glass
or other small object would be firmly pressed into the tire by four
revolutions and all exposed glass edges chipped off. By the time the
other tire is wiped several more seconds will have passed. If the
glass is not thoroughly embedded by then it will not enter the tire.
This is not to say that particles embedded in a tire always cause a
leak immediately, but that they are irrecoverably in the tire at that
time. Those who have patched flats from glass will recall that the
piece of glass is not easily found, especially if the location of the
puncture is not known. The embedded chip is usually imperceptible
when wiping the hand over the place even when known.
On the other hand, the rear wheel is more subject to flats than the
front, because flat objects must first be tipped up to engage a tire
to have any effect. Wiping the rear tire on common short frame
bicycles is hazardous, because the fingers can be sucked into the
narrow gap between tire and seat tube to cause serious injury.
Carefully considered, tire wiping is an idle gesture, reassuring to
some riders, and impressive to others if deftly executed. I recall as
a beginner that learning all the tics of bicycle racing was important.
Wiping tires was one of these. Forget it.