Subject: Wiping Tires
From: Jobst Brandt
Date: October 13, 1997
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.
I'm not entirely convinced by Jobst's argument above. At 3.5 revolutions per second, the bike will move roughly 18 feet per second. Glass accumulations can often be spotted at such distances. A tire wiping cyclist will initiate the wipe when the glass is observed, not when the bike runs over it.
There's also the issue of how long it takes for the glass to penetrate sufficiently to cause a leak. It is not clear to me that the glass goes in all at one go as Jobst seems to believe. Here's my reasoning:
Pavement is granular, so the glass sliver is likely to begin in one of the concavities of the road's texture. As the tire rolls with the sliver stuck partway through, it may make repeated contact with the roadway, sometimes hitting low spots, sometimes hitting high spots. In some cases it may not penetrate far enough to cause a leak until it has hit a sufficiently tall projection of the roadway surface.
There's also the issue of banking. If the bike is rolling straight when it picks up the glass, and the glass gets stuck somewhat off-center on the tire, it may not get fully driven in until the bike has leaned over into a turn on that side.
For these reasons I believe that Jobst has not proven his case, and that there may indeed be occasions when tire wiping can prevent a puncture.
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