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8b.3 Snakebite flats


This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.

8b.3 Snakebite flats

From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Mon, 23 May 2001 14:13:14 PDT

Snakebites, otherwise known as pinch flats, are so called because they
usually cause adjacent punctures about 10mm apart (for tires with
about a 25mm diameter cross section). They occur when the tire casing
bottoms on the rim, causing a compression failure in the tube for both
clinchers and tubulars, much like pinching the cheek with thumb and
forefinger. The finger tips simulate the tire casing and the cheek
the tube.

Reasonably inflated tires can bottom when crossing RR tracks, riding
up a driveway with a raised lip at street level, or riding on rough
roads with ruts and rocks. Although higher inflation pressure helps,
it does not guarantee protection. Watching how, and how fast, such
obstacles are encountered helps more.

Because latex rubber of tubes commonly used in better tubular tires is
several times more stretchable than common butyl rubber, such tubulars
are less susceptible to snakebites. When sheet rubber is compressed,
it stretches laterally like a drum skin, and the farther it can
stretch the less likely it is to tear. In contrast, when ridden over
such obstacles, tubular rims are often dented without the tire going
flat. However, because thin latex tubes hold air so poorly that they
must be inflated daily, snakebites from under-inflation were more
common in the days when most riders rode tubulars.

Snakebites can be identified by inspecting the tube under grazing
light that will reveal diagonal tire cord impressions at the
perforation. This is especially important when only one hole occurs,
the other not penetrating. Riders have claimed that the hole occurred
spontaneously on the underside of the tube and demand reimbursement.

Underside snakebites, the least common, occur mostly on fat MTB tires
that are often ridden with low pressure on soft terrain. At low
pressure, such a tire can roll to one side and pop back, without
disengaging the rim. A snakebite caused by this mechanism appears on
the underside of the tube similar to laying your head to one side
while pinching the skin at the Adam's apple. Such flats are
erroneously attributed to rim tape failure and other obscure causes,
when in fact it was under-inflation that can no longer be assessed.
Here cord impressions also give evidence of a snakebite.


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