Subject: Snakebite Flats
From: Jobst Brandt
Date: May 23, 2001
"Snakebites," otherwise known as pinch flats, are so called because they usually cause adjacent punctures about 10 mm apart (for tires with about a 25 mm 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 fingertips 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.
A tire can compress until it lies almost flat against the rim without any damage to the casing, inner tube or rim. A larger tire cross-section provides more protection against snakebite flats and rim damage than overinflation of tiresdoes. -- John Allen
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