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Subject: Blowouts and Sudden Flats
From: Jobst Brandt
Date: August 11, 2004

Bicyclists often report tube failures that they believe occurred inside a tire casing. They believe these are blowouts caused by faulty tubes that split or were cut by the rim tape. However, they also heard a bang, after which the tire was flat. On removing the tire casing from the rim with tire irons, the burst tube is found to have a long slash.

If there was an audible bang, then the tire was off the rim, exposing the inner tube. However, the undamaged tire usually remains on the rim because tires usually fall back into place after exposing a tube. A tube cannot blow out inside the tire with a bang, because a bang is caused by a sudden change in volume, an expansion. Such an expansion is not possible within a tire casing that is essentially airtight.

The resulting clean slash in the tube cannot occur from rim tape: that would cause a gradual failure along an abraded line extending beyond the end of the split. A burst into a spoke hole in the rim would cause a starburst hole that is smaller than the rim socket because the tube shrinks when no longer inflated.

Tire blow-off occurs most commonly on tandems where substantial energy of descending mountain roads is converted to heat in rims by braking. In contrast, a single bicycle is usually able to dissipate enough of its descending energy by wind drag to not suffer from this. Rim heating with rim brakes on continuous steep descents can increase inflation pressure substantially. For this reason some mountain passes in the Alps prohibit descending by bicycle, while uphill riding is permitted. For instance, Zirlerberg between Zirl and Seebach (Innsbruck), a major road between Germany and Austria, is one of these. The road has several runaway tracks for motor vehicles with brake failure.

Formerly, base tapes made of gauze-like tubes, filled with Kapok, were offered for mountain touring. The padding served as insulation between rim and tube to prevent rim heat from increasing pressure. These rim tapes have not been available lately, probably because bicycle shops did not recognize their purpose.

Short inner tubes which must be stretched to fit on the rim can contribute to tire blow-off, because a stretched tube tends to rest in the space on the bed of the rim where the tire bead should seat for proper engagement with the hook of the rim sidewall. A tube under the tire bead can prevent proper engagement with a hooked rim to cause blow-off even without excess pressure.

Valve-stem separation is less dangerous because it usually occurs during inflation. While riding, it generally causes a slow leak, as the vulcanized brass stem gradually separates from the tube. When this occurs, the stem can usually be pulled out entirely to leave a small hole into which a valve stem from a latex tube of a tubular tire will fit. Stems from tubulars have a mushroom end, a clamp washer, and a locknut that fit ideally. Such a used stem should be part of a tire patch kit.

Tubes with an encircling ribbed zone near the stem are "welded" together at this point and have occasionally developed a leak from no external cause other than tire flexing. Stretching the tube manually at the joint can expose this weakness before installation. Both this defect and stem separations are quality-control problems that in time may be resolved, considering the many tubes of similar manufacture that do not display these faults.

Jobst Brandt

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See also Sheldon Brown on Flats

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More Articles by Jobst Brandt
Next: Snakebite Flats
Previous: Tube Failure in Clinchers

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