Subject: Tube Failure in Clinchers; Valve stem separation flats
From: Jobst Brandt
Date: October 13, 1997

Riders occasionally tell about a tube that blew out with a loud bang INSIDE their tire, leaving the tube with a long slash. The tube blew out, but not as described. If there was a bang, the tube was outside the tire. That is, the tire lifted off the rim and fell back in place after the tube burst.

Tubes do not burst inside tire casings, although they may leak, the most they can do is give off an audible hiss, assuming it is otherwise quiet enough. An un-socketed double walled rim can make a dull pop if the tube is exposed to the inner rim volume. The concept that a tube can explode inside a tire is dangerous, because it leads people to believe that tubes can mysteriously fail without apparent cause INSIDE a tire. With few exceptions, the cause is an improperly mounted tire.

Without understanding the cause, a rider may continue to risk a blowout, without realizing that tire lift-off can be caused by the tube lying between the rim and the tire bead. In this position, the tube prevents the tire from seating properly in the hook of the rim, a condition that, under the right circumstances, will cause a blowout. This cannot occur inside the tire casing. To prevent blow-off, the tire seat must be inspected by pushing the tire away from the rim, upon which the tube should not exposed at any point around the tire.

Valve stem separation is another common failure, but it is less dangerous because it usually occurs while inflating the tire. If it occurs while riding it causes a slow leak, as the vulcanized brass stem separates from the tube. When this occurs, the stem can 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.

  Valve stem separation flats
Date June 16, 2003

A flat caused by valve stem separation, a manufacturing flaw, is less dangerous because it usually becomes apparent during inflation. If it occurs while riding, it causes a slow leak as the vulcanized brass stem gradually separates from the tube. When this occurs, the stem can be pulled out of the tube 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 into the hole left by stem separation. Such a used stem should be part of a tire patch kit. Any good bicycle shop that handles tubular tires or latex tubes should have used ones if they weren't thrown away.

In a self accusative manner, riders often place blame for this failure on errant inflation, the use of the anchor nut on the stem, or some other feature of the rim that they failed to ameliorate. On close inspection, separated stems show that the rubber peeled away leaving only a slight black trace on the stem where the leak began. This isn't caused by any of the usually believed mechanisms. It is a manufacturing flaw.

See also: Sheldon Brown on Flat Tires

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