Date:    Wed, 18 Aug 2004 09:21:09 -0700

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 innertube.  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 air tight.

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 up hill 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 tubes, that 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 exposes 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.


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