8b.4 Blowouts and Sudden Flats

Date:    Fri, 11 Jun 1999 7:42:45 PDT

Bicyclists often report tube failures that they believe occurred
inside the tire casing.  They believe these are caused by a faulty
tube that split or that the rim tape failed.  However, they also heard
a bang after which the tire was flat.  On removing the tire casing
from the rim with tire irons, they discover a burst tube with a large
slash.

If there was an audible bang, then the tire was off the rim.  That the
undamaged tire is still on the rim afterwards proves only that tires
usually fall back into place after exposing the 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.  Beyond that, the resulting clean slash
in the tube cannot occur from rim tape that would cause a gradual
failure along an abraded line that extends beyond the end of the
split.  A burst into a rime hole 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 to a single bicycle, where most of the energy is
dissipated by wind drag.  Rim heating has two effects, of which
increased pressure is probably the lesser one.  Heat softens the bead
of the tire so that it can squirm out of its clinching seat in the
rim.  Tire casing flex at the load point works the tire so that it
squirms out of engagement.  Heat also increases lubricity of the bead
against the rim to facilitate creep.  Formerly, base tapes made of a
dense gauze-like tube, filled with Kapok, were offered for mountain
tourists.  This padding served as insulation to prevent the rim from 
heating the tube  and increasing pressure.

Short tubes, that must be stretched to fit on the rim, can cause tire
blow-off.  A stretched tube will occupy the space on the bed of the
rim where the tire bead should be to make proper engagement with the
hook of the rim sidewall.  The tube under the bead of the tire can
prevent proper engagement with a hooked rim to cause a blow-off even
without great heat or pressure.

Valve stem separation is a failure that is less dangerous because it
usually occurs during inflation.  It causes a slow leak when occurring
while riding, as the vulcanized brass stem gradually 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.