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Subject: 8b.11   Talcum Powder for Tubes and Tires
From:  Jobst Brandt <jbrandt@sonic.net>
Date:    Tue, 04 Nov 1997 16:54:17 PST

> I've been told since my first bike that I should liberally dust the
> tube in talcum powder before installing it.  I've believe that this
> may have reduced the number of flats I've had recently.

Talcum is one of the more durable urban legends.  There is no benefit
in putting talcum or substitute powder on a tube or in a tire.  The
practice has come to bicycle tires the same way tire treads that are
miniature replicas of automobile treads have... if it's good for cars,
it must be good for bicycles.  Trucks (and formerly cars) use talcum
or graphite powder between tire and tube, because without it, the two
can vulcanize from the heat of rolling.  This often makes tube removal
destructive, leaving tube fragments stuck in the tire casing.

Bicycles do not generate enough heat to vulcanize tubes, so they can
be removed from the tire without problem.  Other than that, talcum has
no effect on punctures other than to release air faster when one
occurs.  A tube stuck to the casing will retain air for a considerable
distance after a thorn penetration because the thorn that penetrates
plugs the casing hole leaving the tube hole with no outlet.  This is
especially true for snake bites.  I have found such flats the day
after when they have gone flat over night.  Without powder, a tube
will stick adequately to most clincher tires in about 100 miles.

Corn starch is no better than talcum powder, the only difference being
that it is water soluble, but then who cares.  Talcum also cakes up
when wet, although it doesn't dissolve.

A tube cannot move in a tire when inflated, regardless of what powder
is used, because, no translational forces exist, on top of which the
holding force between tube and casing is large.  That talcum prevents
damage when mounting a tire is also not the case, because the pinch
occurs when the last part of the bead is being popped onto the rim.
This can cause a pinch with or without a tire iron, and powder will
not protect a tube from lying in the gap if it hasn't been pushed into
the tire adequately.

The reason tubes have talcum powder inside is that in manufacture,
they become hot enough that, otherwise, they could become inseparably
stuck when folded.  That is why most butyl tubes have talcum inside.

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Next: 8b.12 ETRTO numbers for tire sizes
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