Subject: 8b.1   Patching Tubes
From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Tue, 2 May 2006 10:25:45 -0700

The question often arises whether tubes can be practically and safely
patched.  I suppose the question comes up because some people have had
leaky patches or they consider it an arcane exercise.  Either way, it
need not be difficult if simple rules are followed.

WHY PATCHES COME LOOSE

Tubes are made in metal molds to which they would stick if mold
release were not sprayed into the mold.  The release agent is designed
to prevent adhesion and it can do the same for patches, some of it
having transfered on and into the surface of the tube.  To make a
patch stick reliably, mold release must be removed.  For this reason
patch kits have sand paper that is not there to roughen the surface
but to remove it.  Failure to remove the 'skin' of the tube is a main
cause of leaky patches.

Tube surfaces often have ridges at mold joints and near the valve stem
that prevent effective sanding.  This usually presents no problem
because rubber glue and the plasticity of the orange (REMA) patch
material are enough to seal those gaps.  However, a plastic disposable
(BIC) razor works well to remove such ridges and, with handle removed,
makes a handy addition to a patch kit.

Once mold release has been removed, rubber solution can be applied
with the finger by wiping a thin film over the entire area that the
patch is to cover.  After the glue has dried, with no liquid or jelly
remaining, leaving a tacky sheen, the patch can be pressed into place.

Although patches can be made from tube material, this must be done
carefully following the same procedure as preparing the tube.
However, butyl tube material, unlike commercial patches, is impervious
to rubber cement solvents and will not cure if the glue on the tube
and patch is not completely dry.  This presents a substantial problem.

PATCHES
 
Patches commonly have a metal foil cover on the sticky side and a
cellophane or impervious paper cover on the back.  The foil must be
pulled off to expose the adhesion surface before pressing the patch
into place.  The backing paper or cellophane often has perforations so
that it will split in half when tube and patch are manually stretched.
This makes peeling the cover of the patch from inside to outside
possible and prevents peeling a newly applied patch from the tube.

REMA patches, the most commonly available in bicycle shops, have a
peculiarity that not all have.  Their black center section exudes a
brown gas that discolors light colored tire casings in daylight.  This
causes the brown blotches often seen on sidewalls of light colored
tires.

LEAKY PATCHES 

Assuming a patch was properly installed, it may still leak after a few
miles, if used immediately after patching.  Because tubes are
generally smaller than the inside of the tire to prevent wrinkles on
installation, they stretch on inflation, as does the patch.  The
stretched tube under the patch wants to shrink away from the patch,
and because there is no holding force from inflation pressure at the
hole, the tube can gradually peel away from the patch starting at the
hole, while the tube under the remainder of the patch is pressed
against it by air pressure.

Flexing of rolling bias ply tires also loosens patches.  Laying a
standard 3.5x2 inch paper business card between tire and tube will
show how severe this action is.  After a hundred miles or so, the card
will have been shredded into millimeter size confetti.

If the puncture is a 'snake bite', chances of a leak are greater.
Pinch flats from insufficient inflation or overload are called snake
bites because they usually cause two holes that roughly approximate
the fang marks of a snake.  Although a single patch will usually cover
both holes, these will be closer to the edge of the patch and have a
shorter separation path to its edge.

In a rolling tire, the patch and tube flex, shrink, and stretch making
it easier for the tube to separate from a partially cured patch.  To
test how fast patches cure, a patch can be pulled off easily shortly
after application, while it is practically impossible after a day or
so.  For reliable patches, the freshly patched tube should be put in
reserve, while a reserve tube is installed.  This allows a new patch
more time to cure before being put into service.

A tube can be folded into as small a package as when it was new and
practically airless, by sucking the air out while using the finger
opposite the stem to prevent re-inflation.  This is not done by
inhaling but by puckering the cheeks.  Although the powders inside
tubes are not poisonous in the mouth, they are not good for the lungs,
but then that's obvious.

PATCH REMOVAL

The best remedy for a leaky patch is to remove it and start over.
However, after several days of curing, a patch is hard to remove.
With heat supplied by a hot iron or heated frying pan at moderate
temperature, patches come off easily.  Pressing a patch against a hot
surface with the thumb until the heat is felt will allow the patch be
pulled off easily.  Patch remnants can be cleaned off with rubber
solution (patch glue) or sand paper.

Minutia
 
Separating patches are often hard to find because separation always
stops at the edge, air pressure preventing further separation.  Slow
leaks that occur, often close when the tube is inflated outside a
tire, so the offending patch cannot be found.  Old tubes to be
discarded often reveal patch separation when cut through the center of
a patch with shears, to reveal talcum powder from the inside of the
tube under most of the patch.
  
Although talcum powder on the outside of tubes does nothing useful, it
is essential on the inside, where it is found in any butyl tube.
Without it tubes would adhere to themselves after manufacture and not
inflate properly.  Externally, talcum may prevent adhesion to the
tire, slight as it is, and may help prevent sudden air loss in the
event of a puncture but it does nothing for the wellbeing of the tube.
When inflated, tubes act like an integral part of tire casings with or
without talcum.

Tires are less flexible at a patch so tread may wear slightly faster
there, but patches have no effect on dynamic balance since wheels
naturally have a greater imbalanced than patches can cause and have no
effect on the heaviest position of the wheel which is either at the
valve stem or the rim joint.  Heat from braking can accelerate
separation of a fresh patch but this generally does not pose a hazard
because leaky patches usually cause only a slow leak.