Subject: Patching Tubes
From: Jobst Brandt
Date: March 15, 2005
The question often arises whether tubes can be practically and safely patched. I suppose the question comes up because some riders have had leaky patches or they consider it an imprecise 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.
Patches can be made from tube material but 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 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.
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.
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.
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.