Bicycle tires are more prone to flats than other types of tires. They are abused, stressed, at times punctured and patched, and are under a high amount of pressure. A flat at the wrong time may be inconvenient, as well as dangerous. Routine observation and maintenance of your bicycle's tires and tubes should be included as part of your preride checklist.
Inner Tube
The majority of bicycle tires consist of an outer rubber shell with an inner tube that holds the air, allowing the tire to maintain its shape. Typically, when a bicycle tire loses air, it is actually the inner tube that is compromised in some way. When diagnosing a flat tire, this is usually the first place to assess. The smaller the width of the tire, on road bikes for example, the higher the pressure within the tube. The ability of the tube to retain its prescribed air pressure is important for proper handling of the bicycle. Decreased air pressure limits the tire's traction to the ground, especially on turns and in inclement weather conditions.
Puncture
Punctures can occur for a variety of reasons. Nails, thorns and glass are among the most common reasons for tube failures. Internal punctures can also occur from an uncovered spoke-end. Liners are available to cover the spoke-ends from direct contact with the tube. Special tubes are available with thicker outer walls to help prevent punctures. Some tubes have a special adhesive within the tube that can potentially seal a small hole.
Finding the Leak
When attempting to find the source of a leak, first view the external tire, looking for any object that may be sticking through to the inner tube. If one is found, continue on around the entire tire to ensure that there is only one compromised area. At times, it may be necessary to first remove the tire and inner tube to discover the leak. Add air to the tube and listen or feel for a hissing sound. Placing your finger over the hole should stop the noise as well as the leak. For hard-to-find leaks, submerge the inflated tube in a large bucket of water and look for bubbles. This will be the source location. Determine if the puncture seems repairable or if it is better to replace the tube. Large tears or blowouts may occur from hitting a hard or sharp object, such as a curb or rock, but are less likely than sharp punctures.
Temperature
During the off-season, when your bicycle is sitting dormant in the garage for many cold months, there is likely to be some incidental air loss from lack of use. Temperature changes cause expansion and contraction of the air in the tubes, resulting in some leakage. Prior to riding your bicycle, always check the air pressure in both tires with a gauge. Refill to the manufacturer's specification, which is usually printed on the side of the tire.
Valve Stem
Another common source of air leak is the valve stem. The two most common valve types, Schrader and Presta, are both prone to leakage. Schrader valves, most common on American bicycle tubes, may leak if the innermost valve core is not screwed in completely, or if dirt gets trapped within the air inlet area. Presta valves are seen more on higher-end road bikes and mountain bikes. This valve is smaller in diameter and lighter in weight. A poorly fitting upper nut on an external thread is usually the reason for air leakage in the Presta. More uncommon would be a compromise in the integrity of the valve seating at the base of the tube, where there may be a leakage. Listening for the air leak is usually the best way to discover where this type of leak is coming from.
Be Prepared
Carry, and know how to use, a patch kit that can be used to fix a small puncture leak. Always have tools available to remove the rim, the tire and the tube from the bicycle. Having a portable pump or air cartridge is especially handy when riding off-road or in rural areas. The best practice is to carry at least one extra inner tube to cover you in an emergency. All of these can easily fit in an under-the-seat bag for storage. If your tires or tubes are old and show signs of wear or age, such as cracks in the side of the tire, replace them prior to riding. Always do a thorough review of your bike's systems, making sure the brakes and gears are functioning prior to riding.
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