A flat tire often relegates an otherwise good bike to the back of the garage. Fixing flat tires may be intimidating at first, especially if you can't find the source of your flat. Consider some potential causes of your flat tire, and always stay alert for glass, potholes and obstacles in the road.
Inflation Problems
If you inflate your tire far beyond its rated capacity, the excess air has to find a way out. The tire may begin to come off the rim, with the inner tube bulging out between the tire and the rim, causing the tube to blow out. On the other hand, underinflating your tire leaves it susceptible to pinch flats, where the inner tube is pinched between the the rim and an obstacle or debris in the road. Pinch flats often cause a snakebite-style set of two holes in the inner tube.
External Punctures
Punctures from outside the tire are caused by glass, thorns, wire and other metal bits, plastic shards, gravel and more, though glass, thorns and metal are the most likely culprits. When debris impales the tire, the tire is often thick enough to stop it from going all the way through. When the tire isn't thick enough, the sharp point encounters the inner tube. Depending on the size of the debris, the flat can be sudden and explosive, or gradual and almost unnoticeable at first. To prevent additional flats, inspect the exterior of the tire closely and remove all debris first before replacing or repairing the inner tube.
Internal Punctures
A rim strip over the inside of the rim protects your inner tube from jabs from your spokes. In addition, though rarer, scrapes and scratches on the inside of the rim can create sharp metal edges. If you find such a sharp edge, use a metal file or very fine steel wool to gently remove it, and clean the rim thoroughly to remove all metal bits. Always use a rim strip to guard your inner tube.
Other Factors
An inner tube valve that doesn't seal properly can cause a slow leak that's very difficult to detect. Since the majority of the time flats are caused by punctures, you can waste a lot of time looking for pinholes in the tube if the actual problem is the valve. Another common cause of flat tires is time -- it's common for a bike stored inside for several weeks or a season to lose air from its tires without having any leaks in the inner tube.
Expert Insight
Bicycle Tutor recommends keeping track of the inner tube's orientation relative to the tire and rim when you remove it from the bike for inspection. If you can find a hole in the tube, you'll be able to determine approximately where on the tire or rim the puncture came from, and inspect that area closely for potential issues. Sometimes, however, you won't be able to find anything that explains the flat.
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