What Causes Flats?


If you ride a bicycle in Chicago, chances are you have some experience with getting flats. While there are not guarantees against flats, there are simple things you can do to make their occurence less likely.

Regardless of the size or width of your bicycle wheel, the fist step in flat prevention is to invest in a set of high quality puncture-resistant tires. Two of our top recommendations at Cosmic Bikes are Continental Contact Plus tires ($35) available in most road bike and hybrid sizes, and Schwalbe Marathon tires ($42-48), available in sizes for almost all types of bikes.

Even with the best tire, you may get occasional flats. And, let me just say that yes, there are "mystery" flats. There probably is a cause, such as the tiniest of punctures, or a flaw in the valve, but it cannot be readily identified by either you or the mechanic. However, in most cases, the cause can be identified and mitigated to minimize the incidence of it happening again.

If you are new to fixing flats on a bike, you may want to see our earlier posts on parts of a bicycle wheel, and common types of bicycle valves.

When you pull your punctured tube out of the tire, go ahead and overinflate it using your pump or an air compressor, and find the hole where the air is escaping. Look closely at that hole. Where on the tube is it located: top, side, bottom or at the valve? Is it a single hole, or a double? Is it a tear?

Let's begin by identifying five major types of flats, and we'll go over them and their probable causes in more detail:

  • Outer surface punctures

  • Sidewall punctures or "snakebites"

  • Inner surface punctures

  • Blow outs

  • Valve problems

Outer surface punctures:
As you look at your overinflated tube, you will see a single hole on the outer surface of the tube (the surface that corresponds to to the part of the tire that rolls along the ground). That indicates that a sharp object penetrated the tire and punctured the tube. The object may have fallen out, but just as likely, it is still embedded in the tire.

Before replacing the inner tube, run your fingers very carefully along the inside of the tire, feeling for anything sharp that may be protruding. At the same time, carefully eyeball the outside of the tire for embedded pieces of glass and other debris, and poke them out as you go along (a paper clip or sharp tweezers work well).

If your tire feels thin and flimsy, or has a lot of slits from previous contact with glass, consider replacing it with a puncture-resistant tire.

Sidewall punctures:
All tires have their recommended inflation pressure embossed on the sidewall, usually in units of PSI (which stands for Pounds per Square Inch). That number can seem incredibly high, especially if you are used to filling up car tires. Bicycle tires take much higher pressure than any car tire, and generally, the skinnier the tire, the higher the recommended pressure.

Many people are reluctant to fill their bike tires to recommended pressure, perhaps because they are afraid of blow-ups. As you increase the amount of air in the tire, it gets harder and harder to pump, which can make you feel like you're overdoing it.  Yet, filling up tires to the correct pressure will protect you against one of the most common types of flats: the snakebite.

This type of hole is usually caused by riding on underinflated tires. When you hit a bump in the road, the lower-than-recommended pressure allows the tire to bottom-out against the sharp edge of the rim, and the inner tube becomes pinched in the process. You will recognize a snakebite if  you examine your overinflated tube, and discover two small holes, about 1/4" apart, usually on the side of the inner tube.

Inner surface punctures:
If your overinflated inner tube is losing air from its inner surface (the surface that would have been facing the rim), you will need to examine the inside of the rim, and the tire beads (see Anatomy of a Bicycle Wheel). Typical causes of this type of puncture include: a faulty rim strip, resulting in exposed spoke nipples or sharp rim surfaces, a burr on the inner seam of the rim, or a broken tire bead, with exposed wires digging into the tube.

Blow outs:
Sudden blow outs happen when the tube, instead of being safely encased inside the tire, becomes exposed to the outside world.

This can happen if, when changing a flat, or filling the tire, a piece of tube inadvertently gets pinched between the rim and the tire. Sometimes it blows right away, but not always. Similarly, you can get a blow-out is the tire bead isn't correctly seated inside the rim, and the inner tube begins to protrude out. Generally, you will feel thumping for the short distance you may be able to ride.

Blow-outs can also occur when the integrity of the tire becomes compromised, and the tube starts to protrude. This can happen as a result of dry rot, a brake pad, fender or other mis-adjusted part rubbing over the tire, or the bead separating from the tire. Inspect your tires periodically to make sure they are in good condition.

Valve problems:
Occasionally, small amounts of debris get jammed inside the valve, causing it to be stuck in open position and leak air. If used improperly, certain flat-prevention products sealants can have the same effect.

Another type of valve failure can occur when you ride on underinflated tires. Every time you brake, the tube and tire continue to move just a little bit even as your wheel stops. This causes the valve to come through the rim hole at an angle. If this goes on long enough, the valve an separate from the body of the tube at the base.