It happens to every cyclist. You're pedaling along, enjoying a peaceful ride when you suddenly hear the dreaded "psssss!" of air leaking out of your tire. When you're riding a bike with tube tires, this indicates that something has punctured the wall of your tire and the rubber tube inside of it. If you're carrying a spare tube and bike tools, it's relatively painless to change the tubes and get back to your ride. Bike tubes come in different widths and circumferences to match various bike tire sizes. It's important to know which sizes you need so you can purchase the correct tubes and prevent being stranded when a flat sneaks up on you.
The French system uses a number to designate diameter and a letter to designate width. The letter code no longer responds to width because tires are now available in much more narrow sizes. For example, a 700C tire was the traditional size for road bikes and hybrids. Today, there are too many various widths available for 700C tires, so they are represented as fractional figures to designate the tire width, such as the common road bike sizes, 700 x 23C and 700 x 25C.
ISO vs. Fractional
Each major bicycle-producing country has its own system of measurement for tire sizes, which has resulted in a lot of confusion. In response, the International Organization for Standardization developed a universal system to cut down on confusion. The first number in the ISO system designates the tire width, and the second indicates the bead seat diameter. For example, a 700 x 38 tire would be 38-622 ISO. If you're not sure which size tube you need, you can make sure all these crazy different measuring systems don't throw you off by simply referring to the tire's ISO size. It should be in parentheses following the common measurement. If you use the ISO size to purchase a tube, you can rest assured it is the correct fit for your tire.
Decimal vs. Fraction
The second number of a tube dimension designates the tire's width. For example, on a 26 x 1.5 tire, the "1.5" represents the width -- but here's where it gets tricky. If two tires have mathematically similar dimensions, but one is listed as a decimal and the other as a fraction, they are not equal. A 26 x 1 1/2 is not equivalent to the aforementioned 26 x 1.5, and these two different tubes are not interchangeable. Pay attention to how the dimensions are listed on your tire to make sure you get the right tube.
Two other important differences to be aware of pertain to the valve of a tube. There are two valve styles, Schrader and Presta. A Schrader valve is the standard, automotive sized valve that you usually find on the fatter tubes of mountain and some touring bikes. The skinnier Presta valve is what most road bike wheels accept. It's important to know which valve type your tire is drilled for to make sure your tube fits correctly. Presta tubes come as threaded and unthreaded. This matters when it comes to inflating the tire. If you have a bike pump or CO2 adapter that will only work with one or the other, you need to purchase the compatible valve. Also, threaded stems have a nut that you can place on the stem next to the rim to prevent the stem from pushing into the rim. Finally, some bikes, such as triathlon bikes, occasionally have front and back tires that are different sizes. In these cases, it's important to identify the two different tube sizes and carry one of each when you ride.
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