Conventional bicycle tires, which are the most common, consist of an outer tire and a separate inner tube. The edges of the tire hook over the edges of the rim with the air-filled inner tube inside the tire. The inner tube has a valve for inflation, is made of rubber and shaped like a doughnut. Tubes have different specifications depending on the type bike and tire.
Size
It is best to get the tube size that matches your tire size. The diameter size is the most critical. Street bikes typically measure 27 inches and mountain bikes 26 or 29 inches, depending on the model. The width is not as critical, but still important. If your tire width is 2 inches and you purchase a 2.5-inch wide tube, it will be underinflated. This will not cause harm to your bike, but it will be heavier because of the extra rubber. A tube that is narrower than your tire will be overstretched and have reduced performance and greater chance of punctures. The tire size will be noted on the side of the tire. It will be listed as diameter by width, such as 26 x 2 inches.
Tube Pressure
Tube pressure is commonly referred to as tire pressure, but it is the inner tube that is inflated. The tire width is what determines the tube pressure. Wider tires require less pressure and narrower tires need higher pressure. Underinflation can cause more rolling resistance, be more prone to pinch flats and may come off the rim. Overinflation can cause less rolling resistance, be more prone to damage, have a harsher ride and can bounce.
Valve Stem Type
Bicycle tire tubes have a stem for connecting to an air hose or bicycle pump. There are three different types of valve stems -- Presta, Shrader and Woods/Dunlop. The Woods valve, also called a Dunlop valve, is rare and not common in the United States. The bottom of the Woods valve was popular in Britain and Asia. You can use a Presta pump with the Woods valve. The Schrader valve is the standard automotive type of valve, which can be filled at gas station air pumps. The Presta valve is smaller and has a tapered end. It is used mostly on high-performance bicycles. It has a built-in valve cap that must be opened before filling the tube and needs to be securely tightened after filling. They are easier to pump with a hand pump, but cannot be used with gas station air hoses without an adapter.
Material
Bicycle tire tubes are made of latex or butyl. Latex is natural rubber harvested from tropical trees. It is lighter than butyl, but is more porous and needs to be topped off with air more often. Butyl is the most commonly used material for tubes and less expensive than latex. Butyl tubes are less porous and hold air better than latex.
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