Bike Terms

In a bike wheel there are three different parts (two on high end/custom bikes).

Rims: The metal frame with spokes that holds the shape of the tire and inner tube. 

Tires: This is the outside rubber portion on the rim, typically made out of a thicker rubber. 

Inner tubes: The inflatable ring inside the tire itself - it holds the air, keeping the tire inflated. Some bikes do not have inner tubes. These bikes are considered to have Tubeless tires. They require tire sealant.  

See Figure 1

There can be a few numbers/letters thrown around when talking about sizing for a bike.

Tire Size: Typically, this is set of numbers that are located on the sidewall of both the front and rear tires, for example, 27.5x2.20. The first number describes the diameter of the tire. The second number is the width of the tire. This number has nothing to do with the frame size* 

See figure 2

Frame Size: This could either be a number in inches or it could be a sizing letter (XS, S, M, L, XL). This is referring to the length and height of the frame itself.* 

*  There is one exception, Kids bikes 20" wheels and smaller typically only come in one size so they will be referred to as the wheel sizes. 

On a lot of mountain bikes, there are multiple speeds (gears) that the bike can be switched to. The mechanism that does this is called a derailer. On some mountain bikes there can be One (Rear) and on others there can be two (Front and Rear). 

See Figure 3