With bicycle wheels varying in size from one type bike to another, measurements must be taken to ensure that tires fit properly. Mountain bikes are no exception. A tire will match both the bike rim width and the bead seat diameter. The bead seat of the rim acts as a shelf on which the tire sits. The bead seat diameter is the distance of the shelf around the entire rim. For accuracy, the rim is measured off the mountain bike.

Place the bare mountain bike rim upright on a mar-proof surface. A mar-proof surface ensures that the rim does not become scratched.

Locate both flanges that run along the top edges of the bicycle rim.

Place the tip of a measuring tape against the inside of one flange. Measure across to the inside of the other flange. The distance between the two is known as the inner rim width.

Place the tip of the measuring tape against the surface of the bead seat. Anywhere along the bead seat will be fine.

Wrap the measuring tape around the diameter of the bead seat. Check that there is no slack in the tape.

Connect the two ends of the tape. Make note of the figure on the measuring tape where the two ends meet. This figure is the rim's bead seat diameter.

Use the two measurements when selecting a new tire for your mountain bike rim.