Position the bike vertically, with both wheels as close to in line as possible. (Use the center stand, a bike lift, jack or even a milk crate to support the bike.)
Place an 8-foot fluorescent tube along either side of the rear wheel just above the bottom of the rim, making sure that the tubes touch the wheel in two spots. Use two large rubber bands or small bungee cords to secure them.
- With the front wheel centered between the tubes, measure the gap at the front and rear of the tire. All four gaps should be equidistant. Of course, if both tires are the same size, the tubes should touch at all four spots.