A bicycle bottom bracket consists of a spindle, two threaded cups and two sets of bearings. The bottom bracket is installed inside a portion of the bicycle frame known as the bottom bracket shell. The chief function of the bottom bracket is to allow the attached crank arms to rotate. When the bicycle is pedalled, the spindle inside the bottom bracket turns simultaneously. To determine the compatibility of a bottom bracket with a bottom bracket shell and/or crank set, two sets of measurements are taken: respectively, the bottom bracket width and bottom bracket spindle length.
Locate the bottom bracket shell on the bicycle frame. The bicycle down tube, seat tube and chain stays all intersect here. The bottom bracket shell is cylindrical.
Place a measuring tape on the bottom side of the bottom bracket shell. Measure from one end of the shell to the other.
Make note of the measurement. Common sizes include 68 mm and 70 mm, while some mountain bikes will measure 73 mm. Bottom brackets sizes are listed according to the width of the bottom bracket shell--not the width of the actual bottom bracket.
Place the bottom bracket on top of a piece of paper. The spindle passes through the centre of the bottom bracket and will be difficult to measure directly. Instead, use a piece of paper to notate the ends of the spindle.
Mark the ends of the spindle on the piece of paper.
Measure the distance between the two marks. This is the length of the bottom bracket spindle. Spindle lengths vary in order to accommodate different sizes of crank sets.