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The bottom bracket is the part of a bicycle frame around which the pedal cranks revolve. When measuring the bottom bracket, it is important to remember that it consists of two parts. The first is the part of the frame itself, usually called the bottom bracket shell. The second is the component that goes inside the shell, which is often referred to simply as the bottom bracket, but is also known as the spindle.
The spindle component of a bicycle's bottom bracket occasionally needs to be replaced, in which case you will need an accurate measurement of both parts. The bottom bracket is measured in millimeters, so you will need a set of calipers or a metric ruler.
Measuring the Bottom Bracket Shell
To measure the bottom bracket shell on the bike frame, flip the bicycle upside-down and hold the ruler or calipers against the shell. Measure the length of the shell from end to end, not including the length of the spindle inside the shell. Most bottom bracket shells measure 68, 70 or 73 mm long.
Measuring the Bottom Bracket Spindle
To measure the spindle, you need to first remove the crank arms. Once the crank arms are removed, measure the length of the spindle from end to end in millimeters. If the cranks on your bicycle attach with nuts instead of bolts, do not include the threaded parts of the spindle in your measurement.
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