The frame of a BMX bike has five key measurements: the top tube length, the head tube diameter, the bottom bracket shell diameter, the brake mount post height and the dropout axle slot size. These five measurements will indicate which height of a rider the frame is intended for, which size headset is needed, which style of bottom bracket is needed, which style of brake is needed and which size axle is needed. Both inch and millimetre measurements are standard use in the cycling world and measurement devices used should contain both standards.
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Things you need
- Measuring tape with inches and millimetres
- Caliper with inches and millimetres
Measure the top tube of the bicycle from the centre of the joint between the top and seat tubes and the centre of the joint between the top and head tubes with the measuring tape in inches to determine the top tube length.
Measure the inside diameter of the head tube in inches with the caliper to determine the headset size.
Measure the inside diameter of the bottom bracket shell in millimetres with the caliper to determine the bottom bracket or bracket bearing size.
Measure the distance between the brake mounting post and a perpendicular point on the frame parallel to the centre of the rim braking surface in millimetres with the caliper to determine which style of brake is needed. If no post mounts are on the frame, it will only accept a single bolt side pull-style brake.
Measure the open axle slot in the frame dropout in millimetres with the caliper to determine the axle size.
Tips and warnings
- Go to the local bike shop for measuring assistance or examples of size options available.
- Measurements must be exact. Many parts in the bicycle world differ by only one or two millimetres.
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