How to Measure a BMX Bike Frame

The average BMX bike looks something like this.

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 millimeter measurements are standard use in the cycling world and measurement devices used should contain both standards.

Things You'll Need

• Measuring tape with inches and millimeters
• Caliper with inches and millimeters

Instructions

1. Measure the top tube of the bicycle from the center of the joint between the top and seat tubes and the center of the joint between the top and head tubes with the measuring tape in inches to determine the top tube length.

2. 

   A bicycle top tube is the tube the fork passes through at the very front of the frame.

   Measure the inside diameter of the head tube in inches with the caliper to determine the headset size.

3. 

   The bottom bracket shell is the part of the frame housing the pedal crank bearings.

   Measure the inside diameter of the bottom bracket shell in millimeters with the caliper to determine the bottom bracket or bracket bearing size.

4. 

   Brake mounting posts extend from the seat stay through the center of the brake arm mounting hole.

   Measure the distance between the brake mounting post and a perpendicular point on the frame parallel to the center of the rim braking surface in millimeters 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.

5. 

   The axle slot is the place in the frame dropout the hub axle slides into.

   Measure the open axle slot in the frame dropout in millimeters with the caliper to determine the axle size.

Tips & 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 millimeters.

References

• Bicycles Inc.