Bicycle Drivetrain Guide
fixed) and hub spacing (the width from axle end to axle end). Modern road bikes are spaced to 130mm, mountain bikes to 135mm and track bikes to 120mm. Older bicycles were spaced to 126mm. They were typically designed for use with freewheel-type rear hubs that carried fewer gears.
With the advent of the freewheel hub and more gears, the rear dropout spacing was increased to accommodate. Another thing to consider when choosing a hub (if it is already built as part of a wheel) is the wheel diameter. The rim must be the appropriate size to fit into the frame and line up with the brake pads.
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Jon Carter is an avid cyclist and loves to write on the subject. Make sure to go check out Bike Wagon for all your biking needs.