Things You'll Need:
- Mountain bike frame, Tape measure
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Step 1
Identify the parts of the bike that need to be measured. Bike manufacturers size bikes based upon a single frame measurement. An "18 in." bike refers to the length of the seat tube, the part of the frame's triangle that extends beneath the seat. The horizontal tube it intersects with at the top is called the "top tube". At the bottom of the seat tube is a round hole called the "bottom bracket". The crank arms are attached to the bottom bracket and are what your pedals are screwed in to.
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Step 2
Once you've identified the parts of the bike use your tape measure to measure from the center of the bottom bracket to the top of the seat tube. Another way to take the measurement is from the center of the crank arms to the top of the seat tube. This measurement is the normal way of measuring frame size.
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Step 3
Compare your measurement with the manufacturer's sizing system. Many manufacturers measure their frame sizes in increments on one inch or one half inch. If it seems like you obtained a strange measurement it may be because the manufacturer of your frame uses a different method to measure the frame size. An alternative measurement used is from the center of the bottom bracket to the center of the intersection of the seat tube and top tube.
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Step 4
Contact the manufacturer. If you cannot determine how your bike's manufacturer measured the frame, it may be necessary to consult with the manufacturer either by phone or by reading their documentation, as there are several methods of frame measurement that are peculiar to specific manufacturers.
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Step 5
Check the fit of any mountain bike before buying it. Because of potential discrepancies in how frames are measured from one brand to the next you always need to check the fit of a bike before buying it and riding it. An 18 in. mountain bike from one manufacturer may fit you completely different than a similar 18 in. bike from another.
