How to Measure a Crank Size
Knowing how to determine bicycle crank size is important if you're planning on replacing your current crank or if you're buying your first bicycle. Crank size can imply either the length of the crank arms, to which your pedals are attached, or the size of the chain rings attached to the crank. Not all cyclists are built alike, and not all crank arms are the same length. Likewise, chain rings come in a variety of sizes and allow for different gearing options. Once you determine which sizes work best for you, measuring the crank will ensure proper fit and optimal performance from your bike.
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Instructions
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Crank Arm
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1
Locate the bicycle crank arms. There is a crank arm on either side of the crank. The pedals are attached to the crank arms. Either arm will do for measurement.
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2
Measure the crank arm from the center of the bottom bracket to the center of the pedal spindle. The bottom bracket is the very center of the crank, where the crank attaches to the bicycle. The pedal spindle attaches the pedal to the crank arm. Measure in metric, as most crank lengths are listed as such.
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3
Record the number. According to sheldonbrown.com, the vast majority of crank lengths are between 165 mm and 175 mm. Expect your measurement to lie somewhere in that range. Crank arms longer than 175 mm are rare.
Chain Rings
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4
Locate the chain rings. The chain rings are the set of "rings" attached to the inside of the right, drive-side crank arm.
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5
Measure the chain rings. Your bicycle will contain between one and three chain rings. The chain rings are measured by their number of "teeth." These teeth are the pointed surfaces over which the chain rides.
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6
Note your measurement from largest to smallest chain ring. For example, a double crank (a crank with two chain rings) that has a 53-tooth, large chain ring and a 39-tooth, small chain ring will be listed as a 53/39.
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