How to find the correct crank length for your bicycle

Crank arm of a bicycle is the one which connects the front chain ring to the pedal ( Identify the bicycle parts ). When I bought my bicycle, I was not aware that choosing the correct crank arm length went a long way in ensuring the smoothness and comfort of your ride on the bicycle.

My bicycle Avon Roadster AB-310, comes by default with the largest crankarm. I measured it to be exactly 185 mm. This is true for all Roadster bicycles manufactured in India – be it BSA, Hercules, Avon or Atlas. When it comes to bicycles, the manufacturers in India use the one-size-fits-all technique.

But if you want your bicycle to suit you, then choosing the correct crankarm length is very important. This is more true when you ride sitting down on the saddle rather than standing. The following is a chart which gives the approximate crankarm length you should choose for your bicycle based on your leg length.

Table showing the correct crankarm length based on your leg length
Leg Length Crank Length
60 to 65cm 150mm
66 to 70cm 155mm
72 to 75cm 160mm
75 to 78cm 162.5mm
79 to 81cm 165mm
82 to 83cm 167.5mm
83 to 86cm 170mm to 172.5mm
87 to 90cm 175mm to 177.5mm
91 to 94cm 180mm
94cm plus 185mm

How to measure your leg length

Myra VanInwegen has a very detailed article giving the specifics of choosing the crankarm length of your bicycle. In the article, she points out how to measure your leg length, and I quote :

Measuring your leg length is easy. Take a big book and push it up into your crotch as you’re standing against a wall. Make sure the back edge of the book is flat against the wall. Mark the wall at the top of the book (at the arrow in the diagram) and measure to the floor. This is your leg length.

Chris Juden of the CTC has a different method for finding leg length. He measures height to top of head when sitting on the floor and subtracts that from standing height (no shoes). He finds this produces a more reliable method than the “book between the legs” method described above.

Using one of the above two methods of choosing ones leg length, I measured my leg length as 83.5 centimeters. So looking up the preceding chart, I figured the correct crankarm length for me should be 167.5 to 170 mm. But my bicycle has a crankarm length of 185 mm which is too large for me.

After visiting a few bicycle shops, I was told by one shop owner that the crank arm of BSA SLR is exactly 170 mm in length. So I replaced the 185 mm crank arm on my Avon Indian roadster with the 170 mm BSA SLR crankarm. My riding comfort has improved since then.

If you are very serious about riding a bike (bicycle), it is very important to choose the correct crank arm.