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I'm interested in the design of cottered cranks. The cotter-pin joint as seen in vintage bicycles is not a very widespread type of joint in modern times, so it's hard to find information about this in machinery handbooks, etc.

For example, given a common 5/8" diameter crank spindle, and 3/8" diameter cotter pin, how far is the cotter pin bore drilled from the spindle center? And how deep are the flats on the spindle? Is this described in any standards document, or at least is there a rule of thumb, like the cotter pin bore should overlap the spindle by X percent of the cotter diameter, etc? Are there any resources available to describe this mechanism or can we only reverse-engineer vintage bike cranks?

For reference, here are some dimensions that I measured with a digital caliper, from a random crankset.

Spindle diameter at the crank-arms: 0.0620 (nominal size is 5/8" or 0.626, so the spindle is about 0.006" undersize.

Spindle thickness at bottom of the cotter-notch: 0.505. Therefore the notch is cut about 0.115" deep, or about 18% of the diameter.

Notch is about 3/8" wide and centered about 3/8" from the end of the spindle.

Hole in crankarm for the spindle is about 0.652"...quite a lot oversize! I suspect better-quality cranks had tighter tolerances.

Hole in crankarm for the cotter is about 0.380"...0.005" over the nominal 3/8" size.

The cotter-hole appears to be drilled so that the edge of the spindle-hole touches the axis of the cotter-hole...in other words, if the spindle were fully round, it would cut the cotter right in half...+/- 0.020" or so that I'm able to measure.

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  • Does this answer your question? Cottered crank dimensions – Daniel R Hicks Apr 29 at 21:05
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    FWIW Sutherland's 6th and earlier has a lot of data on cottered BB spindle dimensions in terms of diameter and length/spacing dimensions (left/right/center) for different brands and nationalities, and interpreting the codes on them. It basically has the kind of data that's needed to deal with replacing BBs and cranks on cottered crank bikes. I've never spent time with editions earlier than 6th; there could be more in some of them. Sharp has data and formulas useful for arriving at a spindle diameter, but I don't believe it has anything on the exact questions you're asking. – Nathan Knutson Apr 30 at 1:48

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