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Cotter Pins

Posted Jan 14 2011 1:25pm

The racing bicycle developed through the 1920s and 1930s, becoming increasingly sleek and lightweight.

However, aluminum alloy was not widely used for bicycle components as it was considered unreliable at that time.

All this changed in the 1940s due to WWII and aircraft production; huge strides were made in metallurgy and improvements made in aluminum alloys.

However, there was reluctance immediately after the war, both by manufacturers and consumers to use these new alloys for high stress bicycle components like cranksets.

My first race bike that I bought in the early 1950s had steel cranks, held to the bottom bracket axel with cotter pins; a somewhat crude method of attachment that had been around since the early days of the bicycle.

Never-the-less the cotter pin was cheap, simple and reliable, providing they were fitted correctly. I used to buy my cotter pins “Plain,” and file my own flats on them.

The procedure was to file a flat, tap it into the hole in the crank arm using a hammer; the relatively soft cotter pin would butt up against the hardened surface of the flat on the bottom bracket spindle.

This would create a shiny spot on the flat of the cotter pin; the pin would be removed and the shiny spot would then be filed off, and the pin refitted.

This was repeated until there was perfect contact along the entire flat portion of the cotter pin.

Periodically, usually before an important race the cotter pins were driven out, making sure there was a block of wood under the crank arm, (Right.)

This was so the frame or the BB bearings would not be damaged by hammering on the cotter pin unsupported.

The cotter pin was touched up with a file again and refitted. After several refits it was necessary to buy new cotter pins and start over.

The rule I was always taught was that “When the pedal goes down, the cotter goes up.” On the downward stroke of the crank, the nut on the cotter pin was on the top.

The idea is, when the left crank is driving the tendency is for the cotter pin to be pulled in tighter. However, the cotter pins have to be installed in opposite directions; otherwise the cranks will not be in the exact same plane.

With the BB spindle now driving the chainwheel from the left, the right cotter pin is tending to be pushed out and the only thing stopping it is the nut. So one could argue that it doesn’t matter which direction the cotters go, one of them is bound to be in the wrong direction. 

All I know is I always fitted them the way I was taught, I checked them regularly and I never had a cotter come loose while I was riding.

Most people reading this will probably never have to concern themselves with a cotter pin; unless you are into collecting vintage bicycles.

But you never know you might find yourself working on an old roadster bike, and now instead of staring at a cotter pin and asking, “What do I do with this?” You at least have the basics. 



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