Here is a scan showing one of the earliest depictions I've seen of a classic "5-pin" chainring set. It is shown in The Data Book among a page of Cyclo-Tank innovations, dated 1935.
http://farm4.static.flickr.com/
This eventually became extremely popular on Stronglight (mod 49) cranks during the 1950s - 70s and on TA's Pro 5-vis cranks introduced in 1963. But the design had already been quite commonly used on many different manufacturers' steel cranks throughout the 1940s and was clearly used earlier.
There is really no way of accurately determining the bolt circle diameter used on the example pictured. Later cranks and chainrings from MANY manufacturers had settled on 50.4 mm. for the smaller circle connecting rings to cranks. This is an extremely odd size, which also has no obvious direct decimal or fractional BSC (British) equivalent.
Does anyone know why that diameter first adopted... and who had first used it? Was it perhaps simply carried over from other machinery completely unrelated to bicycles?
BOB HANSON, ALBUQUERQUE, NEW MEXICO, USA
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