aarn: 42th prototype track chainring [bicycle]
let me start this post by mentioning that i have little to no interest in going into production of bicycle components, especially in something like chainrings. this was more of a study in manufacturing and design and something that i completed just to see how difficult it would be, and find out what goes into the design of a mechanical component like a chainring. i started my research by tracing back the roots of the bicycle chain to an old ANSI standard for transmission roller chain, determining the foundation, which is a 0.500” pitch (ANSI #40) and 0.3125” roller diameter. searching in the machinery’s handbook, i was able to find the ANSI sprocket tooth form for a roller chain (ANSI/ASME B29.1M-1993), and design a custom chainring based on some specs that i had determined (and a whole sheet of equations). i designed a 42-tooth sprocket, and not surprisingly, it looked nothing like the tooth pattern on a modern bicycle chainring. unable to find any information on the evolution from the old mechanical standard to the modern chainring, i decided to push on and choose which tooth features i thought were critical, and design the intricacies of the pattern myself. the guys at open were kind enough to let me borrow a chainring, and a i had a few on hand at my office, and by way of mechanical measurement, and my fine-tuned “eyeballing” skills, i came to a design with which i was comfortable. originally designed around a 144-BCD (for most track cranksets), i designed a version for a 135-BCD that would fit on my miche crankset so i could test it out.
the profile of the prototype chainring was waterjet cut from a sheet of 6061 aluminum, and then the counterbores and facing operations were done using conventional milling. the machinist kind of rushed through this, and subsequently messed up MOST of the machining operations. it did not make me happy, but the prototype is good enough to test out the tooth pattern on my bike. i also had a problem with different seat diameters on two different versions of the 135-BCD miche cranks, so i had to take a file to the inside seat diameter on the chainring, which also did not make me happy (especially because it fit fine on the drive-side crank arm that WAS NOT on my bike, figures.) so basically, i have noticed that the teeth look a little thinner than some of the more beefy track chainrings, the styling looks pretty great, it would look a lot better if the machinist had been a little more careful. the chainring has been on my bike for two days, and is running very smooth and quiet. this makes me happy. i think i’m going to try and optimize the tooth pattern and make some more of these, at least for my own bikes, maybe a few 144-BCD, who knows?!
