Oval Chainrings

This isn’t really cheating, is it?

My original skip-tooth chainrings and sprockets were pretty knackered. Having been able to acquire NOS (new old stock) chains for both bikes from eBay listings, I felt that getting the gearing the same way would just be a poke and hope process. One could never know the actual wear and trueness of parts until after receiving the purchase. So, I set about to make my own.

Immediately, I remembered my inability to pedal the heavy beast up a local 9% grade, maybe I’m in need of gearing alternatives, but is that a cheat? I’ll still have to exert the same energy, right, (conservation of energy and all that)? Nuff thinking on that, comfort is king.

Even though I’m a low cadence (65-75rpm) rider the stock ratios are daunting. So some quick calculations gave up the few possible alternatives, alternatives severely limited by the 1″ pitch chain (fewer teeth — fewer combinations).

Once the new ratios were chosen, I was forced to ponder other opportunities for improvement… So, first thing, lighten the gearing using 6061 aluminum which will gain some ease in the milling process. Next up, using the 6061 will allow for cool customization of styles and finishes. Lastly, there’s that need to revisit oval chainrings.

Image embed from: www.oniebicyclemuseum.co.uk
Read more… 1893 Columbia ‘Century’ Model 32

Originally oval sprockets were a cycling innovation of the 1800’s, later they began to appear in many engine designs on camshafts, where they are very effective in lessening stresses in the timing line. Even saw them experimented with for taking the vibration out of “imbalanced by design” motorcycle engines (you know what brand I’m talking about). Currently there is a resurgence in the MTB world of oval chainwheels, if its making climbing easier for the pros, its for me and I need all the help I can get. And after all, we’ve already determined… its not cheating — I’ll still have to exert the same energy, right, you know, that conservation of energy thing. Furthermore, who am I to disregard an opportunity to dip into truly vintage and modern cycling tech? So let the ovalling calculations begin.

Draw it up
Print a test
Model it
Model it

The 1” pitch makes these easy to design and mill, the only oddity was the rear sprocket threading at 24tpi (threads per inch) x 1.375. These sprockets screw on to the Bendix hub with only 3 threads securing the connection. If you’ve ever spun a sprocket during a climb you know the pain if this connection fails. So, after a stress study in Fusion, I was confident that my power output matched against the all the resistive forces could not overcome the connection of a properly machined and installed sprocket/hub combination.

Being that I’m a whore of free promotion, I couldn’t pass up a naming opportunity, adding the logo while reducing the weight (hopefully the balance isn’t to affected). In a future post, I’ll discuss the results on the road and how my knees felt. In the meantime take a look at how the reimagined skip tooth gearing came out in the photos below. Enjoy your ride.

Mill and finish Oval
Mill and finish Round

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