Variable speed bike sprocket

Give your single speed bike a boost with some parametric goodness

If there was an award for parametric design that made riding your single speed or fixie bike usable on gradients greater than 5 degrees for people other than Olympic athletes, then Jason DeRose would surely take it out with his variable ratio mechanical gear design.

DeRose, a software developer used Python and employed mathematics and geometry to work out the position of the sprocket teeth to craft his design. As part of DeRose’s design process, he then extruded the linework into 3D in Blender. He has also released the project files as open source on launchpad to allow others to build upon it.

DeRose notes in his project rational, several advantages in efficiency, that his concept has over the common bicycle derailleur system:

“…in most [derailleur] sprocket combinations, the input and output sprockets are not coplanar. In these cases, some of the rider’s input torque is wasted as lateral forces upon the frame, doing no useful work in terms of propulsion. Second, the output side (rear) derailleur involves the use of two pulleys to properly position the chain relative to the selected output sprocket and to absorb any excess chain length created by the particular sprocket combination. These pulleys double the total chain articulation (from 360° to 720°), introducing additional frictional losses. This increased chain length adds additional mass, disadvantageous on its own. This additional chain mass increases the effective combined moment of inertia of the rotating masses…”

The concept seems pretty sound and similar in principle to other variable gear mechanisms that I’ve seen. Perhaps, 3D printing a prototype would be a good methodology to test out the design? Although, with such a mechanism fixed to your ride, it could no longer be technically deemed a single speed, I doubt that anyone would notice as you effortlessly fly pass the exhausted and out of breath hipsters.

Via Blender Nation


David is an industrial designer from New Zealand. He contributes a weekly article on personal fabrication for Ponoko. Follow him on Twitter!

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