VECTr – Variably Expanding Chain Transmission

In 2008, I started working in earnest again on the bike transmission. My wife and I had bought a home in the suburbs by now and were both busy with our careers. I ended up working at Sam Houston State University in campus ministry and teaching Philosophy, though the mechanical problem-solving part of me was still largely unfulfilled.  I channeled that desire into the bicycle transmission.

For my part, I kept trying to solve the problem of making a chainring expand by designing a device that had the sprocket broken up into gear segments hinged at one end and simultaneously pivoting them to increase the radius at which they would engage the chain.  As I looked into prior patents on expanding chainrings (and earlier “expanding pulleys”), I found that people have been attempting to solve this problem for almost as long as bicycles have existed.  Frank Berto, on p. 53 of his magisterial book The Dancing Chain, cites the Protean device from 1894 as the earliest commercial attempt to manufacture an expanding chainwheel system. (In another post, I will outline other earlier attempts.)

Examples of historical patents

For over 100 years, many previous inventors had attempted various ingenious ways of expanding a gear’s radius, yet almost all of them involved making all the chain-engaging elements move at the same time. Apparently none of them had been commercially successful, however, since none of them were available today.  And my own limited tests with the crude models I could construct seemed to indicate that there was a problem in having gear segments move or expand simultaneously.  While distance between the teeth of the gear segments remained fixed relative to each other, the distance between the gear segments (and so the teeth of adjacent gear segments) would change while the pitch of the chain would not.  (To address this issue with VECTr I recently made a particular refinement to my present design.)

This new insight into the limitation of the idea I was pursuing allowed me to frame in greater specificity the expanding chainring problem: how could the radii at which the gear segments engage the chain grow or contract while still fitting into a chain whose links did not vary their pitch?  In one moment, while driving down a back road in Central Texas, the idea hit me that the gear segments could change their radial position (in fact they would have to do so) only when they were not engaging the chain.  I realized that on every turn of the crank, there was a quarter of the revolution when the gear was free of the chain.  This is the space between where the chain comes to the chainring from the rear sprocket and where it leaves the chainring to return to the rear sprocket.  It had to be in this quarter turn of the crank that the gear segments could change position without hindrance from the chain, but they could still engage the chain with properly pitched teeth.  This flash of insight sent my design of expanding chainrings into a new direction, one that would ultimately culminate in VECTr.