While the original video of VECTr garnered positive feedback and interest, there were many questions about its operation as well as suggestions for improvement. (You can see a representative sample in the comments on various pages of the this website.) Easily, the number one criticism was that the expanding chainring design has been tried and found commercially wanting. As I continue to point out, there have been many expanding chainring designs that have not had sustainable commercial success, but the differences among them probably account for their varying degrees of profitability. Trying to find the simplest, most reliable design producible for the least cost is the true challenge in design in general, and specifically in the expanding chainring transmission game.
There were concerns specific to the VECTr design which I have spent my time trying to address. The first had to do with the smoothness of the operation. It has always been an aim of the design process to make a device which is compatible with current bicycle standards, and so I made a four gear segment mechanism which could be fitted to the 64 BCD fittings of a MTB crank arm.
It was noted, however, that this made the operation of VECTr appear “lumpy” and uneven, and viewers thought it would make for an uncomfortable ride. Yet, increasing the number of gear segments would increase steps between the gear ratios, and thus limit the range of settings for the device.
Also, the design of the locking/adjustment mechanism of the gear segment consisted of three pieces for the sliding motion, plus a pin/bolt and spring for the locking motion, in addition to the actual chain-engaging gear segment. These were all held together with simple blind rivets, which I knew were none too strong, and probably would not hold up to the stresses of actual road use. In working up some cost estimates for the manufacture of VECTr in that iteration of the design, it became clear that it would be difficult to entice a manufacturer to license the design, or for me to make them out of my garage. I thought, too, that simplifying the design of the gear segment mechanism would decrease the weight and offset the additional mass of added gear segments.
The result of rethinking these issues led me to design VECTr with five expansion arms (fitted for 110 BCD cranks) and five gear segments which would slide within narrower channels, still having notches on one edge into which a locking pin would fit. In a prior blog post I detailed how I had to call upon my trigonometric acumen (latent and seldom used as it was) to work out the proper spacing of the notches for the five arm design.
In this new design, I replaced the three-piece sliding parts with simple bolts (which also took the place of the rivets) and the locking bolt sliding with the rest; it would be biased into locking position by an external elastic band or other small spring.
I spent much of my time working on VECTr this past year (when I could work on it) trying to get this design to work, as well as working to make the control mechanism compatible with a Shimano indexed shifter. I detail some of that work in a prior blog post, too.
Given that I say that I tried to get this new design to work is a pretty good indication that it was not successful. I will have to let you know the insights this process gave me that led me to the current design, which I think shows even greater promise.
VECTr - Variably Expanding Chain Transmission 