My design sense often leads me just outside the proverbial box. Just enough to create something unique that no one else has. I’m not into freak bikes but bikes that are functional, enjoyable to ride and to look at (for me). I make bikes from older cut up frame sections, reimagined into a fun to ride bike that attracts some attention (Frankenbikes). Lately I’ve been designing bikes with two different sized wheels. As in my front-loading Bakfiet with a 20” front and a 26” rear, it makes sense. But with my other mismatched wheeled creations, 700c in front with a 20” rear, the combination tends to get people scratching their heads and asking the same question, “Why a small wheel in the rear?” To this I usually reply, “I dunno, I think it looks cool.”
Although this design is more compact, as well as lighter in weight and quick initial roll, having a small wheel in the rear makes for a bumpier ride and lowers all the gearing which requires some special attention to get that tall gear back. I can either install a larger chainring or fewer teeth at the cog. A 60t chainring requires a new bottom bracket, spider and crank arms. Or a new wheel can be laced with a Caprio hub and cassette fitted with a 9t cog. Or for a really tall gear, I can do both. In any case, with all that is available, it’s possible for me to make a bike with a small rear wheel and have gearing as tall as I can handle.
In order to go to these extremes with the gearing, I need to make those decisions before I start building the frame. For a 60t chainring, chain stays need to be designed to accommodate the larger diameter. The bottom bracket shell needs to be compatible with components to handle a 110 bcd or more for the rare 60t chainring. If I choose to go with a specialty hub that can take a 9t cog, the dropout width needs to be known and addressed.
The bike's gearing is based on a 20” wheel with a 7 speed 14t-28t Shimano freewheel from a mini velo. The frame was made from two different bikes. An old Nishiki road bike, a kid’s 20” wheeled mountain bike and bits of metal scraps from other projects. They come in handy when I need a small piece of curve or taper for supports and details.
For this project, I used the original cotter crank with a 53t chainring. After the frame was fabricated, painted and all the components I intended to use were installed, I realized my first problem. The left side crank arm was not clearing the chain stay and was rubbing. I could do one of three things. First, I could replace the bottom bracket with a longer spindle, but that would require a whole new bracket and crank assembly (I’m too cheap to buy them). Secondly, I could locally modify the chain stay at the point of contact with the crank arm by dimpling the tube for clearance, but that would require repainting or even re-welding in the event I messed it up (at the time I was pretty sure I’d mess it up). And thirdly, I could grind the heel of the arm just enough to clear. Because the cotter crank is made of forged steel, I thought this to be the better and most painless option.
I’m happy to report that it worked well and has not compromised the pedal attachment at all.
After all was finished, I took it out on many rides but soon realized my high gear was too low for more high speed downhill pedaling (53t-14t). But I was stuck. I couldn’t go with a 60t chainring without reworking the frame or have another wheel made requiring a wider dropout. I could have installed a 20” wheel with a Nexus 7 hub from a Dahon that I subsequently bought on Craig’s List for another project, but I had done and purchased so much for a derailleur system with shifters and all, that I was determined to make this setup work. After some research I found a compromise. Turns out, there is a 7 speed 11t-28t freewheel made by Epoch that would replace the 14t-28t Shimano. I bought one online along with a thin-walled, deep-spline removal tool.
The only issue was that it did not go on to the exact position as the original Shimano. It was a few millimeters shifted to the right and brought the chain much too close to the inside of the dropout.
I then rearranged and replaced the spacers on the axle to bring the entire wheel a few millimeters to the left thus giving me the correct chain clearance and smooth shifting. I did the same to the v-brakes for symmetrical action. While it is true that my wheel is slightly off center, it is barely noticeable and hardly requires a re-dishing of the wheel.
Now I feel the bike’s gearing (53t-11t) is as normal as that of any bike I have in my collection. I am not a clipped-in spandex cyclist but more of a pleasure rider. Sure, I could go stiffer but that will be for another project.
In solving this problem, I’ve learned a lot and feel I’ve solved future problems as I continue to design and build just outside the box.
P.S. As to that question of “Why?”, I now simply say,
“Because I can”.
