Towards an ergonomic gearing system.

[zerodish]

I have every gear shimano makes including the rare 29 and have been making 38 39 and 40 teeth rear gears out of chainrings. What I did was carefully note at what speed I shifted and adjusted the gear ratios so that there was no speed gaps between the gears. The theory is simple at high speeds aerodynamic forces predominate so the change in energy is proportional to the cube of the change in ratio of the gears. At low speeds gravity predominates so the change in energy is directly proportional to the change in the ratio of the gears. So far the best is 39 30 24 20 17 15 13 on the rear. The next gear would be a 11.5 since this is impossible I use a 12. The middle gear on the front is 36. With a crossover a natural shift works out to 15 percent so the high gear would be about 42 which is what I use. This allows me to ride at my normal top speed of 28 mph with a optimum cadence though I can go 50 percent faster with a hill or a strong tail wind.  An optimum shift for the low front gear would be 40 percent this works out to a 26 tooth chainring. If you need more range than this you will have to put up with gaps in the gearing or go to a quad front chainring setup. I have built many of these though they are to wide to shift properly. It is possible to put four chainrings in the space used by 3 now. I think the best touring crank would have a 58mm bolt circle diameter for the inner an 94mm bolt circle for the next gear and a 110 bolt circle diameter for the outer two. This would allow for no gaps in the gearing. Note I use 8 gears with 9 gear spacing which allows me to build dishless wheels. All photos are available on flickr by searching zerodish. 

[Old Guy New Hobby]

What a post! I don't understand why gears are so close together. I guess that 90% of all bikers don't race. For us, there is a fairly broad power-cadence curve. We could easily get buy with the gears further apart. Often, by the time I shift, I want to shift two gears. We put triples on the front but get only a small increase in range. (Changing the front chainring typically adds only 2 or 3 gears.) We add more gears on the cassette and space the gears closer together. Most of us would get by fine with 7 or 8 gears, if they were spaced further apart to give decent range.

[canalligators]

There are many theories on gear spacing and patterns.  What John Forrester says applies handily for touring.  You need a low enough low, a high enough high, useable spacing and an easily remembered pattern.  Sacrifice higher gears for lower if you need to.  An easy pattern is important when you're riding a loaded bike, especially on uphills at low speeds.

In practice for touring, you want gears with spmewhere around 10% differences.  Closer spacing at the high end is most useful for racing, it is not terribly important for touring.