Before there were derailleurs there were internal-gear hubs. I was brought up appreciating the low maintenance needs of Sturmey-Archer 3 speed hubs. Around 1999 I couldn’t get one where I lived, ended up riding a Shimano Nexus Inter 7-speed. That 7 speed is dead now in 2013, a can of metal shavings, possibly worsened by unwisely riding it under water this winter. This creek hasn’t stopped me yet, but after hydrolocking a truck engine around 2003 it contributed to an early demise of my hub this time around – same creek, different crossing. For me to learn limits.
Now I’m working on replacing the hub with a NuVinci brand continuously variable transmission. Fascinating different mechanical design. I did not see it coming. Reviews by owners seem to be positive. Putting it into my existing bike. We’ll see how that performs, ask a few months from now.
I never was into working on bikes over and over, weekend after weekend. Some use of time yes, the fun of achievement, but then I want years of trouble-free use. I like these guys’ theme
“riding bikes was a ticket to freedom and fun; it was more about the experience, riding with friends, where you’re going and with whom, exploring;
that does not seem to exist today as bicycling seems more focused on your gear, not on you”
but for myself for now I don’t like their beautiful product depends on electronics. I’m keeping mine to a simpler manual shifter: Manual still would work after a solar storm and even in a non-apocalyptic world gives an amount of control I prefer.
Regarding ratios, buried in an article at Wikipedia is a simple truth:
In bicycle hub gears, the sun is usually stationary, being keyed to the axle or even machined directly onto it. The planetary gear carrier is used as input. In this case the gear ratio is simply given by (Ns+Na)/Na. The number of teeth in the planet gear is irrelevant.
That is for input planet gear carrier to output ring gear. The other way around is reciprocal. Fixed is one. Hence classic Sturmey-Archer ratios are 3/4, 1, 4/3 (75%, 100%, 133%) for a range of 177%. I was able to come to the same result.
The math of planetary internal-gear hub ratios bicycles also is discussed in an excerpt of a good engineering textbook, linked from a blog. While their formulas are right, a keen observer may notice they’ve apparently not looked at classic Sturmey-Archer model ratios, which doesn’t diminish the correctness of their math.
There is a practical list of internal gear ratios.
There is a great website about internal-gear hubs by Sheldon Brown.
Not in the habit of writing about sports and recreation, this qualified for an exception: Gears, water damage, and a new kind of transmission – exposing stuff hidden inside casings.