Hi Matt!
Here is how it all lays out...
• Cranks are made for internal square-taper bottom brackets or for external-bearing bottom brackets.
• Typically, the cranks often paired with Rohloff drivetrains have BCD/PCD measurements (distance between chainring bolts) of 110mm (and five arms) or 104BCD/PCD (and four arms).
• Chainrings are available in a variety of sizes to fit cranks for either BB, BCD/PCD, and either number of arms.
The actual combination of chainring and cog depends a) on your needs and b) on the minimum ratios approved by Rohloff to avoid overstressing their hubs. Rohloff revised their allowable ratios only recently (about 10 months ago).
I am currently running the minimum Rohloff-approved combo, consisting of a 36T chainring and a 17T cog. I am using a 104BCD 4-arm crank and external bottom bracket. This same chainring would fit the new Thorn-branded 104BCD crank which uses an internal BB, a design which often results in greater bearing durability. 36T chainrings are also widely available to fit 110BCD 5-arm cranks.
I have been very pleased with the 36x17 combo for my needs because it offers a low gear of 15 gear-inches, which I find very useful for carrying very heavy loads up very steep hills. I can pedal that low gear with no problem, but sometimes people have trouble balancing at the low speeds that result from such a low gear (5-5.5mph/8-9kph @ my 120RPM, I can balance fine even at 2.5mph/4kph, YMMV). The high gear of "only" 80 gear-inches is fine for my needs because my bad knees won't pull higher gears without causing me pain and injury. As with the low gear, my fast-light "hummingbird" 110-120RPM pedaling cadence allows for a top speed of 26-28mph/42-48kph. While this is plenty fast for me while pedaling -- I simply coast downhill if my speed exceeds that -- many people would find an 80 gear-inch high to be too low, causing them to "spin out".
Rohloff hubs have a gear range of 526%. Through different combinations of chainring and cog, you can bias it toward a low gear range for climbing, medium gear range for all-'round use, or a high gear range if you're a demon for speed. Regardless, the spread between low and high gears remains the same at 526%. Given my intended use of the Nomad and where I ride it (and the state of my knees, my preferred fast cadence, steep hills, and often very heavy expedition loads including lots of water and food stores), it made sense for me to bias the range to the lowest possible. I often find myself riding in Gear 1 or Gear 2 under such circumstances.
Not everyone has the same needs or would be happy with the same choice. That said, I agree with Andy Blance: It sure is nice to have low gears available for use when you're already tired at the end of a long day and come up against a very steep hill; it makes touring life so much easier and more pleasurable to have those "bailout" gear just a wrist-twist away
All the best,
Dan.
