Traditionally gearing is measured in inches, but I’m not going down that trail a) because it means nothing to the beginner, and b) it’d take longer to explain than the gearing itself, so here’s an idiot’s guide.
The Basic Rule
Basically if you are going to do loaded touring and don’t like pushing, a good rule of thumb is that if you can still keep riding then the bottom gear isn’t too low. I for example can ride happily down to gentle walking place, about 3-4 MPH, and so my bike is geared so that at this speed I’m still turning my legs over at a reasonable one revolution per second. This is a much lower bottom gear than most off the peg bikes, but it’s always nice to have such a “Granny gear” in reserve even if you use it once in a blue moon. On my bike, which is equipped with 700c wheels, I get this sort of bottom gear using a 24 tooth inner chainring and a 32 tooth rear sprocket. A few years ago such a set up would be difficult to achieve but now those nice mountainbike people have provided us with “microdrive” chainsets, typically 42/32/24 which make super low bottom gears pretty easy to obtain. As a rule of thumb your biggest rear sprocket should be bigger than the inner chainwheel. If you ride mountainbike sized 26 inch wheels these automatically drop your gearing by a couple of teeth over a 700c wheel by virtue of their smaller circumference.
Gears When Climbing
The use of a microdrive set also means that a lot of your gears will be pretty low and for loaded touring this is how it should be. When working hard on a climb small gear differences can make a big difference to how tired you get, a choice of several low ratios means you can keep your climbing rhythm going as the hill varies in steepness. On the other hand bowling along the flat you’re generally working less hard so need a lesser choice of ratios, if your top gear isn’t big enough to pedal downhill then big deal – just stop pedalling and coast, it’s not an option up hill!
So to put my money where my mouth is, for 700c wheels a 24 or 26 inner chainwheel and 30 or 32 rear sprocket is ideal. With 26 inch wheels a rear sprocket of 28 will probably be as low as you can ride without wobbling into a ditch…
The Best Climbing Gears
Of the available sets I have one favorite and that’s Shimano’s STX especially the triple chainset. It’s well made and offers ideal ratios of 42/32/24. It also uses steel chainrings which in my experience last about 3 times as long as aluminium – using such small chainrings does make this important. The matching derailleurs are of good quality and I like the 7-speed hubs which build a stronger wheel than more exotic 8 speed hubs. As for the freehub casette, Shimano do a lovely 14-32 seven speed set which is hard to get but perfect. The new cassettes with a 34 tooth biggest sprocket are not as good as there’s too much gap between bottom gear and the next – remember you need closely spaced ratios for climbing. With this set up you should have set of ratios that will happily handle speeds between 3 and 20 mph, even if you’re a plodder like me…