exupero's blog

Bike gear ratios

I have a 21-speed bike with three gears on the pedals and seven on the back wheel. I've ridden it for two decades and always assumed the gearing ratios produced by using a bigger gear at the petal surpassed all the ratios produced by using a smaller gear, but riding recently, I wondered if it might be better to have overlapping ranges of gearing ratios. Then the bike chain wouldn't have to switch as often between the big gears on the pedal, since one could pick whichever gear was most suitable for the current terrain. Also, when the chain does have to switch between pedal gears, the transitions could be smoother as the chain wouldn't have to jump all the way to the opposite end of the rear gears to reach the next highest gearing ratio.

To check, I counted the teeth on each gear assembly. On the pedals, the smallest gear has 22 teeth, the middle gear 32 teeth, and the largest 42 teeth. On the rear wheel, the gears have 11 teeth, 13 teeth, 15 teeth, 18 teeth, 21 teeth, 24 teeth, and 28 teeth. In combination, that produces the following (approximate) gearing ratios:


Sure enough, the ranges of each pedal's gear ratios overlap. It's nice to know the sweet spot for any given terrain isn't going to be on a transition between one pedal gear and another.