The number of gears on a bike can range from just one (singlespeed) to thirty or more. The gear is changed by shifting the bike chain between different cogs. At the rear wheel, cogs are generally attached together as a unit called a cassette. The number of cogs on cassettes has steadily increased over the years. At the cranks (what the pedals attach to), cogs are referred to as chainrings.
At any given time, a bike rider obviously wants to choose a gear appropriate for the terrain. When we say choose a “gear” in this context, what we really mean is choose a gear ratio. The current gear ratio is just the number of teeth on the currently selected chainring (up front) divided by the number of teeth on the currently selected cog (in back). For example, a 32-tooth chainring and a 16-tooth cog result in a get ratio of 32/16 = 2.
For any bike with more than one gear, there will be a range of gear ratios available. Hang in there, we’re going to do a little bit of math although it’s very easy math. 🙂 For example, a bike with a single chainring and a 10-speed cassette will have 10 distinct gear ratios. If the chainring has 32 teeth and the cassette is an 11 – 36 (meaning that the smallest cog on the cassette has 11 teeth and the largest cog has 36 teeth), then the gear ratios range from 32/11 (2.9) to 32/36 (0.9). Ultimately, it’s this range of gear ratios that are the best measure of the bike’s gearing.
However, keep in mind that any bike that has more than one chainring up front generally has more than one gear choice that will result in a very similar gear ratio. Let’s take a look at at a classic 3-chainring setup where the chainrings have 22, 32 and 42 teeth respectively. Sticking with the same 11 – 36 cassette, we already know that the middle chainring (32 teeth) provides gear ratios from 0.9 to 2.9. The small chainring (22 teeth) is going to provide gear ratios from 22/11 (2.0) to 22/36 (0.6) and the big chainring is going to provide gear ratios from 42/11 (3.8) to 42/36 (1.2). Notice that there is considerable overlap between these three ranges.
Road vs. Mountain
Road bikes have traditionally come with two chainrings up front so the total number of gears is just twice the number of cogs on the rear cassette.
Mountain bikes all used to come with three chainrings up front but, as the number of cogs on a cassette has increased, there has been a recent trend to reduce the number of chainrings to two or even one. The primary advantage of having a single chainring is that, in addition to eliminating the weight of one or two chainrings, it also eliminates the need for a front derailleur and the front shifter thus saving a significant amount of weight. The downside of a single chainring is that it ultimately results in reduced range of available gears. To accommodate for this, cassettes are now available with an increased range of cog sizes.
How should all of this impact my bike buying decisions?
The good news is that bike gearing is fairly easy to modify after you buy a bike (by replacing the cassette and/or chainrings), so it doesn’t need to be your primary concern when choosing a bike. However, going from a single chainring to multiple chainrings is a bigger deal since it requires adding a front derailleur and front shifter and in some cases a new set of crank arms. So, if you’re pretty sure you want a particularly wide range of gears you might want to steer clear of bikes that come with just a single chainring.