AKA: Gear Ratios
Understanding gearing and how it affects the way your bike rides isn't hard; you just have to know a few basics. Of course, things can get pretty technical in this area too, but for our purposes, the basics are exactly what we’ll stick with. First and foremost, if you want your bike to accelerate quickly and feel easy to pedal, decrease the number of teeth on your front sprocket, or increase the number of teeth on your freewheel or rear cog. If you want more top-end speed (even though it’ll be harder to get started), put on a bigger front sprocket, or go to a smaller freewheel. Going up (or down) a tooth on your freewheel will have a much bigger effect on how your bike accelerates than going up or down on your front sprocket, so if you're looking to fine-tune, start up front.
For racers, the challenge has always been finding a gear that will get them up to top speed the fastest, yet won't leave them “spinning out” once they get there. Take a 42/16 for instance; that's a great gear set-up for the first 30-40 feet of the track, since it lets you accelerate quickly, but unless you can pedal really fast (and smoothly), you're going to get smoked around the rest of the track. If you put on a 45 or a 46-16, you’ll probably be slow out of the gate, but you'll have more speed available to you once you get moving to work your way through the pack. You’ll have to experiment to see what works best with your strength and riding style.
It used to be that virtually everyone ran a 44/16, but within the last two years, that's been thrown out the window, especially for freestyle. Street riders wanted smaller sprockets that didn't get caught during grinds, and they wanted to shave ounces wherever possible. That's why you'll find smaller gear ratios, even as small as 25-9, on a lot of bikes today. So, as the sprocket sizes drop, how are we supposed to know what gear set-ups to use to maintain the same pedaling power as a 44/16?
If we divide the number of teeth on the sprocket by the number of teeth on the freewheel, we get what is called a gear ratio, which is the number of revolutions the rear wheel makes for each revolution of the front sprocket. Therefore, the gear ratio associated with a 44/16 is 2.75 (44÷16), meaning the rear wheel will roll two-and-three-quarter times for every rotation of the sprocket. As the tooth counts on freewheels and cassette hub drivers go down, we simply need to pick a front sprocket which will allow us to stay as close to that 2.75 gear ratio as possible to keep everything feeling the same (see chart).
Since smaller sprockets and shorter chains equal weight savings, you might be asking yourself why everyone isn't running smaller gears. There are two primary reasons: At present, freewheels only go down to 14-tooth models, so if you want to go smaller than that, you’ll need a cassette hub, which is typically heavier and more expensive. Secondly, as the sprockets get smaller, there's more stress on your chain when you start laying down the horsepower. Smaller rear cogs mean less chain engagement–there are fewer teeth (roughly half of the cog) locked into your chain, so you might suffer some chain skip. The smaller gear set-up also dramatically increases the load on your chain, so chain breakage becomes more likely, and who wants to break a chain while blasting out of the gate or cranking toward a gap? Before you start micro-sizing your drivetrain, make sure it can handle what you dish out.
Well, now you should know why that 44/9 you’re running isn’t quite working out. Cut out those gear charts and put them in your wallet; next time that cute girl in math class wants to know what 38 divided by 15 is, whip ’em out and you’ll be her knight in shining armor.
Chart 1: Gear Ratios
The numbers inn this chart are gear ratios, which are obtained by dividing the number of teeth on the front sprocket by the number of teeth on the rear. For instance, the gear ratio for a 44/16 set-up is 2.75. The highlighted ratios in the chart correspond with other gear set-ups that produce a ratio closest to 2.75.
Chart 2: Inch Gears
The numbers in this chart are obtained by multiplying the gear ratio by the wheel circumference (which is 20 x * for a BMX bike). The result is the distance you will travel, in inches, with each full revolution of the front sprocket.