Read any bicycling magazine and you will find references to numbers: tire size, frame size, fork travel, bike gears. Except for bike gears, the numbers are easy to understand. Gears, for most riders, are what they hear about at the bike shop. But what do they mean? What difference does it make to you, the rider?
The numbers that you hear (52/39) refer to the number of teeth on the chain ring. The chain ring gears are a set of two or three numbers. A road bike generally has two chain rings, but can have three. A mountain bike will most likely have three (52/42/30). Larger numbers have more teeth on the gear. Different sized chain rings and gears have different numbers of teeth. A compact double chain ring has two chain rings; a triple, three chain rings. The large chain ring is the outboard ring, the small one is inboard. If it is a triple, the mid-sized ring is in-between the large and small rings. Chain rings can have as few a 20 teeth and run up to 56 or more.
The gears on the rear of the bike (the cassette) can come in sets of 6, 9, 12, or 10, or whatever someone who is experimenting with gears can come up with. On the rear of your bike, the big gear is in board, the small one, outboard. A compact double chain ring, with 10 gears on the back, is a 20 speed bike. Put a triple chain ring up front, and it turns into a 30 speed bike. More teeth on the chain ring gears means more revolutions of the derailleur gears, which accounts for the harder high gear–big chain ring, small cassette gear, and the easy granny low–small chain ring, big cassette gear.
As with so many things, the gearing on your bike depends upon some variable factors: where you are going to be riding (flat, hilly, mountains), your strength, your experience, mountain bike, road bike, or hybrid. When buying a bike, road, mountain, or hybrid, it’s a good idea to discuss with the shop tech what your main interest is and what the terrain is like where you will be riding the most. For the most part, new bikes, out of the box, have a good set of gears on them for most riders. If you are a racer, a ranked cat rider, or someone in the elite class, you’ll have different needs and will probably have a very specialized set of gears all around.
Your bike gears are an important part of enjoying your ride. The best place to take your questions about the gearing on your bike, or the new one you’d like to buy, is your local bike shop. Not only will you learn about the gears, you’ll get to look at all the great new bikes and gear on display. Just don’t drool too much on the new bikes.