Bike Gearing, part 1

I'm considering changing the gearing on my race bike. What does that mean?

First, a picture (of my training bike):

Chainring and Cassette


No big mystery here.

Chainrings come as doubles or triples, which is just how many there are. Many race bikes have a 53x39 chainring, and another popular chainring is the 52x42x30. The numbers are simply how many teeth are on a chainring. Thus, the 53 has 53 teeth, and is obviously bigger in circumference than a 39 chainring. A 53x39 chainring has one chainring with 53 teeth and one chainring with 39 teeth.

Cassettes are numbered in the same way, except usually there are so many it isn't convenient to list them all. For example, a popular cassette is a 9 speed 12-25. This means there are nine gears, the smallest has 12 teeth, and the largest has 25 teeth. The ones in between are 13, 14, 15, 16, 17, 19, 22. Basically, you can look up what the middle gears are in a catalog. Other popular cassettes are the 12-27 or the 11-23.

With this knowledge, you can describe your gear combination as two numbers: the first is the chainring, and the second is the cassette. For example, you may choose to pedal downhill in the 53x12, on the flats in the 53x19, and uphill in the 39x23.

When someone mentions 53x17 or 53x39, how can you tell if they are talking about a chainring-cassette combination (53x17) or talking about two chainrings (53x39)? This is by context, as the largest common cassette gear is 27, and the smallest common chainring (that I've heard of) is a 34. So while is it possible someone talking about a 53x17 means two gears on the chainring and 53x39 to mean a 53 tooth chainring and 39 tooth cassette (and presumably this person own a lot of metal working gear and machines their own parts), it isn't likely.

What is important is the gear ratio which is easy to calculate: it is just the front gear divided by the back gear. Say I'm in the 53 up front, and the 17 in the back. I'd say that I was riding in the 53x17, and my gear ratio would be 53/17 = 3.12. If I were in the 39x25, my gear ratio would be 1.56, and so forth.

Gear ratio matters because from this, plus some other info (wheel diameter and cadence), you can calculate how fast you will go for a given gearing. Wheel diameter is fixed for a particular bike (well, tire thickness can make it vary slightly), so the only thing that normally varies while you are riding, is cadence, which is how many revolutions per minute you pedal.

The lower the gear ratio, the easier it is to pedal. Of course, it will be easier to pedal on a flat surface versus a hill, but in either case, a lower gear ratio is easier to pedal than a higher gear ratio.

Part of gear selection is picking a chainring and cassette so the gear ratios overlap as little as possible, and also provide coverage for the speeds you plan to travel at. I'll go into this in another post.