Oh, and regarding chain ring size, one should start out with a Gear Inch ratio in mind, and then pursue that Gear Inch ratio by selecting a chain ring, cog, and tire size in combination.

Most factory bikes get delivered at 78 gear inches (a very high gear inch ratio, in my mind).

A 17t or 19t cog (prime numbers) will give the rider 17 or 19 skid patches, respectively, if the rider cares (I don't skid and I counsel against skidding).

A 47t chainring and a 17t cog with 23mm tires will give a very useful ratio of 72.7 gear inches; not to mention the charm of two prime numbers (it doesn't matter).

Go to the following site and play with the gear calculator:

http://software.bareknucklebrigade.c...it.applet.html