kerank

I've seen some posts related to bicycle gearing, and am trying to figure out how to relate each chain-ring / cog combination to each other. I think I read it has something to do with a ratio... would that be chain-ring/cog or vise-versa? An example of a problem I'm trying to solve: I have a bike with a double and one with a triple chain-ring. On the double, if I ride a 39 chain-ring with a 23 cog, to get the same "gear" using my triple 42 chain-ring, what cog would I have to use?

Could anyone enlighten me?

bac

You need to convert apples to apples. Gear inches is what you need. However, this site gives you even more:

Sheldon Brown Gear-Inch Converter

Good luck!

KevinF

The link to Sheldon's website is definitely a good one, but it all comes down to the ratio of the chainring (the one up front) to the cog (the one in back). Your proposed 39x23 gear essentially means that the rear wheel will turn 39/23 = 1.69 times for each revolution of the cranks. How far that will actually take you, or how fast you can get going, etc., is dependent on your tire size and how fast you are pedalling.

So... If you're in a 39x23 gear, and you want to know its equivalent on a 39-tooth chainring, it's simply:
39/23 = 42/x... Solve that for "x" and you get x = 24.7, meaning a 42x25 is roughly equivalent to a 39x23 assuming you're using the same tire size.

Retro Grouch 

To understand gear-inches, think back to the days of high-wheeler bikes. A larger wheel would obviously go faster but the size of the wheel that you could ride would be limited by the length of your legs.

A chain drive bike solves that problem. Divide the front chainring teeth by the rear cog teeth and multiply by the diameter of the drive wheel and you'll know what size high-wheeler would be equivlent. The same formula is commonly used to compare the gearing on any bike.

zacster

I've always used 27" as the rear wheel size, even though I've been riding 700c for longer than a lot of you are alive. So 39/23*27 = 45.8. It doesn't really matter what you use as the rear wheel size if you always use the same size, its all relative, but if you use 650c on one bike and 700c on another it does.