Bicycle Gearing

People often talking about how hard or easy it is to pedal their bike, but really don't understand how all the gearing works.

While the cyclists most interested in gearing tend to be single speed or fixed gear riders, everyone should know that not all cranksets and rear cassettes (the pieces of your bike that actually create all the gear inches / ratios) are created equal.

Let's defined a few terms that you will need to know.
1) Chain ring: The ring(s) that are attached to the cranks. Bikes will have from 1(single), 2 (double), or 3 (triple) chain rings.
2) Cassette, Freewheel, Cog: All terms used to describe the gearing rings used on the rear wheel. Each is similar, but different.
~ A cog is single ring, that usually attaches to the rear wheel and does not have a ratcheting mechanism. Without ratcheting, your pedals will constantly turn even if you try to coast, and use of this system is called riding "Fixed gear."
~ A Freewheel is a cog mounted on a ratcheting mechanism, which allows you to coast when you stop pedaling.
~ A cassette is a cluster of cogs mounted to a freewheel, offering you the ability to switch between gears. (cassettes can range from 2 -10 gears)

There are two terms used in gearing, Gear Ratio and Gear Inches.
Ratios take into account the direct relationship between your chainrings & the rear cogs; while Gear inched is similar, it also takes into account the size of the wheels/tires that you are using too.

It may sound complicated, but it really isn't.

With so many wheels sizes being used these days, Gear Inches is the most accurate way to make sure you are gearing all your bike alike.

For the sake of time and ease, we'll just cover a Single Speed / fixed gear ratio and inches.

~ Gear Ratio = Chainring size / rear cog size.
~ Gear Inches = (Chainring size / Rear cog) * wheelsize in inches

Here are a few examples:
A 43 tooth chainring and a 20 tooth cog being used on a bike with 29" wheels would look like this:
~ Gear Ratio = 43/20 = 2:51 ratio.
~ Gear Inches = (43/20)*29 = 49.4 gear inches.

So what's the difference?
Let's say you feel comfortable pedaling your current bike that is using a 32t chainring, 16t cog, and 26" wheels; but now you bought a new bike and want to have the same gearing.
If you do the math on your current bike:

~ Gear Ratio = 32/16 = 2:1 ratio.
~ Gear Inches = (32/16)*26 = 52 gear inches.

But what if your new bike has 29" wheels?
If you use the 2:1 "Gear Ratio", You'll find that it's not the same pedal stroke. In fact it'll be much, much harder.

Using a 29" wheel and 32t/16t = 58 gear inches!
So now all you need to do is change a few values until you locate a Gear Inches for the new bike that is similar to your old bike.
In this case, using a 32t chainring, 18t cog, & a 29" gives you a gear ratio of 51.5 gear inches, which is very close to your old bike.
For fun, you can do a few options +/- to fine tune your choice.
( 32/17 )*29 = 54.6")
( 32/18 )*29 = 51.5")
( 32/19 )*29 = 48.8")