By Bryan Buckalew

Mike Larson is a good guy to talk to about gear ratios. He teaches mechanical engineering at Seattle University. He also used to run production engineering and development at SRAM. Plus, of course, he commutes by bike every day – Ballard to Capital Hill. “I think it’s seven miles with two good hills,” Larson said.

Larson reduced the concept of gear ratios to this: “What does your rear wheel do for every revolution of the crank?”

Seems simple enough, right?

“It’s basically tooth count. A one-to-one gear ratio would be: every time you pedal, your tire goes around once.” So, “how many teeth do you have on the front and how many teeth do you have on the rear? You basically just take the teeth count at the input and divide it by the output.”

That’s chainring teeth count divided by rear cog teeth count. Here’s an example: Let’s say your bike has a 48-tooth chainring and a 24-tooth cog. Write that down. Forty-eight divided by 24. The answer? Two. That means you have a two-to-one advantage. Every time you pedal once, your rear wheel goes around twice. Now imagine you’re pedaling and you come to a hill. You switch the chain down to your 28-tooth chainring in front and up to your 28-tooth cog in back. Twenty-eight divided by 28 equals one. You pedal once, your rear tire goes around once.

Pretty simple really.

Great, you say, but what do gear ratios have to do with me? I’m a commuter. I use about five different gears on my daily ride. Why should I care?

The industry trend today is to add more gears, kind of like injecting cheese into the crust of a pizza.

“You’d be hard pressed to find a decent touring bike today that has only 24 speeds: three in front and eight in the rear. They’re going to 10 in the rear now. Some people are pushing it to 11. It’s crazy.”

But more cog/chainring combos (gears) don’t necessarily mean more options. Gear ratios overlap.

“I think somebody has said that for a 27-speed bike, three chainrings in the front and nine cogs in the rear, there’re really only thirteen or fourteen real different gear ratios.”

So don’t get fooled by a large number of gears, but do pay attention to the actual ratios to get the most out of your bike.