What Are Gear Inches?

So, what are “gear inches” and why should you care about them?

I got to thinking about them on one of my bike rides and wondered, if I was in my small chainring (34 teeth) and in my 13-tooth rear cassette what gear would be equivalent to that in my large chainring?

Well, the answer is below. If I was in my 34 tooth chainring and in my 13-tooth cassette it would be equivalent to my 50 tooth chainring and my 19-tooth cassette at 71 inches.

I’ve got a Shimano Dura-Ace cassette with the following gears: 11-12-13-14-15-17-19-21-24-27-30.

The 71” figure is the distance your wheel will travel with one full pedal turn at the prescribed chainring and cassette teeth. Therefore, if you’re in your 50-tooth chainring and your 11-tooth cassette, one revolution of your pedal will result in 123” of travel on the ground. Conversely, if you’re in your 34-tooth chainring and your 30-tooth cassette you’ll travel 31”.

How do you arrive at this? The calculation (based on a 27” wheel/tire diameter – this is approximate based on a 700mm wheel) is: Drive Wheel Diameter (27”) X # Teeth in Chainring/# Teeth in Cassette).

As you can see in this illustration below, being in the 34/13 is the same as being in the 50/19 at 71”. Additionally, this gearing results in nine truly unique gears (shaded blue, with four unique gears in the 34 and five in the 50 – all the rest basically overlap (out of 22 gears).

What about bikes that use a single chainring? This is becoming more popular but only on flatter courses because the compromise is either reducing the range of gears or having huge jumps between gears (like five or more gears at a time).

Who cares? I don’t know. But I do know it’s best NOT to cross-chain when possible. Cross-chaining is when you’re in your small chainring and your smallest gear on your cassette or the large chainring and your largest cassette gear. This stretches your chain and increases friction, making its life shorter and decreases your wattage by an estimated five watts (and who needs that).

There you go; don’t say I never gave you some basically useless information.

See you at the races!

Cheers!

Coach Ron Saetermoe

www.wiisecoaching.com

https://app.tridot.com/onboard/sign-up/ronsaetermoe

Gear Inches
Cassette	34	50
11	83	123
12	77	113
13	71	104
14	66	96
15	61	90
17	54	79
19	48	71
21	44	64
24	38	56
27	34	50
30	31	45
Wheel/Tire Size	27”