Gear Inch Question

[TT Tandem]

I was doing the calculation for my new set-up. Campy 12-29 cassette and a 50-34 chain ring. The calculator I was using changed the calculation slightly when the tire width was changed from 23 to 25.
I was wondering why the tire width would have an effect?

[nooneline]

Because increasing your tire width also increases the circumference of your wheel. And so one pedal rotation results in a longer roll-out of the wheel - that is, the length of one complete rotation of the wheel.

FYI,
Gear inches is a shorthand term that refers to the size of the wheel that you'd need in order to have a penny farthing with the exact same gearing. Sort of a silly term, but it's stuck.

[TT Tandem]

You beat me to it! I just realized that!!!
Thanks

[palincss]

I was doing the calculation for my new set-up. Campy 12-29 cassette and a 50-34 chain ring. The calculator I was using changed the calculation slightly when the tire width was changed from 23 to 25.
I was wondering why the tire width would have an effect?

Because when you increase the width you also increase the height, which increases wheel diameter. Recall, the formula for gear inches is #teeth in front / #teeth in back * wheel diameter in inches.

[bluesea]

I was doing the calculation for my new set-up. Campy 12-29 cassette and a 50-34 chain ring. The calculator I was using changed the calculation slightly when the tire width was changed from 23 to 25.
I was wondering why the tire width would have an effect?



I went through that exercise a couple of weeks ago. The 11sp 12-27 has a tighter cogset which is a real improvement over a 10sp 12-27. I decided to try the 12-29 for the bailout gear for these steep gradients here in HNL.