Gear Inches - Layman's Explanation

[Wesh-Laurence]

Is this simply the distance in inches you travel for one revolution of the pedal cranks?

[al_yrpal]

No, Pi is left out of Pi*D, its a simplification with D being the only variable. Multiply the inches quoted by Pi and you have an accurate distance. These figures are just a quick comparison.

Al

[stoobs]

It's the equivalent diameter of the wheel that you'd be riding if you had no gears but still went the same distance in one revolution of the cranks. That is to say, it's the size of an equivalent penny-farthing. At very low gears it's smaller than the wheels on your bike, and at high gears, the penny farthing would be much too large for your legs to reach the pedals.

Multiply this number by Pi (3.141) to get how far you progress with each rev of the cranks.

[Wesh-Laurence]

Thanks for the replies.

I've just found the explanation on Wikipedia. I think the Sheldon Brown's idea of using Gain Ratio far more useful.

[hartleymartin]

I'm too used to thinking in terms of gear inches, that gain ratios don't really make that much sense to me.

I know that I like my touring bicycles to have gears ranging from 20 to 95 gear inches, and that my favourite set-up for a 3-speed is 45/60/80 gear inches.

[thirdcrank]

.... I think the Sheldon Brown's idea of using Gain Ratio far more useful....

The point is 'useful for what?' IMO the main value of measuring a gear like this is to have some basis of comparison (eg so you know that 52 x 13 with be the same as 48 x 12 - picked for easy mental arithmetic. ) Beyond that, people can argue endlessly about the best steps between gears (less easy now with the demise of the sprocket board) or they can explain to people what gear inches are to people who don't know but it's a narrow field of study.

Since crank length tends to be arrived ot on the Ford system (anything you want so long as it's either 170 mm or 175 mm) the SB method doesn't really take things much further forward for a whole extra layer of complication. (And it's rare I risk being accused of apostasy )

[gaz]

.

[hubgearfreak]

my favourite set-up for a 3-speed is 45/60/80 gear inches.

yes. the holy trinity of ratios