Originally Posted by
VegasJen
OK, I have *no* idea what you're talking about.
Not surprising, I realized later that gain-ratio is not just referenced on Sheldon Brown, he invented it, so may be a bit obscure.
Gear ratio, chainring to cogs, is just what it says, and doesn't take into account different size wheels. The same gear ratios on my 20" wheels will be inherently "lower" geared versus 700c.
Gear-inches takes gear ratio, AND wheel/tire size into account.
GAIN RATIO also takes crank arm length into account, along with gears and tire size; Longer crank arms give more leverage ("lower" gearing), but then the pedals have a larger circle to make (more displacement), just like with anything that makes the gearing lower, you need to move your feet farther (more revolutions), but not push as hard. So that's what gain ratio does, it also takes crank arm length into account.
Whole explanation on Sheldon Brown of current gearing descriptive systems, like Gear-Inches, Development in meters, etc., plus gain ratios (semi-long):
https://sheldonbrown.com/gain.html
Short form of above, if you already know the other systems:
What About Crank Length?
All of these systems share a common inadequacy: none of them takes crank length into account! The fact is that a mountain bike with a 46/16 has the same gear as a road bike with a 53/19 only if they have the same length cranks. If the mountain bike has 175's and the road bike 170's, the gear on the mountain bike is really about 3% lower!
A New Standard Proposed
I would like to propose a new system, which does take crank length into account. This system is independent of units, being expressed as a pure ratio.
This ratio would be calculated as follows: divide the wheel radius by the crank length; this will yield a single radius ratio applicable to all of the gears of a given bike. The individual gear ratios are calculated as with gear inches, using this radius ratio instead of the wheel size.
The gear calculator on Sheldon Brown has a gain ratio option under "gear units":
https://www.sheldonbrown.com/gear-calc.html
I don't know my needs in gain-ratios, as I've never tried different arm lengths. I just know that with 170 arms, I need 21-85 gear-inch range, at least, at both ends. Touring with load and long climbs, I might need lower than 21.