Originally Posted by
jimmuller
"Gear inches" refers to the effective wheel diameter you would have if your pedals were connected directly to the drive wheel. That's how it was with a high-wheeler - you needed a larger wheel to go faster.
You calculate gear inches by multiplying the wheel diameter times the chainring size, then dividing by the cog size. Or as a formula:
G = d * F / R.
A 52T ring with a 14T cog would be 27 * 52 / 14 = ~100 inches. (A 700c wheel is close enough to 27" for this purpose.)
The ideal gear depends on terrain and riding style, of course. I find I ride mostly in the range of 50"-60". For the most common hills around here it is nice to have a low around 32". I almost never use anything higher than, say, 80".
Originally Posted by
ThermionicScott
Usually anything over 25 is with some degree of downhill or a tailwind, but I can still apply power up to ~30 MPH or so. Being able to spin is a useful thing if you're in your highest gear.
The others explained it pretty well, but one nice thing about gear inches is that it removes wheel size from the measurement so you can compare gearing across different bikes. 72" is 72", whether you're on a BMX, an MTB, or a road bike with 27" wheels.
Well, I don't know about all that. All I know is on my 1979 Sekini 10speed racer, the wheels are 27in dia. the crank set has 52teeth on the largest sprocket and the cassette has 14teeth on the smallest sprocket. On flat ground I peddle about about 7 revolutions per 5 seconds (I just estimated this at my desk), that's 84rpm. Something like 36rpm uphill and I've done uphill on 10th gear.
So at 84rpm I don't know how those calculations would have me going at over 100.