Originally Posted by
User1
Great article, but the one thing the author failed to explain is the relationship of inches to rpm. For example; "On a touring bike, 18″ is a great low gear and 113″ is a good high gear." 18" = one rpm of the crank, and 113" = one rpm of the crank.
Sorry to say but you aren't really helping explain the relationship of gear inches to crank revolutions. First, "rpm" stands for revolutions per minute. Gear inches really don't need a time component to explain them. I think you really mean
18" = one revolution of the crank, and 113" = one revolution of the crank
But even this is confusing and wrong. One turn of a crank in an 18" gear will move the bicycle forward 35". One turn of a 113" gear will move the bike forward 354" or 29 feet. A 700C X 32 tire/wheel combination has a circumference of 85". A 34 tooth chainwheel and a 34 tooth cassette cog, for example, is a 1:1 gear which means that one revolution of the crank moves the bike forward 85".
Gear inches is mostly an anachronism left over from the days of the "
ordinary" where the crank was attached directly to the axle of the wheel. One revolution of that kind of crank resulted in one revolution of the wheel. An 18" ordinary was an 18" wheel that moved forward 35" for one turn of the crank. A 113" ordinary would move forward 29 feet for each turn of the crank. The problem is that you can't go very fast on an 18" wheel unless you spin it at an insane rate per minute. A cadence of 120 rpm (which is about the limit of how fast humans can spin a crank with their legs) in an 18" gear results in a speed of about 3 mph. So you didn't want to ride an 18" ordinary anywhere.
The other end of the spectrum...the 113" ordinary...would move you don't the road at a good brisk clip even at a low rpm. At 60 rpm, a 113" ordinary would do 20 mph. The problem is you can pedal a 113" ordinary unless you have legs that are about 4.5 feet long. A "normal" size for an ordinary was 60" to 70". The average man...and ordinarys were almost exclusively ridden by men...has a 30 to 34" inseam and can reach the pedals on that size wheel.
Gears, gear reduction and derailers have allowed bicyclists to ride bikes that are like riding an "ordinary" from 15" (possibly less) to, well, the sky's the limit. But the relationship of revolutions of the crank to forward motion is a bit more complex. Europeans tend to use meters development which can be expressed as
Meters development = drive wheel circumference in meters X (number of teeth in front chainring/number of teeth in rear sprocket)
If you multiply gear inches by 0.08, you get meters development. You could also get "feet development" by substituting "meters" with "feet" in the above equation.