Calculating Gear Inches
Gear inches express gear ratios in terms of the diameter of an equivalent directly driven wheel, and are calculated as follows:
Diameter of drive wheel in inches × number of teeth in front chainring / number of teeth in rear sprocket.
where
- gi = gear inches
- dwd = drive wheel diameter
- fct = number of front chainring teeth
- rct = number of rear sprocket teeth
For example, suppose the drive wheel is actually 26 inches in diameter. If the front chainring and rear sprocket have equal numbers of teeth, one turn of the pedals produces exactly one turn of the drive wheel, just as if the pedals were directly driving the drive wheel. That combination of gears and wheel is said to be "26 gear inches." If the front chainring has 48 teeth and the rear sprocket has 24 teeth, then each turn of the pedals produces two turns of the rear wheel. This is equivalent to doubling the size of the drive wheel; that is, it is like a directly driven bicycle with a 52-inch wheel. That gear is said to be "52 gear inches."
A bicycle with a 26-inch wheel, a 48-tooth chainring, and a cassette with gears ranging from 11 to 34 teeth has a lowest gear of 26 × 48 / 34 = 36.7 gear inches and a highest gear of 26 × 48 / 11 = 113 gear inches.
For bicycles with 700c wheels, most cyclists quote gear inches based on a nominal wheel diameter of 27 inches, corresponding to the old British tire size of 27 x 1¼" (ETRTO 630). This means that a 48/18 setup is usually considered to be 72". Strictly speaking, the rolling diameter of a 700c wheel is significantly lower, at about 26" for a 20mm tire or 26.3" for a 23mm tire. This means that the true gear on a 700c wheel can be as low as 69", which is the equivalent of only 46/18 on an actual 27" wheel, and can be the source of some confusion when comparing gears unless it is clear whether gear inches have been calculated using the actual wheel size or a conventionalised 27".
The accurate way of calculating true wheel diameter is to take the rim diameter and add twice the tyre diameter. For example, 700c × 23mm. 700c wheels have a rim diameter of 622mm. Add 46mm (2 × tyre diameter of 23mm) + 622mm = 668mm which is equal to 26.3 inches (rounded to 1 decimal place). 26 inch mountain bicycle wheels have a rim diameter of 559 mm.
Read more about this topic: Gear Inches
Famous quotes containing the words calculating, gear and/or inches:
“[The] elderly and timid single gentleman in Paris ... never drove down the Champs Elysees without expecting an accident, and commonly witnessing one; or found himself in the neighborhood of an official without calculating the chances of a bomb. So long as the rates of progress held good, these bombs would double in force and number every ten years.”
—Henry Brooks Adams (1838–1918)
“But Lord, remember me and mine
Wi’ mercies temporal and divine!
That I for grace and gear may shine,
Excelled by nane!
And a’ the glory shall be thine!
Amen! Amen!”
—Robert Burns (1759–1796)
“The daily arguments over putting away the toys or practicing the piano defeat us so easily. We see them coming yet they frustrate us time and time again. In many cases, we are mothers and fathers who have managed budgets and unruly bosses and done difficult jobs well through sheer tenacity and dogged preparation. So why are we unable to persuade someone three feet tall to step into six inches of water at bathtime?”
—Cathy Rindner Tempelsman (20th century)