The main concept in speaking the language of gears is the difference between a physical gear (i.e. chainring or cog) and the gear ratio. The physical gears are what you shift to determine the gear ratio you pedal against. You can shift the front or rear derailleur to select a different chainring or cog. These shifts change the gear ratio. The trick is that moving to a bigger chainring makes a bigger gear ratio (a "harder" gear), while shifting to a bigger cog makes the opposite, a smaller gear ratio (an "easier" gear). This is where the confusion arises.
So let's lay out the common terms used in describing the range of gears.
- Chainring. The large gear in front attached to the crankset; either 1, 2, 3 different chainrings depending on the style of bike. Also referred to as the front (e.g. "shift up in front") or something related to the front derailleur; or a ring (i.e. big ring or small ring).
- Cog. The smaller series of gears in back; ranges from 1 on a single-speed bike up to 10 or 11 on modern bikes. Also referred to as the back or rear (e.g. "shift up in back"); sprocket; cassette; or cluster.
- Teeth/tooth. The number of teeth on a gear. Commonly 50 or 53 teeth are on the big chainring in front, and 11 or 12 teeth are on the smallest cog. Often abbreviated as "T" as in 53T chainring, but also often omitted to call a chainring simply a 53. Chainring combos are usually referred to as 53/39 (big ring, slash small ring), and cassette series are usually referred to as a range from the smallest to bigger cog, as in 12-23.
- Gear combination. The number of teeth of the chainring and cog that is being used. It's usually expressed as 53×12 if the chain is resting on the 53T chainring and the 12T cog.
- Gear ratio. The mathematical ratio that compares the combination of the front and rear gears: Gear ratio = Chainring ÷ Cog. For example, if the chain is running on a 53T chainring and a 12T cog, the gear ratio = 53 ÷ 12 = 4.4167. Note: the nomenclature for gear combination makes it look like you multiply the numbers (e.g., 53x12), but in reality you divide them to figure gear ratio.
- Gear development. Also commonly referred to as roll out, this is the distance the bike travels with one full revolution of the pedals. It is the product of the gear ratio (chainring ÷ cog) and the circumference of the rear wheel (wheel diameter × π). Therefore gear development = (chainring ÷ cog) × (wheel diameter × π). For example, riding in a 53x12 with a 700/23 rear wheel wheel results in (53 ÷ 12) × (26.3 inches × π) = 4.4167 × 82.6 inches = 364.8 inches, or 30 feet 4.8 inches. The reason for calculating gear development is to account for different wheel sizes that bikes use, such as the variety of common wheel sizes on mountain bikes.
- Gear inches. Still used to refer to gear sizing in track racing, this concept is a throwback to the days of the ordinary bicycle and indicates the size of wheel that would be necessary on an ordinary bicycle to match the gear development of a modern bike. That means, instead of using a combination of gears like on a modern bike, what size wheel would be required on an ordinary bicycle to cover the same distance with one revolution of the pedals. It is calculated as: gear inches = (chainring ÷ cog) × wheel diameter. For example, (53 ÷ 12) × 26.3 inches = 116.2 or just short of 10 feet. Since you'd need legs more than 5 feet long to pedal an ordinary bike this big, you can see why gears were invented!
To summarize the key parts of those terms, the chainring and cog combination determines the gear ratio (gear ratio = chainring ÷ cog). Because people use similar wheel and tire sizes, gear development is not often used when taking about gears, and gear inches are usually reserved for track racing.
So why all the terms, definitions, and math? As I pointed out earlier, we still haven't gotten to what it means when someone says, "Shift up!" or "Use a smaller gear." So let's get to the point.
If someone says smaller or bigger, take this to mean a smaller or bigger gear ratio; unless chainring or cog is specifically stated, bigger and smaller gears refer to bigger or smaller gear ratios. That means that a bigger gear means a harder gear to push and a smaller gear is an easier gear to push.
Keeping on this same theme, shifting up means shifting to a bigger gear ratio—a harder gear to push—and shifting down means a smaller gear ratio—an easier gear.
The trick is to pay attention for references to the cogs, though. If someone says, "Shift your cassette up," this does not mean the same as, "Shift up." Instead "Shift your cassette up," refers to going to a bigger cog in back, which is actually shifting down to an easier gear.
So why don't cyclists use the terms easier and harder to describe gears? Who knows!?
They just don't, and it's terminology that usually is reserved for beginners, so as you get used to the phrases and the meaning becomes clear, it's no longer a problem.
But to give some examples of the many variations, here are a list of terms and phrases that refer to bigger and smaller gears.
Shifting to a harder gear (bigger gear ratio)
- Shift up
- Put it in the big chainring
- Drop down your cassette
- Use a bigger gear
- Use a smaller cog
- Put it in the 12 (or 11; i.e. use the smallest cog)
- Use the big meat (big chainring)
Shifting to an easier gear (smaller gear ratio)
- Shift down
- Put in the small ring
- Go up your cassette
- Shift to a smaller gear
- Use a bigger sprocket