Bike Drivetrain Glossary: Chainrings, Cogs, Chain and Derailleurs

The parts that carry power from your legs to the rear wheel: cranks, chainrings, chain, cassette, derailleurs and shifters. It's described by the number of chainrings and cogs, for example 2 × 12.

See also: groupset, speeds (e.g. 12-speed), 1x drivetrain Gear Ratios

The drivetrain and brakes from a single brand and product line (Shimano 105, SRAM Rival, Campagnolo Chorus…). You can mix parts from different groupsets, but the number of speeds and the cable pull have to match.

See also: drivetrain, speeds (e.g. 12-speed)

The toothed ring attached to the cranks that drives the chain. More teeth means a harder gear: 50/34 or 52/36 teeth are typical on road bikes; mountain bikes usually run a single 30-34 tooth chainring.

See also: cog (sprocket), crankset, bolt circle diameter (BCD), rollout (gear development) Gear Ratios

Each of the toothed rings on the cassette. More teeth means an easier gear: the smallest usually has 10 or 11 teeth and the largest 28 to 34 on road bikes and 50 to 52 on mountain bikes.

See also: cassette, chainring, rollout (gear development) Gear Ratios

The cluster of cogs that slides onto the freehub body of the rear hub. It's named by its smallest and largest cog: an 11-34 goes from 11 to 34 teeth. It must have the same number of speeds as the chain and derailleur.

See also: cog (sprocket), freehub body, speeds (e.g. 12-speed) Gear Ratios

The two crank arms with their chainrings and, on many models, the spindle that joins them. You choose one by crank length, chainring tooth counts and spindle standard.

See also: crank arm, chainring, bottom bracket Crank Length

Each of the two arms that connect the bottom bracket spindle to the pedal. Its length is measured from the center of the spindle to the center of the pedal; 170-172.5 mm is the most common size for adults.

See also: crankset, clipless pedal Crank Length

The spindle and bearings the cranks turn on, installed in the frame's bottom bracket shell. If you feel side-to-side play or hear creaking when you push hard on the pedals, it's usually the first thing to check.

See also: bottom bracket shell, crankset, bearing

Connects the chainrings to the cogs. Each number of speeds has its own width (the more speeds, the narrower the chain), and length is counted in links: most chains are between 106 and 126 links.

See also: quick link, chain wear (chain stretch), chain tool Chain Length

A link you can open and close by hand or with special pliers to install or remove the chain without breaking it. It must match the chain's number of speeds.

See also: chain, chain tool Chain Length

A tool that pushes out the pin of a link to shorten or open the chain. You need it to size a new chain before closing it with its quick link or connecting pin.

See also: chain, quick link Chain Length

The mechanism that moves the chain from one cog to another and keeps it tight. Each model has a maximum cog size (for example 34 or 52 teeth) and a total capacity in teeth that you shouldn't exceed.

See also: derailleur hanger, jockey wheels (pulleys), derailleur clutch, limit screw

The mechanism that moves the chain from one chainring to another. It disappears on single-chainring (1x) drivetrains, which are more and more common on mountain and gravel bikes.

See also: chainring, 1x drivetrain, cross-chaining

The handlebar control that operates the derailleur, either by pulling a cable or by sending an electronic signal. On mountain and city bikes it's separate from the brake lever; on road bikes it's usually built into it.

See also: brifter (shift/brake lever), cable housing, electronic shifting

A road bike lever that brakes and shifts. Each brand has its own name for it: STI or Dual Control at Shimano, DoubleTap or eTap at SRAM and Ergopower at Campagnolo.

See also: shifter, brake lever, drop handlebar

How far the bike travels with each full turn of the cranks. With a 50-tooth chainring, a 25-tooth cog and a wheel with a 2.10 m circumference: 50 / 25 × 2.10 = 4.2 m (about 13.8 ft). Americans often compare gears in gear inches instead.

See also: gear ratio, chainring, cog (sprocket), wheel circumference Gear Ratios

Chainring teeth divided by cog teeth: with 50/25 the ratio is 2, so the wheel turns twice for every pedal stroke. Unlike rollout, it doesn't take wheel size into account.

See also: rollout (gear development), cross-chaining Gear Ratios

A drivetrain with a single chainring and no front derailleur, written 1x (1 × 12, for example). It's simpler, lighter and more reliable; the trade-off is bigger jumps between cogs than with two chainrings.

See also: front derailleur, cassette, chainline Gear Ratios

A road crankset with 50- and 34-tooth chainrings, easier for climbing than the classic 53/39. The semi-compact (52/36) sits in between and is the best seller on race bikes.

See also: chainring, crankset, rollout (gear development) Gear Ratios

Using the big chainring with the biggest cogs, or the small chainring with the smallest cogs. The chain runs at an angle: it rubs, gets noisy, loses some efficiency and wears faster.

See also: chainline, front derailleur Gear Ratios

The distance from the center of the chainring (or chainrings) to the frame's centerline. Ideally it lines up with the center of the cassette; on mountain bikes with Boost hubs it's usually 52 mm.

See also: cross-chaining, Boost, 1x drivetrain

The two small toothed wheels on the rear derailleur: the upper one guides the chain and the lower one keeps it tight. They run on bearings or bushings and are worth cleaning often because they collect grease and grime.

See also: rear derailleur, derailleur clutch

Each of the two screws marked H and L that limit how far the derailleur can move, so the chain doesn't drop into the spokes or off the outside.

See also: rear derailleur, front derailleur, barrel adjuster

A hollow screw the cable runs through; turning it changes cable tension. A quarter or half turn is usually enough to fix a derailleur that hesitates between two cogs.

See also: cable housing, rear derailleur, limit screw

The outer tube that the derailleur or brake cable runs through. Shift housing has lengthwise strands so it doesn't compress; brake housing is spiral-wound to handle hard pulls.

See also: barrel adjuster, shifter, internal cable routing

A mechanism in the rear derailleur that resists cage movement so the chain doesn't slap the frame or drop on rough terrain. Almost every mountain and gravel derailleur has one, often with a lever to switch it off when you remove the wheel.

See also: rear derailleur, jockey wheels (pulleys)

The part of the rear hub the cassette slides onto, which lets you stop pedaling without the cranks turning. There are several incompatible standards: Shimano HG and Microspline, SRAM XD and XDR, and Campagnolo N3W.

See also: cassette, hub

The number of cogs on the cassette: a 12-speed drivetrain has 12 cogs. Chain, cassette, derailleur and shifter all need the same number of speeds to work properly.

See also: cassette, groupset, chain Gear Ratios

Chain elongation caused by worn pins and rollers. Check it with a chain checker about every 300 miles (500 km), or once a month: the usual rule is to replace it at 0.5% on 11- and 12-speed and at 0.75% on 10-speed or fewer; past 1%, you'll usually need a new cassette too.

See also: chain, cassette Chain Length

The diameter of the circle formed by the bolts that hold the chainrings to the crank spider. A chainring only fits a crankset with the same BCD (110 or 130 mm, for example); direct-mount chainrings don't use bolts.

See also: chainring, crankset