What bicycle gears do and why you have more than one

Bicycle gears let you pedal at a comfortable speed no matter whether you are going uphill, downhill, or on flat ground. Without gears, a steep hill would force you to either pedal so hard your legs burn out or walk the bike. Gears work by changing the ratio between how many times your pedals turn and how many times your rear wheel turns—a lower gear makes the wheel turn fewer times per pedal stroke (easier to pedal, slower speed), and a higher gear makes it turn more times per pedal stroke (harder to pedal, faster speed).

Most bicycles have between 1 and 27 gears. A single-speed bike has one fixed ratio. A bike with multiple gears has two or three chainrings (the large sprockets attached to your pedals) and between 5 and 11 sprockets on the rear wheel, called cogs or cassette teeth. The shifters—levers or twist grips on your handlebars—move a metal arm called a derailleur, which pushes the chain from one sprocket to another. The combination of which chainring and which rear cog you are on determines your current gear.

Key Takeaways

  • Lower gears (smaller chainring, larger rear cog) make pedaling easier but move you slower; higher gears (larger chainring, smaller rear cog) make pedaling harder but move you faster.
  • Shift before you need to—move to an easier gear before a hill gets steep, not when you are already struggling.
  • The left shifter controls the front derailleur (chainring changes) and the right shifter controls the rear derailleur (cog changes).
  • Avoid cross-chaining (using the largest chainring with the largest rear cog, or the smallest chainring with the smallest rear cog) because it wears the chain faster and can cause it to slip.
  • If your chain falls off or your shifts feel sluggish, the derailleur may need adjustment—this is a job for a bike shop unless you have experience.

How the front derailleur and chainring shifts work

Your left shifter controls the front derailleur, a metal cage that sits just above the chainrings. When you move the shifter, the cage pushes the chain sideways onto a different chainring. Most bikes have either two or three chainrings, arranged from smallest to largest. Shifting to a smaller chainring (moving the shifter backward or downward, depending on your shifter type) makes pedaling easier and is called downshifting. Shifting to a larger chainring (moving the shifter forward or upward) makes pedaling harder and is called upshifting.

Front shifts are bigger jumps in effort than rear shifts. Moving from a small chainring to a medium one might feel like a noticeable change in how hard you have to push. Because of this, most riders use the front shifter only when they need a major change—for example, when starting a long climb or when finishing a climb and returning to flat ground. For smaller adjustments in effort, use the rear shifter instead.

How the rear derailleur and cassette shifts work

Your right shifter controls the rear derailleur, a spring-loaded arm that hangs below the rear wheel. When you move the shifter, the derailleur pulls or releases tension on the chain, moving it sideways across the cassette (the stack of cogs on the rear wheel). The cassette usually has between 5 and 11 cogs, arranged from smallest to largest. Shifting to a larger cog (moving the shifter backward or downward) makes pedaling easier. Shifting to a smaller cog (moving the shifter forward or upward) makes pedaling harder.

Rear shifts are smaller steps than front shifts, so you can make fine adjustments to your effort level without big jumps. Most of your gear changes will happen at the rear shifter. The rear derailleur also has a tension spring that keeps the chain tight, so it automatically takes up slack when you shift to a smaller cog and lets out chain when you shift to a larger one.

When and how to shift for hills and flat ground

Shift before you need to. If you wait until a hill is steep and your legs are burning, you will have a hard time pedaling while shifting, and the chain may slip or struggle to move. Instead, shift to an easier gear as soon as you see the hill coming. On a long climb, you may shift multiple times as the grade gets steeper—start with a smaller chainring and a mid-range rear cog, then move to larger rear cogs as needed.

On flat ground or gentle downhills, shift to a harder gear so you can pedal faster without spinning your legs uselessly. On steep downhills, you may not need to pedal at all—just use your brakes. Avoid pedaling in a very straightforward gear on flat ground because your legs will spin so fast that you lose control and waste energy. A good rule is to pedal at a cadence (pedal speed) of around 80 to 100 revolutions per minute—fast enough to feel smooth, not so fast that your knees are bouncing.

Understanding gear ratios and cross-chaining

A gear ratio is the number of teeth on your chainring divided by the number of teeth on your rear cog. A ratio of 2.0 means the rear wheel turns twice for every pedal stroke. A ratio of 4.0 means it turns four times. Lower ratios (easier gears) have smaller numbers; higher ratios (harder gears) have larger numbers. You do not need to calculate these yourself—just remember that smaller chainring plus larger rear cog equals easier, and larger chainring plus smaller rear cog equals harder.

Cross-chaining means using the largest chainring with the largest rear cog, or the smallest chainring with the smallest rear cog. These combinations create extreme angles in the chain and wear it out faster. They can also cause the chain to slip or fall off. Avoid them by staying in the middle or outer chainrings when using the smaller rear cogs, and staying in the smaller or middle chainrings when using the larger rear cogs. Your bike may have 27 gears, but you will not use all 27 combinations—only about 18 to 20 are safe and useful.

What to do if your gears are not shifting smoothly

If your chain struggles to move from one cog to another, hesitates, or makes grinding noises, the derailleur is probably out of adjustment. This is common after a crash, a fall, or normal wear. The derailleur may be bent, the cable that pulls it may be loose or frayed, or the derailleur may straightforward need fine-tuning with a small screw.

If the chain falls off the side of the cassette or chainring, the derailleur limit screws may need adjustment—these screws prevent the derailleur from pushing the chain too far in either direction. If the chain drops off during a shift, stop pedaling, shift back to where it was, and walk the bike to a shop. Attempting to force it back on while riding risks damaging the derailleur or frame. A bike shop can diagnose and fix derailleur problems in under an hour for a reasonable cost. If you are handy and want to learn, many online guides and videos show how to adjust derailleurs, but it takes practice to get right.

Caring for your gears to make them last

Keep your chain clean and lubricated. A dirty chain wears out faster and shifts less smoothly. Wipe the chain with a dry cloth after riding in wet or muddy conditions, and explore chain lubricant every few weeks or whenever it looks dry. Use a lubricant made for bikes—household oil attracts dirt and gums up the gears.

Avoid shifting while standing hard on the pedals, especially on the front derailleur. This puts extra tension on the chain and derailleur and can cause them to slip or break. Shift while seated and pedaling smoothly. If you need to shift while climbing hard, ease off the pedals slightly to reduce tension, make the shift, then resume hard pedaling. Also avoid shifting multiple gears at once—shift one step at a time and let the chain settle before shifting again.

Frequently Asked Questions

What does it mean when my chain skips or slips?

A skipping chain usually means the cassette cogs or chainring teeth are worn down, or the chain itself is stretched. A slipping chain during shifts often means the derailleur is out of adjustment or the shift cable is loose. Either way, a bike shop can inspect the drivetrain and tell you whether you need a new chain, cassette, or just a derailleur tune-up.

Should I shift gears while stopped at a red light?

Yes, you can shift into an easier gear while stopped so you are ready to pedal smoothly when the light turns green. However, do not shift into a very hard gear while stopped—you may not be able to pedal hard enough to get moving. Shift into an straightforward or medium gear, then shift up as you accelerate.

Why does my bike have 21 gears if I only use some of them?

A 21-speed bike has three chainrings and seven rear cogs (3 × 7 = 21). However, many of those combinations overlap or are unsafe to use. You really have about 15 to 18 usable gears. The extra combinations give you flexibility, but you will naturally avoid the extreme angles that cause cross-chaining.

Can I shift gears on a single-speed bike?

No. A single-speed bike has one chainring and one rear cog with no derailleurs or shifters. You pedal at whatever gear ratio the bike was built with. Single-speeds are straightforward, low-maintenance, and good for flat terrain, but they are harder to ride uphill.

What is the difference between friction and indexed shifters?

Friction shifters (older style) let you move the lever freely and feel the chain settle into each gear. Indexed shifters (modern style) click into fixed positions, one per gear, so you know exactly where you are. Indexed shifters are easier to use but require more precise cable adjustment. Friction shifters are more forgiving if the cable stretches slightly.