The basic principle: pedals turn the chain, the chain turns the wheel

A bicycle moves because your legs push the pedals, which are attached to a crankset — the metal arms and gears near the center of the frame. When you pedal, the crankset rotates. A chain sits on the largest gear of the crankset and wraps around a smaller gear attached to the rear wheel. As the crankset turns, it pulls the chain, which pulls the rear wheel around. One full rotation of the pedals does not equal one full rotation of the wheel — the gear ratio determines how many times the wheel spins for each pedal stroke.

The smaller the rear gear relative to the front gear, the more the wheel turns per pedal stroke, which means you go faster but have to pedal harder. The larger the rear gear relative to the front gear, the less the wheel turns per pedal stroke, which means you go slower but can pedal easier. This is why bicycles have multiple gears: they let you match the pedal effort to the terrain.

Key Takeaways

  • Pedals drive a crankset, which pulls a chain, which rotates the rear wheel — the gear ratio determines how many wheel rotations happen per pedal stroke.
  • Larger front gears and smaller rear gears create higher speeds but require more leg effort; the opposite creates lower speeds with easier pedaling.
  • Brakes work by friction: rim brakes squeeze the wheel rim, while disc brakes squeeze a rotor attached to the wheel hub.
  • The frame, fork, and wheels are rigid structures that support your weight and absorb impact, while the handlebars let you steer by turning the front wheel.
  • Bearings at the wheel hubs, bottom bracket, and headset allow parts to spin freely with minimal resistance.

How gears multiply your pedal power

The crankset typically has two or three large gears (called chainrings), and the rear wheel hub has four to eleven smaller gears (called a cassette or sprockets). When you shift gears using the shifters on the handlebars, a derailleur — a spring-loaded arm — moves the chain sideways to sit on a different gear. Shifting to a smaller rear gear or larger front gear makes pedaling harder but speeds you up. Shifting to a larger rear gear or smaller front gear makes pedaling easier but slows you down.

The mechanical advantage works the same way as a lever: a larger gear has more teeth, so the chain travels a greater distance with each rotation, which means the wheel attached to that gear spins faster. A smaller gear has fewer teeth, so the chain travels a shorter distance, which means the wheel spins slower. On a flat road you might use a large front gear and small rear gear for speed. On a steep hill you would shift to a small front gear and large rear gear so your legs can push the pedals without exhausting you.

Brakes: stopping through friction

Bicycles use two main types of brakes. Rim brakes have rubber pads mounted on the frame that squeeze against the outer edge of the wheel rim when you pull the brake lever. The friction between the pads and the rim slows the wheel down. Disc brakes have a metal rotor (a flat disc) attached to the wheel hub and brake pads mounted on a caliper near the rotor. When you pull the lever, the caliper squeezes the rotor, creating friction that slows the wheel.

Disc brakes are more powerful and work better in wet or muddy conditions because the rotor stays cleaner than a rim. Rim brakes are lighter and cheaper. Both types rely on the same principle: friction between two surfaces slows the rotation of the wheel. The harder you pull the lever, the more force the pads explore, and the faster you stop. Brake cables or hydraulic lines connect the lever to the brake mechanism, so pulling the lever at the handlebar translates into squeezing force at the wheel.

The frame and fork: structure that holds everything together

The frame is the skeleton of the bicycle — typically a triangle made of steel, aluminum, carbon fiber, or titanium. It connects the front wheel, rear wheel, pedals, and handlebars into one rigid structure. The fork is the part that holds the front wheel and connects to the frame at the top. When you turn the handlebars, the fork rotates, which steers the front wheel left or right.

The frame must be stiff enough to transfer your pedal power efficiently to the rear wheel without flexing, but light enough that you can move the bicycle easily. Different frame materials have different trade-offs: steel is durable and cheap but heavy; aluminum is light and affordable but can crack; carbon fiber is very light and stiff but expensive and can shatter if damaged; titanium is light, strong, and durable but the most expensive option.

Wheels: how they stay round and roll smoothly

A bicycle wheel is a circle of metal rim with spokes connecting the rim to the hub at the center. The hub is where the wheel attaches to the frame. Bearings inside the hub are small metal balls held in a race (a circular groove). These bearings let the wheel spin freely around the axle with very little resistance. Without bearings, friction would slow the wheel dramatically.

The spokes are under tension — they pull inward on the rim while the rim pulls outward on them. This tension keeps the wheel round and rigid. If a spoke breaks or loosens, the wheel becomes wobbly and inefficient. The tire is a rubber covering that grips the ground and absorbs bumps. A tube inside the tire holds air pressure, which cushions impacts and reduces rolling resistance. Some bicycles use tubeless tires, which seal directly to the rim without an inner tube.

The bottom bracket and headset: where rotation happens

The bottom bracket is the bearing assembly where the crankset connects to the frame. It allows the crankset to spin freely as you pedal. The headset is the bearing assembly where the fork connects to the frame. It allows the fork to rotate so you can steer. Both use the same principle as wheel bearings: metal balls in a race that let metal parts spin with minimal friction.

If the bottom bracket or headset becomes loose or damaged, you will feel grinding, clicking, or play (movement where there should be none) when you pedal or steer. Keeping these bearings clean and properly adjusted is essential for smooth operation. Over time, dirt and water can enter the bearings and cause rust or wear, which is why regular maintenance extends the life of a bicycle.

How pedaling translates into forward motion

When you push down on one pedal, the crankset rotates. The chain pulls the rear gear, which rotates the rear wheel. The tire grips the ground, and friction between the tire and ground pushes the bicycle forward. The faster the rear wheel spins, the faster you move. The harder you push the pedals, the more force you explore to the chain, which applies more force to the wheel.

Your body weight sits on the saddle, which is supported by the seat tube of the frame. The frame transfers this weight to the wheels through the seat tube and chain stays. The wheels support your weight and roll forward. Steering happens when you turn the handlebars, which rotate the fork, which turns the front wheel. The front wheel then pulls the rest of the bicycle in that direction because the frame is rigidly connected to the fork.

Frequently Asked Questions

Why do some bicycles have more gears than others?

More gears give you finer control over the effort required to pedal in different conditions. A bicycle with 21 gears has more gear ratios to choose from than a bicycle with 7 gears, so you can find a ratio that matches the terrain more precisely. However, more gears also mean more weight, more complexity, and more maintenance. For casual riding on flat terrain, fewer gears are often sufficient.

What happens if the chain falls off?

If the chain falls off the gears, it will not transfer power from the pedals to the wheel, and you will not be able to move forward by pedaling. The chain usually falls off because the derailleur is misaligned, the chain is worn, or you shifted gears too abruptly. You can put the chain back on by hand and then check that the derailleur is properly adjusted so it does not happen again.

Why do bicycle wheels have spokes instead of being solid?

Spokes are much lighter than a solid wheel while still being strong enough to support your weight. The tension in the spokes keeps the wheel round and rigid. A solid wheel would be heavier and would not absorb impacts as well. Spokes also allow air to flow through the wheel, which reduces wind resistance at high speeds.

How does the chain stay on the gears?

The chain sits in the teeth of the gears, and the teeth grip the chain links. The derailleur keeps the chain aligned with the correct gear as you shift. Tension from the chain itself and the derailleur spring keeps the chain tight so it does not slip. If the chain becomes loose or worn, it can slip or fall off.

What is the difference between a fixed-gear and a freewheel bicycle?

On a fixed-gear bicycle, the pedals are always connected to the rear wheel, so the wheel cannot spin faster than the pedals. You cannot coast — if the wheel is moving, the pedals are moving. On a freewheel bicycle, the rear wheel can spin independently of the pedals, so you can coast without pedaling. Most modern bicycles have freewheels because coasting is more comfortable and safer.