The basic parts and what they do
A bicycle moves forward because you push pedals that turn a chain, and that chain turns the rear wheel. The frame holds everything together, the handlebars let you steer, and the brakes slow you down by pressing rubber pads against the wheels. That is the whole system: pedals, chain, wheels, brakes, and a frame to connect them.
The frame is the skeleton. It is usually made of steel, aluminum, or carbon fiber, and it carries your weight while you ride. The two wheels are connected to the frame by metal rods called axles, which let them spin freely. The front wheel is attached loosely so you can turn it left and right with the handlebars. The rear wheel is fixed in place so it only spins forward and backward.
The seat sits on top of the frame and holds your weight. The handlebars stick up from the front and let you steer and balance. Everything else—pedals, chain, brakes, gears—attaches to this frame and works together to move you forward.
Key Takeaways
- Pedals turn a chainring, which pulls a chain that spins the rear wheel and moves the bicycle forward.
- The frame is the structure that holds the wheels, seat, handlebars, and all other parts in place.
- Brakes press rubber pads against the wheel rims or discs to slow down or stop the bicycle.
- Gears change how hard you have to pedal and how fast the rear wheel spins, making hills easier and flat ground faster.
- Bearings are small metal balls inside the wheel hubs and pedal cranks that let parts spin smoothly with less friction.
How the pedals and chain move the wheel
When you push down on a pedal, you turn a large toothed ring called a chainring (or sprocket). The chainring is bolted to the pedal crank, so they spin together. As the chainring spins, it pulls a metal chain that wraps around a smaller toothed ring on the rear wheel hub, called a cog or sprocket. The chain pulls the cog, the cog spins, and the rear wheel spins with it.
The chain is just a loop of metal links held together by pins. Each link can bend slightly, so the chain can wrap smoothly around the chainring and the cog without jamming. The teeth on the chainring and cog are shaped so the chain sits in the grooves and does not slip off when you pedal hard.
The harder you push the pedals, the more force you put into the chainring, and the more force the chain passes to the rear wheel. If the chainring is large and the cog is small, one turn of the pedals spins the rear wheel many times, so you go fast but have to pedal harder. If the chainring is small and the cog is large, one turn of the pedals spins the rear wheel fewer times, so you go slower but pedaling feels easier. This is why bicycles have gears.
How gears make pedaling easier or faster
Most bicycles have multiple chainrings in the front and multiple cogs in the back. When you shift gears, you move the chain from one chainring or cog to another. A derailleur is a small metal arm that pushes the chain sideways to guide it onto a different gear. The front derailleur moves the chain between the front chainrings. The rear derailleur moves the chain between the rear cogs.
Shifting to a smaller chainring or a larger cog makes pedaling easier because the rear wheel does not spin as many times per pedal stroke. This is useful on hills, where you need less force to keep moving. Shifting to a larger chainring or a smaller cog makes the rear wheel spin more times per pedal stroke, so you go faster on flat ground, but pedaling feels harder.
The chain tensioner (part of the rear derailleur) keeps the chain tight so it does not sag or fall off. Without it, the chain would go slack when you shift to a smaller cog, and it would not grip the teeth properly.
How brakes slow you down and stop
Brakes work by creating friction between a moving part and a stationary part. There are two main types: rim brakes and disc brakes.
Rim brakes have rubber pads on a metal arm. When you squeeze the brake lever, a cable pulls the arm, and the pads press against the metal rim of the wheel. The friction between the pads and the rim slows the wheel down. The harder you squeeze, the harder the pads press, and the faster you stop. Rim brakes are lighter and cheaper, but they do not work well when the wheel rim is wet or muddy.
Disc brakes have a metal disc (called a rotor) bolted to the wheel hub. When you squeeze the brake lever, hydraulic fluid or a cable pushes brake pads against both sides of the disc. The friction stops the wheel. Disc brakes work better in wet and muddy conditions because the rotor is not exposed to the rim, and they give you more stopping power. Most modern bicycles use disc brakes.
How wheels and bearings spin smoothly
A bicycle wheel is a circle of metal spokes connected to a metal hub at the center. The hub is where the axle passes through, and it is the part that actually spins. Inside the hub are bearings—small metal balls held in a metal cage. The balls sit in grooves and let the hub spin around the axle with very little friction.
Without bearings, the hub would rub directly against the axle, and friction would slow you down and wear out both parts quickly. The balls in the bearing roll smoothly, so the hub can spin freely. Over time, bearings can wear out or get dirty, and the wheel will feel rough or wobbly. When that happens, the bearings need to be cleaned or replaced.
The spokes are thin metal rods that connect the hub to the rim. They are under tension, which keeps the rim round and strong. If a spoke breaks or loosens, the wheel can become uneven and rub against the brakes. Spokes can be tightened with a spoke wrench if the wheel starts to wobble.
How steering and balance work
The front wheel is connected to the frame by a fork, which is a metal piece that looks like a tuning fork. The fork is attached to the frame at a joint called the headset, which has bearings inside so the fork can turn smoothly. When you turn the handlebars, you turn the fork, and the front wheel turns with it.
Steering a bicycle is not just about turning the handlebars. When you lean into a turn, the bicycle naturally wants to turn in that direction because of the angle of the fork and the way the wheel contacts the ground. This is called trail, and it is why bicycles are stable even when you are not holding the handlebars tightly. The fork is angled slightly forward, so the wheel naturally follows the direction your body is leaning.
Balance comes from the speed of the wheels and the position of your weight. When you are moving, the spinning wheels act like gyroscopes and resist tipping over. When you stop, you have to put your foot down or you will fall. This is why bicycles are easier to ride fast than slow.
What can go wrong and when to get help
Most bicycle problems are small and can be fixed at home with basic tools. A loose chain can be tightened by adjusting the rear axle or the derailleur. Brake pads that are worn down can be replaced. A flat tire can be patched or the tube replaced. Rust on the chain can be cleaned off with a brush and oil.
Some problems need a professional. If the wheel rim is bent, it needs to be trued (straightened) with a spoke wrench, which takes skill and the right tool. If the bearings are damaged, the hub may need to be rebuilt or replaced. If the frame is cracked, the bicycle may not be safe to ride. If you are not sure whether something is safe to ride, take it to a local bike shop and have them look at it.
Regular maintenance keeps a bicycle working well. Clean the chain every month, check the tire pressure before each ride, and inspect the brakes and gears to make sure they shift smoothly. A bicycle that is well cared for will last for years.
Frequently Asked Questions
Why does my chain keep falling off?
A chain falls off when it is too loose, when the derailleur is not adjusted correctly, or when the chainring or cog teeth are worn down. Check that the chain is tight by pushing on it with your finger—it should move about half an inch. If the derailleur is bent, it may need to be straightened or replaced. If the teeth are worn, the chainring or cog needs to be replaced.
What does it mean when my brakes feel spongy?
Spongy brakes usually mean there is air in the brake line (on hydraulic disc brakes) or the brake cable is stretched (on rim brakes). Hydraulic brakes need to be bled to remove the air, which is a job for a bike shop. Cable brakes can often be tightened by adjusting the barrel adjuster on the brake lever or the brake arm.
How often should I replace my chain?
A chain typically lasts between 1,000 and 3,000 miles, depending on how often you ride and how well you maintain it. A dirty or rusty chain wears out faster. You can check chain wear with a chain checker tool, which measures how much the links have stretched. Most bike shops can do this check for a small fee.
Why does my wheel wobble when I ride?
A wobbling wheel usually means the rim is bent, the spokes are loose, or the bearings are worn. Spin the wheel and watch where it rubs against the brake pads. If the rim moves side to side, it needs to be trued. If the wheel feels rough or grinding when you spin it by hand, the bearings may be damaged and need replacement.
Can I fix a flat tire myself?
Yes. Remove the wheel, take out the inner tube, find the hole (you can listen for air or feel for a puncture), and patch it with a patch kit or replace the tube with a new one. Reinstall the wheel and pump it up. If the tire has a large gash or the valve is damaged, the tube cannot be patched and must be replaced.