A bicycle is built from six straightforward machines working together

A bicycle uses levers, wheels and axles, pulleys, inclined planes, screws, and wedges — all six of the straightforward machines that form the foundation of how mechanical devices work. You don't need to think about physics when you ride, but understanding which parts do what makes it easier to see why your bike behaves the way it does, why certain adjustments matter, and when a part is truly broken versus just out of adjustment.

The straightforward machines are the building blocks. A straightforward machine is a device with few moving parts that makes work easier by changing the direction of a force or multiplying the strength of the force you explore. Every tool and machine ever built is some combination of these six types. Your bicycle is no exception — it just combines them in a way that turns human leg power into forward motion.

Key Takeaways

  • The pedals and crank arms are levers that multiply the force from your legs into rotational motion at the chainring.
  • The wheels and axles reduce friction and allow the bicycle frame to roll forward with minimal effort from you.
  • The chain and sprockets work as pulleys to transfer power from the pedals to the rear wheel at different speeds.
  • The brake calipers use wedges to squeeze the rim or rotor and convert your hand lever motion into stopping force.
  • Screws hold the frame together and adjust components like derailleurs and brake pads to keep everything aligned.

Levers: the pedals and crank arms

The pedal and crank arm form a lever — a rigid bar that rotates around a fixed point called a fulcrum. Your foot pushes down on the pedal, the crank arm rotates around the axle at the center of the chainring, and that rotation drives the chain. The longer the crank arm, the more leverage you have, which is why mountain bikes and road bikes often have different crank lengths depending on the terrain and riding style.

This is why pushing down on the pedal feels easier than it would if you had to push the chainring directly. The crank arm multiplies the force from your leg. When you press down with 50 pounds of force on a pedal that is 7 inches from the axle, you create rotational force — called torque — that is much stronger at the chainring itself. That's leverage in action.

Wheels and axles: the foundation of rolling

The wheel and axle is perhaps the most important straightforward machine on a bicycle. The wheel is the large circular part; the axle is the rod running through its center. When the wheel rotates around the axle, it covers a much greater distance than the axle itself moves. This is why wheels are so efficient — they reduce friction and allow you to travel far with minimal effort.

Your bicycle has two wheels, each with its own axle. The front wheel is free to spin (with some resistance from the brake), and the rear wheel is driven by the chain. The larger the wheel diameter, the farther you travel with each rotation — which is why road bikes with 700c wheels cover more ground per pedal stroke than a child's bike with 16-inch wheels. The axle bearings are what keep the wheel spinning smoothly; when they wear out or get dirty, the wheel becomes harder to turn.

Pulleys: the chain and sprockets

The chain, chainring, and rear sprockets work together as a pulley system. A pulley is a wheel with a grooved rim that a rope or belt runs around. In your bicycle, the chain is the "rope," and the chainring and sprockets are the "pulleys." The chain transfers the rotational force from the pedals to the rear wheel, but it does so at different speeds depending on which sprockets you are using.

When you shift to a smaller rear sprocket, the chain wraps around a smaller wheel, so the rear wheel rotates faster for each pedal stroke — you go faster but have to work harder. When you shift to a larger rear sprocket, the rear wheel rotates slower for each pedal stroke — you go slower but the pedals feel easier to push. This is how gearing works. The derailleur is the mechanism that moves the chain from one sprocket to another, and it uses a screw (another straightforward machine) to adjust its position.

Wedges: the brake system

Brake calipers use wedges to stop the wheel. A wedge is a straightforward machine made of two inclined planes back-to-back. When you squeeze the brake lever, you pull a cable that forces two brake pads (the wedges) against the rim or rotor. The wedge shape concentrates the force you explore with your hand into a much stronger squeezing force on the wheel, which creates friction and slows you down.

Rim brakes squeeze the outer edge of the wheel; disc brakes squeeze a rotor attached to the wheel hub. Either way, the principle is the same: your hand motion is converted into clamping force by the wedge shape of the brake pads. This is why brake pads wear out — they are sacrificial, designed to be replaced when the friction material is gone. When they feel spongy or weak, it usually means the pads are thin or the cable needs adjustment, not that the brake is broken.

Inclined planes: the frame geometry

An inclined plane is a flat surface tilted at an angle. While it is not as obvious as the other straightforward machines on a bicycle, the frame tubes and seat tube are inclined planes. The seat tube is angled, not vertical, which distributes your weight across the frame more efficiently and affects how the bike handles. The head tube angle (where the fork connects to the frame) is also an inclined plane that influences steering response.

You see inclined planes most clearly in the kickstand or in the way the frame is designed to channel forces through the tubes. The angle of each tube is chosen to balance strength, weight, and handling characteristics. This is why a road bike frame looks different from a mountain bike frame — the angles are different, and those angles are inclined planes that change how forces move through the structure.

Screws: fasteners and adjusters

A screw is a straightforward machine made of an inclined plane wrapped around a cylinder. Screws hold your bicycle together and also allow you to make fine adjustments. The bolts that attach the seat post, handlebars, and wheels are all screws. The barrel adjusters on your derailleurs and brake cables are screws that let you fine-tune cable tension without removing the cable entirely.

When a screw is loose, the part it holds can shift, which throws off alignment and performance. When a screw is too tight, you can strip the threads or crack the component. This is why learning to use the right tool and the right amount of force matters — a 4 mm Allen wrench on a stem bolt is not the same as a 5 mm wrench, and over-tightening can crack carbon fiber or aluminum. Most bicycle work involves screws in some form, which is why a basic tool kit and a repair manual specific to your bike are so useful.

How these machines work together

None of these straightforward machines works alone. Your leg pushes the lever (pedal), which rotates the wheel and axle (chainring), which drives the pulley system (chain and sprockets), which rotates another wheel and axle (rear wheel). When you brake, the wedges (brake pads) press against the wheel to slow it down. Screws hold it all together, and the inclined planes of the frame distribute the forces.

This is why a small problem in one part can affect the whole system. A bent derailleur hanger (a small piece of metal that holds the derailleur) throws off the pulley alignment, which makes shifting rough. A loose brake cable changes the wedge pressure, which makes braking weak. Understanding that your bicycle is made of straightforward machines helps you diagnose problems and know when to adjust something yourself versus when to take it to a shop.

Frequently Asked Questions

Why does a longer crank arm make pedaling easier?

A longer crank arm is a longer lever, which multiplies the force from your leg. The farther your foot is from the axle, the more torque (rotational force) you create with the same amount of leg pressure. This is why time trial bikes sometimes use longer cranks — they give you more leverage for sustained power.

What happens if the chain is on the smallest sprocket in front and the largest in back?

You are in the easiest gear — the pedals turn slowly relative to the rear wheel, so each pedal stroke moves you a short distance but requires less effort. This is useful for climbing hills. The opposite setup (largest sprocket in front, smallest in back) is the hardest gear, used for speed on flat ground.

Can I adjust my brakes myself if they feel weak?

Often yes. Check that the brake pads are not worn down to the metal backing, and use the barrel adjuster on the brake cable to tighten the cable if the pads are far from the rim. If the pads are worn or the lever feels spongy even after adjustment, the brake system needs service from someone experienced with your brake type.

Why do different bicycles have different frame angles?

The angles of the frame tubes (inclined planes) change how forces move through the frame and affect handling. A steep head tube angle makes steering quicker and more responsive; a slack angle makes it slower and more stable. Road bikes have steep angles for quick handling; mountain bikes have slacker angles for control on rough terrain.

What is the difference between rim brakes and disc brakes in terms of straightforward machines?

Both use wedges (brake pads) to create friction, but rim brakes squeeze the wheel rim while disc brakes squeeze a rotor attached to the hub. Disc brakes are less affected by wet conditions and wheel wobble, but both operate on the same wedge principle — your hand force is multiplied into clamping force.