How does a motorcycle work?

A motorcycle is a bicycle's shape with a small engine in the middle. Our model is a typical Indian 150 cc commuter, with the numbers of a Bajaj Pulsar 150.

The steel frame is a double cradle: two tubes run down from the steering head and under the engine like a hammock. The engine is a single cylinder covered in cooling fins, because air, not water, carries its heat away. Fuel injection (older bikes use a carburettor) mixes petrol into air that has passed through the air filter. Burnt gas leaves through the exhaust and the silencer.

Behind the cylinder, in the same metal case, sit the clutch (a stack of plates bathed in oil, so it's called a wet multi-plate clutch) and a five-speed gearbox. A chain and two sprockets take the drive to the back wheel.

Up front, telescopic forks hold the wheel and soak up bumps; at the back, a swingarm and two shock absorbers do the same. A disc brake stops the front wheel and a drum brake the rear. A 12-volt battery and the ECU, the engine's little computer, live under the seat.

A commuter bike's engine is one cylinder with one piston. It works like a car engine: suck in air and petrol, squeeze it, bang (a spark lights it), blow the burnt gas out. Four strokes take two turns of the crank. (For the four strokes in more detail, and why cars use four cylinders, see CarClear.)

With one cylinder there is only one bang every two turns, so a heavy flywheel keeps the crank spinning in between. At 8,500 rpm that is still about 70 bangs a second.

There is no radiator. Instead the cylinder wears cooling fins. Heat flows out through the metal, and the fins give the passing air a big surface to carry it from: about eight times more than a bare cylinder. The faster you ride, the more air sweeps past. Stuck in traffic at full load, an air-cooled engine runs hot.

Petrol must be mixed into the air. A carburettor does it with a narrow throat, the venturi: the air speeds up, its pressure drops, and petrol is sucked out of a tiny jet. Since BS-VI rules in April 2020, new Indian bikes use fuel injection instead: a sensor-fed computer sprays exactly the right amount, which cuts pollution.

An engine is happiest between about 3,000 and 9,000 rpm, but the back wheel turns anywhere from 0 to 1,000 rpm. Three sets of gears close the gap.

First, a small gear on the crank drives a big one on the clutch: the primary drive, 3.47 : 1. The clutch is a stack of plates, friction plates turned by the engine and steel plates that turn the gearbox, squeezed together by springs. Pull the clutch lever and the stack opens, so the engine spins without driving the wheel.

The gearbox has five pairs of gears always in mesh. Only the pair that is locked to the output shaft by a sliding dog ring passes the drive. You pick it with your left foot: each press or lift of the gear lever turns the shift drum one click. Grooves cut round the drum push selector forks sideways, which slide the dog rings. The grooves only allow one step at a time, in order: 1-N-2-3-4-5, one down and four up. This is a sequential gearbox.

Last, the chain runs from a 15-tooth sprocket to a 42-tooth one on the wheel: the final drive, 2.8 : 1. Keep it oiled and with a couple of centimetres of slack: too tight and it wears the bearings, too loose and it can jump off.

A car turns by pointing its front wheels. A motorcycle turns by leaning. In a steady turn, two things pull on the bike: gravity pulls down, and the road's grip pushes the tyres sideways, towards the middle of the turn. The bike must lean until those two balance around the tyres, at an angle where

tan θ = v² ÷ (g × r)

At 60 km/h round a bend 50 m across the middle (radius 50 m), that is about 29°. Twice as fast needs four times the sideways grip. Real tyres are round, so the bike leans a degree or two more than the formula says.

How do you start a lean? Surprisingly, by steering the wrong way for a moment. Push the right handlebar forward and the front wheel points a little left. The tyres run out from under the bike, it tips to the right, and then the bars swing back right to hold the turn. Riders call this countersteering. Above walking pace, every rider does it, mostly without knowing.

Once leaned, the bike holds itself steady. The steering axis is tilted back (the rake, 26° here), so the tyre touches the road about 100 mm behind where the axis hits the ground (the trail). The road drags the tyre into line like a shopping-trolley wheel. The spinning wheels act as gyroscopes too, which helps at speed. CycleClear shows how the same tricks balance a bicycle.

Brakes turn speed into heat. A disc brake squeezes pads onto a steel disc; a drum brake pushes shoes out against the inside of a drum. Discs shed heat faster and fade less, so most bikes have one at the front.

When you brake, the bike's weight surges forward and the forks dive. The front tyre is pressed harder into the road, so it can grip harder, while the rear goes light. In a hard stop the front brake does about 70–80% of the stopping. That's why stopping with only the rear brake takes twice as far.

The shortest stop comes when the tyres are just about to slide. The distance is d = v² ÷ (2 × μ × g), where μ is the tyre's grip: about 0.8 on dry roads, 0.5 on wet ones. Double the speed and you need four times the distance.

Grab the front brake too hard and the wheel locks. A skidding front tyre can't steer or hold the bike up, and it falls. ABS watches each wheel's speed; when one starts to lock, it lets the pressure off and on again about 10 to 15 times a second. CBS, the combined braking system, makes the rear lever also work the front brake a little. In India, since 1 April 2019 every new two-wheeler above 125 cc must have ABS, and smaller ones ABS or CBS.

Indian commuters are famous for mileage: a 100–125 cc bike often gets 60–70 km from a litre of petrol, a small car about 15–20. Why?

Riding at a steady speed on a flat road, the engine only has to beat two things. Air drag grows with speed squared (and the power to beat it with speed cubed). Rolling resistance, from the tyres squashing as they roll, grows with weight. A bike and rider weigh under 200 kg and push aside a small hole in the air. At 45 km/h they need only about 1 kW at the wheel, less than an electric kettle.

A small engine is also a better fit for that little job. Every engine wastes some fuel just turning itself over (friction and pumping). A big engine working gently wastes more of its fuel that way than a small one working harder. That's why a car, CarClear's kind of machine, needs several times more petrol per kilometre.

An electric scooter goes further still: about 25–30 Wh from the socket per kilometre, because an electric motor wastes very little. The same kilometre on a petrol commuter burns about four to five times as much energy.