How Cars Work · From Bang to Wheels

Out to the wheels

The transmission has done its job. We have a shaft coming out the back of it, spinning at the right speed for the gear we are in.

In an old American car, the engine and transmission are at the front, but the wheels that get powered are at the back. So the spinning has to travel from front to rear. It does this through a long metal rod called the driveshaft, which runs along the underside of the car like a backbone.

The driveshaft is spinning. The two rear wheels need to spin. We are almost done. There is just one more clever piece.

When a car turns a corner, the two rear wheels travel different distances. The outside wheel goes around the outside of the curve (longer path), and the inside wheel goes around the inside (shorter path). If both wheels were forced to spin at the same speed, one of them would have to skid sideways through every turn.

So between the driveshaft and the two rear wheels sits a clever box of gears called the differential. It lets the two wheels turn at different speeds while still both being driven by the same driveshaft. The driveshaft turns the differential, the differential drives both rear axles, and the two axles can spin independently if the wheels are on different parts of a curve.

That is the last link in the chain. Crankshaft → clutch → transmission → driveshaft → differential → axles → wheels. The car moves.

Try It: Bicycle Gear Ratios

You will need:

• A bicycle with multiple gears

• A way to flip it upside down (a bike stand, or just balance it on the seat and handlebars)

Turn the bike upside down so the wheels are off the ground and can spin freely. Look at the gears. The pedals turn a big toothed wheel called the chainring. The chain runs back to a smaller toothed wheel on the rear axle, which turns the rear wheel.

Pick the highest gear (largest chainring in front, smallest cog in back). Slowly turn the pedal one full turn with your hand and count how many times the rear wheel spins. Probably four or five times.

Now shift to the lowest gear (smallest chainring in front, largest cog in back). Turn the pedal one full turn again and count. Probably one and a half, maybe two times.

That difference is gear ratio. In the high gear, one pedal turn equals many wheel turns — great for going fast on a flat road, but very hard to start moving from a standstill. In the low gear, one pedal turn equals barely a wheel turn — great for getting up hills or starting from zero, but you can’t go very fast.

Now flip the bike right-side up and ride it in each gear in a flat parking lot. Feel the difference. In the lowest gear, the pedals are easy but you barely move forward. In the highest gear, the pedals are harder to push but you can really cruise once you’re up to speed.

A car works exactly the same way. The transmission is a multi-speed bike for the engine.

Did You Know? The differential — that clever device that lets the two rear wheels turn at different speeds — was not invented for cars. It is much older than that. Around 200 AD in China, engineers built a device called the south-pointing chariot. On top of the chariot was a wooden figure with an outstretched arm. Inside the chariot was a system of gears connected to the two wheels. As the chariot moved and turned, the gears compared the speeds of the two wheels, and used the difference to keep the figure’s arm pointing south, no matter which way the chariot was rolling. It was a mechanical compass made entirely of gears. It worked by exactly the same math as the differential under a modern car. Some Chinese engineer in 200 AD figured out a piece of mechanical thinking that the rest of the world wouldn’t put into a car for another seventeen hundred years.

Now we can go

So the wheels are spinning. The car is moving.

But it can’t yet stop, and it can’t yet steer. Two more things the car has to know how to do, neither of which has anything to do with the engine.

That is Chapter 5.

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