How Cars Work · From Bang to Wheels
A box full of gears
A car has the same problem as a bicycle. Sometimes you need force (starting from a stop, climbing a hill, towing a trailer). Sometimes you need speed (cruising on the highway). One pair of gears can’t do both.
So an old car has a box full of gears, with a way of choosing which pair is in use at any moment. The box is called the transmission, or the gearbox. (Both names are accurate. “Gearbox” tells you what it is. “Transmission” tells you what it does — transmits power from the engine.)
A typical 1960s car has four forward gears and one reverse gear:
First gear: lots of force, very little speed. For starting from a stop.
Second gear: less force, more speed. For getting up to about 20 miles per hour.
Third gear: less force still, more speed. For cruising around town.
Fourth gear: least force, most speed. For highway driving.
To switch between them, the driver moves a stick poking up out of the floor — the gear shift. Inside the transmission, the shift moves a small fork that slides one gear sideways into mesh with another. Different positions of the stick mesh different pairs of gears. The stick also has a position labeled R, for reverse, which engages an extra gear that flips the direction of rotation, so the car backs up.
That is the inside of the transmission. A box of metal gears, with sliders that decide which pair is doing the work at any given moment.
The clutch
There is still a problem. While the engine is running, the crankshaft is spinning all the time, at whatever speed the engine is running. If we hooked it up directly to the transmission, then any time the driver tried to switch gears, the spinning crankshaft would grind against the still gears in the new ratio, and we would shred the teeth off everything.
We need a way to temporarily disconnect the engine from the transmission while we shift gears. That is the clutch.
A clutch is, in its simplest form, two flat discs that can be either pressed together or pulled apart. One disc is bolted to the spinning crankshaft. The other disc is bolted to the input shaft of the transmission. When the discs are pressed together, friction holds them tight, and they spin as one — engine drives transmission. When the discs are pulled apart, they aren’t touching, and the engine spins on its own without doing anything to the transmission.
The driver controls the clutch with a pedal on the floor, called (unsurprisingly) the clutch pedal. Pedal up, discs pressed together, engine connected. Pedal down, discs pulled apart, engine disconnected.
So the procedure for changing gears looks like this. Push the clutch pedal down — engine disconnected. Move the gear shift to the new gear — different pair of gears slides into mesh. Let the clutch pedal back up — engine reconnected, now to the new ratio. All in a second or two. Repeat at every shift.
This is what your mom is doing in an old car when she does a little dance with her left foot every time the car changes speed. (Modern cars handle the whole clutch-and-shift dance automatically, with no clutch pedal at all. We will get to those in Part 2.)