How power gets to the wheels

The engine makes a twist on its crankshaft. Getting that twist to the road takes a chain of parts called the drivetrain. Each part solves one problem the engine can’t solve on its own.

• An engine can’t turn slower than about 800 rpm without stalling, but the wheels have to stop at junctions.
• An engine only works well across a narrow band, from about 1,500 to 6,500 rpm, but the wheels need to turn anywhere from a crawl to 70 mph (113 km/h) and beyond.
• An engine’s twist is far too weak to push a car up a hill on its own.

The drivetrain fixes all three. It can disconnect the engine, it changes how fast the wheels turn for each turn of the engine, and in doing so it multiplies the engine’s torque.

Power flows through six main parts on its way to the road:

1. Flywheel. A heavy disc bolted to the end of the crankshaft. It smooths out the pulses from each cylinder firing and gives the clutch something to grip.
2. Clutch. A friction disc squeezed against the flywheel by a strong spring. Press the pedal and the spring lets go, so the engine spins freely while the car stands still or you change gear. Let the pedal up gently and the clutch slips a little as it grips, which is how the car pulls away smoothly.
3. Gearbox. Pairs of gears of different sizes. When a small gear drives a big one, the big one turns more slowly but with more twist.
4. Final drive. One more fixed pair of gears, usually about 3 to 4 to 1, that slows the drive down again before the wheels.
5. Differential. Splits the drive between the left and right wheels, so the outside wheel can turn faster in a corner.
6. Driveshafts and wheels. Shafts with flexible joints carry the drive to each wheel, and the tyres finally push against the road.

Pick a gear and move the engine speed slider. In first, the engine turns 14 times for every turn of the wheels: the gearbox’s 3.6 to 1, times the final drive’s 3.9 to 1. That multiplies the engine’s torque by about 14 too, so at 2,000 rpm the car creeps along at 11 mph (17 km/h) with a huge push at the wheels.

Change up to sixth and the engine turns under 3 times for each turn of the wheels. Now 70 mph (113 km/h) needs only about 2,750 rpm. The engine is quiet and uses less fuel, but there’s much less push at the wheels, which is why you change down to overtake.

The gears never add power. They swap speed for torque and back, losing a few per cent to friction on the way. The horsepower vs torque lesson explains why that makes power the number that decides how quickly a car can accelerate.

Which wheels the engine drives changes how the parts are laid out.

• Front-wheel drive is the most common. The engine sits sideways at the front, with the gearbox, final drive and differential built into one unit beside it. Two short driveshafts run straight to the front wheels. It is compact and cheap to build, and it leaves more room inside the car.
• Rear-wheel drive usually has the engine lengthways at the front. A long propshaft runs under the car to a rear axle holding the final drive and differential. The front wheels only have to steer, which many drivers think gives a purer feel.
• All-wheel drive sends power to both ends, so the car can use the grip of all four tyres. It needs extra shafts and a way to share the drive between front and rear.

A manual gearbox leaves the clutch and the gear changes to you. Automatics do the same job in different ways:

• Torque converter automatics link the engine and gearbox through spinning oil instead of a clutch, so they pull away very smoothly.
• Dual-clutch gearboxes are two manual boxes in one, one for odd gears and one for even. The next gear is already selected, so changes take a fraction of a second.
• CVTs use a belt running between two cone-shaped pulleys instead of fixed gears, so they can pick any ratio in between.
• Electric cars mostly have a single gear. An electric motor makes strong torque from standstill and spins to well over 10,000 rpm, so one ratio covers every speed.

The drivetrain carries every bit of the engine’s torque, so its parts wear in step with how hard the car is driven.

• Clutch slip. As the friction disc wears thin, it stops gripping under load. The revs flare up when you accelerate hard in a high gear, but the car doesn’t speed up to match. Often there’s a hot, burning smell. A new clutch is one of the bigger routine bills on a manual car.
• Dual-mass flywheel rattle. Many modern cars, especially diesels, have a flywheel built in two halves with springs between them to soak up vibration. When the springs wear, it rattles at idle or judders as you pull away. It is usually replaced along with the clutch.
• Clicking on full lock. The joints at the outer ends of the driveshafts are called CV joints. A worn one clicks when you accelerate with the steering turned hard, such as pulling out of a parking space. The rubber boot around each joint keeps its grease in; if it splits, dirt gets in and the joint wears out quickly.
• Crunching gears. Small brass rings called synchros match the speeds of the gears before they lock together. When they wear, or the clutch doesn’t fully let go, a change crunches. Second gear is often the first to suffer.
• Whine or hum. A whine that rises and falls with road speed can come from worn bearings in the gearbox, the final drive or the differential.

Changing almost anything between the engine and the road changes how the car drives.

• More power needs a stronger clutch. Remaps add torque, and the standard clutch may start to slip. An uprated clutch has a stronger spring and grippier material, so the pedal gets heavier and pulling away can be less smooth.
• A lightweight flywheel lets the engine rev up and slow down faster, so the car feels sharper. The engine idles less smoothly, stalls more easily and can rattle the gearbox.
• A shorter final drive multiplies the torque more in every gear, so the car accelerates harder, but it revs higher on the motorway and uses more fuel.
• Bigger wheels and tyres change the gearing too. If the new tyres are taller overall, the car goes further for each turn of the wheels, and the speedometer reads lower than your true speed. Speedometers are allowed to read high but never low, so keep the overall diameter within a few per cent of standard.
• A limited-slip differential stops one wheel spinning away the power when it loses grip. It is one of the most effective upgrades for a powerful front-wheel-drive car.