Drivetrain

The drivetrain is the collection of components that takes power once it has been processed by the transmission and delivers it onward to the road wheels. It exists because raw engine output is rarely usable as it stands: the rotational speed and torque must be multiplied, divided between wheels, and routed to whichever axle drives the vehicle. The drivetrain is the chain of hardware that performs this routing and distribution, turning crankshaft motion into the rotation of the tyres.

In most cars the drivetrain encompasses the gearbox, the driveshafts or propeller shaft, the differentials, the half-shafts and the axles, along with the joints that link them. The transmission selects a ratio; the shafts carry that motion towards the wheels; the differential allows the two wheels on an axle to turn at different speeds through a corner while still splitting torque between them; and the constant-velocity or universal joints let the shafts flex with steering and suspension travel. Each part is sized to handle the torque it will see, with components nearer the wheels generally experiencing higher torque after the final reduction.

A frequent source of confusion is the line between drivetrain and powertrain. The powertrain includes everything that makes and transmits power, the engine or electric motor included, whereas the drivetrain conventionally excludes the prime mover and covers only the parts that deliver that power from the transmission to the wheels. Put simply, the powertrain is everything from the spark to the road, while the drivetrain is everything after the gearbox.

The arrangement of these components defines a vehicle's drive configuration, and that layout shapes much of how the car feels and behaves. A front-wheel-drive drivetrain is compact and efficient, a rear-wheel-drive one separates the jobs of steering and driving between the axles, and a four-wheel-drive or all-wheel-drive drivetrain adds a transfer case or a second differential to feed both axles. The choice influences traction, weight distribution, cabin packaging and cost.

From an ownership perspective the drivetrain is generally durable but not maintenance-free. Differential and transaxle oils need periodic changing, constant-velocity joint boots must be intact to keep grease in and grit out, and worn joints announce themselves through clicks or vibration. Electrified cars retain a drivetrain even though the prime mover has changed, with the motor's output still passing through a reduction gear, shafts and a differential. Understanding the drivetrain in relation to the powertrain, the transmission and the differential clarifies exactly which parts are responsible for moving a car rather than merely generating its power.