Drivetrain, Brakes, Steering and Suspension

Manual and Automatic Drivetrains

The drivetrain transfers mechanical energy from the engine to the wheels so the vehicle can move. In a manual transmission vehicle, the clutch connects and disconnects the engine from the transmission. When the clutch pedal is released and the clutch is fully engaged, a heavy spring-loaded pressure plate clamps the friction-lined clutch disc firmly against the spinning engine flywheel. When the driver pushes down on the clutch pedal to shift gears, the release bearing, also called a throw-out bearing, pushes against the spring fingers of the pressure plate. This action releases clamping pressure on the clutch disc, disengaging engine power so gears can be selected without clashing.

In a typical modern manual transmission, synchronizers help match the speed of the selected gear to the shaft before locking them together, helping prevent grinding during shifts. Selecting different transmission gears alters the relationship between engine speed and road speed. A low gear features a high numerical gear ratio that provides more turning force, or torque, which is necessary for moving a stopped vehicle or accelerating. In contrast, a high gear has a lower numerical gear ratio that allows the vehicle to achieve greater road speed at a given engine speed. At highway cruising speeds, modern transmissions often engage an overdrive gear ratio. In overdrive, the output speed is faster than the input speed, which significantly lowers engine operating speed to maintain steady cruising.

Automatic transmissions handle power delivery without a mechanical foot-operated clutch. In a conventional torque-converter automatic transmission, a torque converter transmits engine power through moving fluid; other automatic designs may use clutches instead. Inside the sealed housing of a torque converter, three main components work together: the pump (or impeller) driven by the engine, the turbine connected to the transmission input shaft, and the stator redirecting fluid between them. A conventional automatic transmission typically uses planetary gear sets to provide its forward gear ratios and reverse; modern manual transmissions generally use constant-mesh gears and synchronizers, not sliding gear sets. Automatic transmission fluid helps transmit power through the torque converter, supplies pressure used to operate shifting components, lubricates parts, and helps carry away heat.