Four-wheel drive

Four-wheel drive is a drivetrain that can deliver engine power to all four wheels, either continuously or when the driver engages it. It improves traction when some wheels have limited grip.

Transfer case: A gearbox that directs engine power from a transmission to the front and rear axles. It splits power between the axles in many four-wheel-drive vehicles.
All-wheel drive: A drivetrain that supplies power to all four wheels, usually automatically and without driver selection of a low-range mode. It contrasts with traditional four-wheel drive’s selectable modes and common low-range gearing.
Low-range gearing: A drivetrain setting that uses reduction gears to increase torque at the wheels and lower vehicle speed. It enables controlled crawling over steep or obstructed terrain.
Vehicle dynamics: The study of how forces and motion govern a vehicle’s movement and stability. Four-wheel drive changes acceleration and handling without removing the limits imposed by tire grip.
Differential: A gear assembly that allows wheels on the same axle to rotate at different speeds while receiving torque. It lets each axle’s wheels turn at different speeds during a corner.
Two-wheel drive: A drivetrain that sends engine power to either the front or rear wheels, but not both axles. It has fewer driven wheels and generally less traction on slippery surfaces.
Rock crawling: A form of off-road driving that crosses rocky obstacles at very low speeds. Low-range four-wheel drive helps maintain control while climbing large obstacles.
Tire traction: The force a tire can transmit to the road before it slips. Four-wheel drive can use available grip at more tires, but tire condition remains decisive.
Center differential: A differential that distributes torque between a vehicle’s front and rear axles. It allows full-time four-wheel drive to accommodate different axle speeds.
Front-wheel drive: A drivetrain that sends engine power to the front wheels. It is one common two-wheel-drive layout against which four-wheel drive adds a driven rear axle.

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