A vehicle moves because the engine creates rotational power and the drivetrain sends that power to the wheels. The driveshaft and axles are key parts of this path, especially in rear wheel drive, four wheel drive, and all wheel drive vehicles. They must carry torque while allowing the suspension and wheels to move.
Understanding these parts helps explain acceleration, traction, turning, and why different vehicles are built with different drivetrains.
Power usually flows from the engine to the transmission, then to a driveshaft, differential, axle shafts, and finally the wheels. The driveshaft spins at high speed and transfers torque over a distance, while the differential changes the direction of rotation and lets left and right wheels turn at different speeds. Axles carry torque to the wheels and may also help support vehicle weight, depending on the axle design.
Universal joints, constant velocity joints, and bearings keep the system aligned and flexible as the vehicle moves.
Key Facts
- Torque is twisting force: τ = F × r, where τ is torque, F is force, and r is radius.
- Power carried by a rotating shaft is P = τω, where P is power, τ is torque, and ω is angular speed.
- A driveshaft transfers rotational power from the transmission or transfer case to the differential.
- A differential lets the left and right wheels rotate at different speeds during a turn.
- Wheel torque after gearing can be estimated by wheel torque = engine torque × transmission gear ratio × final drive ratio × efficiency.
- CV joints allow axle shafts to transmit torque smoothly while the wheels steer and the suspension moves.
Vocabulary
- Driveshaft
- A rotating shaft that carries torque from the transmission or transfer case to a differential.
- Axle shaft
- A shaft that transfers torque from the differential to a wheel.
- Differential
- A gear assembly that sends torque to two wheels while allowing them to rotate at different speeds.
- Universal joint
- A flexible joint that allows a driveshaft to transmit rotation through a changing angle.
- Constant velocity joint
- A joint that transmits torque at a steady rotational speed even when the axle is bent at an angle.
Common Mistakes to Avoid
- Confusing the driveshaft with the axle shaft. The driveshaft usually carries power to the differential, while axle shafts carry power from the differential to the wheels.
- Assuming both drive wheels always spin at exactly the same speed. In a normal differential, the wheels can rotate at different speeds so the vehicle can turn smoothly.
- Ignoring gear ratios when calculating wheel torque. The torque at the wheels is multiplied by the transmission and final drive ratios, minus losses from friction.
- Thinking a CV joint and a universal joint do the exact same job. Both handle angles, but CV joints are designed to keep output speed more constant, which is important for front wheel drive steering axles.
Practice Questions
- 1 A driveshaft carries 240 N·m of torque and spins at 300 rad/s. Use P = τω to find the power transmitted in watts and kilowatts.
- 2 An engine produces 180 N·m of torque. The transmission gear ratio is 3.00:1, the final drive ratio is 4.10:1, and drivetrain efficiency is 0.90. Estimate the torque delivered to the wheels.
- 3 During a left turn, the right wheel must travel a longer path than the left wheel. Explain why a differential is needed and what problem would occur if both axle shafts were locked together on dry pavement.