The drivetrain takes the rotating power from the engine or electric motor and transfers it to the wheels. It uses gears, shafts, and joints to transfer the rotation of the engine to the rotation of the wheel.
The primary component of the drivetrain is the transmission. Just like a multi-speed bicycle, the transmission changes the rotation speed of the drive axles with the speed of the engine. It allows more torque to start, and then reduces the torque once the car is moving at a higher speed. The following are some of the main components of the drivetrain.
Torque Converter or Clutch
A manual transmission uses a clutch assembly, and an automatic transmission uses a torque converter. Both do the same thing: they allow the transmission to be disengaged from the rotation of the engine and provide a connection between the engine and transmission.
Cars with a manual transmission use a clutch, which is a friction wheel and a spring-loaded metal plate.
When you push on the clutch pedal, you are pulling the spring-loaded pressure plate away from the friction clutch disc, breaking the connection between the engine and the transmission.
A torque converter uses hydraulic pressure to engage the engine. As the engine speeds up, the torque converter creates more pressure and engages the transmission. The torque converter can also act like a gear reducer to increase torque at low speeds.
Transmission or Transaxle
The transmission, also called a transaxle (front-wheel-drive vehicles) or a gearbox, uses different gear ratios to adapt the output of the engine to the speed of the wheels. Low gears turn the drive wheels slower. An overdrive gear turns
the output of the transmission faster than the output of the engine. The transmission also allows the wheels to be spun in reverse.
A transmission sends its power to the drive axles with tubes called driveshafts and axles. A transaxle drives its wheels directly without any additional reductions.
Driveshafts and Drive Axles
In rear- or four-wheel drive cars, the transmission sends power to the axles through a driveshaft. In a front-wheel-drive car, the wheels are driven directly by the drive axles. Both use a type of flexible joint so the wheels can move without breaking the drive shafts.
U-Joints and CV Joints
The use of universal joints, or U-joints and constant velocity joints, or CV joints, allow the drive shafts and axles to rotate at an angle and allow the drive points to move. U-joints are commonly used in rear-wheel and four-wheel drive cars, and CV joints are typically used in front-wheel drive and cars with independent rear suspensions.
CV joints typically are covered with a rubber boot to keep them clean.
Differentials and Axles
On rear-wheel and four-wheel- drive cars, the axle uses an additional set of gears to convert the rotation of the driveshaft 90 degrees and out to the drive wheels. In a “solid” axle, the drive axles are housed inside the axle tubes. In an independent suspension system, the axles are usually exposed and use CV joints to allow each drive axle to move up and down.