Front-Wheel-Drive (Transaxle and CV Joint) Fundamentals Chapter 77

Objectives Describe differences between front- and rear-wheel drivetrains Tell the names of parts of a transaxle Trace the power flow through four- and five-speed transaxles

Introduction Most automobiles today have front-wheel drive FWD car has transaxle Drive axles extend to front wheels out of each side of transaxle Each end of the drive axle is a CV joint Transaxle can be either manual or automatic

Front-Wheel Drive Advantages More efficient drivetrain Better fuel economy Combined with MacPherson struts: less unsprung weight for better handling Transmission hump is eliminated A few FWD engines have been mounted longitudinally Most transaxles mounted sideways

Manual Transaxle Manual transaxles and transmissions Use same kind of clutch Three parallel paths for power flow Input shaft located above intermediate shaft Input shaft gears directly drive output shaft gears Differential assembly Gear shafts Supported by larger ball, roller, or tapered roller bearings End play is controlled by thrust washers

Shift Linkage Transverse transaxles Two shift cables or rods Advantage Shifted by cables or shift linkage Two shift cables or rods One moves a selector on transaxle Other moves shift fork back and forth Advantage Engine shake is not transmitted back to driver’s hand on shift lever

Transaxle Differential Allows wheels to turn at different speeds when rounding corners Same as rear-wheel-drive differential Ordinary helical gearset Used instead of bevel gears Power from differential side gears is transmitted to front drive axles through axle shafts

Transaxle Power Flow Five-speed power flow Fixed gears for first, second, and reverse on input shaft Fixed gears for third, fourth, fifth on intermediate shaft Power flow leaves transmission intermediate shaft Continues through drive pinion to axles Engine is mounted sideways Axles run parallel to input shaft

Automatic Transaxle Combination of automatic transmission and differential Same parts and operation apply Transverse engine Power flow is through gears or sprocket and chain Chain drive Allows transaxle to be mounted slightly below and to the side of the engine

Front Drive Axles Difference between rear-wheel drive and front-wheel drive axles Front-wheel drive axles have CV joints at ends Axles driven at sharper angles Allow steering front wheels during power transmission Universal joint changes output speed twice in every revolution when run at an angle Rear-wheel drive vehicle drive shaft turns very fast Positioned before gear reduction of differential

Axle Shaft Parts Drive axle is called half shaft or axle shaft Stub shaft (stub axle) Short shaft at outside end Splined to front hub so it can drive front wheels CV joint classifications Inboard and outboard Fixed and plunge Ball and tripod

CV Joint Construction Most common joint combination Fixed joints Rzeppa joint for outside fixed joint and a tripod for inside plunge joint Fixed joints Rzeppa joint: ball-and-socket type Types of plunge joints Tripod tulip Double offset plunge joint Cross groove plunge joint

Axle Shafts Characteristics May be solid or hollow May have damper weights to absorb vibration Turn much slower than rear-wheel drive shaft Balance not as important When different lengths: long one twists and lags before puts its torque to the wheel Torque steer is prevented by longer axle shaft of larger diameter tubing

CV Joint Boots Boots at each end of axle contain grease CV joint boot Protect joint from the elements CV joint boot Attached to axle and stub shafts with plastic or steel bands or straps Made of natural rubber, neoprene, silicone, or urethane