Introduction:

The drive axle assembly transmits torque from the engine and transmission to drive the vehicle’s wheels. It changes the direction of power flow, multiplies torque, and allows different speeds between the two drive wheels. Drive axles are used for both front wheel drive and rear wheel drive vehicles. The drive axle assembly integrates numerous components CV joints, drive shafts, U‑joints, differentials, and axle shafts to efficiently transmit engine torque to the wheels while accommodating steering, suspension movement, and speed differences. Constant velocity joints enable front wheel drive vehicles to steer and drive simultaneously, while U joints and slip yokes handle angular and length changes in rear wheel drive layouts. The differential, whether conventional or limited slip, ensures proper power distribution and wheel speed differentiation during turns.

Front Wheel Drive Axles:

Front wheel drive (FWD) axles, also called axle shafts, transfer engine torque from the transaxle’s differential to the front wheels. A key component is the constant velocity (CV) joint, which transfers uniform torque at constant speed through a wide range of angles. On FWD or four wheel drive cars, operating angles up to 40 degrees are common. The axles must transmit power to wheels that drive, steer, and cope with suspension movement. CV joints also allow the axle length to change as the wheel travels up and down.

Types of CV Joints:

CV joints are classified by position (inboard/outboard), function (fixed/plunge), or design (ball‑type/tripod).

Inboard and Outboard Joints:

On FWD vehicles, two CV joints per half shaft are used. The joint nearer the transaxle is the inner (inboard) joint; the one nearer the wheel is the outer (outboard) joint. In RWD independent rear suspension, the joint nearer the differential is inboard, and the one closer to the wheel is outboard.

Fixed and Plunge Joints:

A fixed joint does not plunge, a plunge joint allows in and out movement. In FWD, the inboard joint is a plunge joint, allowing effective length change. The outboard joint is fixed and handles steering angles up to 40 degrees. In RWD IRS, one joint may be fixed and the other plunge or both plunge operating angles are smaller because wheels do not steer.

Ball Type Joints:

Two basic varieties: ball type and tripod type. Both can be inboard/outboard and fixed/plunge.

Fixed Ball Type:

Consists of an inner race, six balls, a cage and an outer housing with machined tracks. The balls transfer torque and act as bearings. The balls always bisect the shaft angle, reducing effective angle by half and eliminating vibration. Input speed equals output velocity. The cage holds balls snugly, worn cage windows cause clicking noise when turning. Opposing balls work as a pair wear in one track matches the opposite track. The dish style joint, similar to Rzeppa, is used on Volkswagen and many German RWD models.

Plunging Ball Type:

Two styles: double‑offset and cross groove. These are compact with flat outer housings and angled grooves. The double‑offset joint has a cylindrical housing with straight grooves, handles angles up to 25 degrees and plunge up to 60 mm. Used as inboard on some FWD half shafts and on propeller shafts. The cross groove joint is flatter, handles plunge up to 46 mm in short distance, with inner/outer races sharing plunge equally. Handles angles up to 22 degrees. Used as inboard on FWD half shafts or at either end of RWD IRS axle shafts.

Tripod CV Joints:

Tripod Plunging:

Central tripod (spider) with three trunnions fitted with spherical rollers on needle bearings and an outer housing (tulip) that can be closed (tracks enclosed) or open. Commonly used as FWD inboard plunge joints.

Fixed Tripod:

Sometimes used as FWD outboard joint. Trunnion mounts in outer housing; three roller bearings turn against an open tulip on the input shaft. A steel locking spider holds it together. It has greater angular capability. The fixed tripod cannot be removed from the shaft or disassembled the complete joint and shaft assembly must be replaced if it fails.

Rear Wheel Drive Shafts:

A drive shaft must smoothly transfer torque while rotating, changing length, and moving up/down. The engine and transmission are bolted to the frame, while the differential is mounted on springs. As suspension moves, drive shaft angle and length change. The Hotchkiss type shaft uses U joints for angle variation and a slip joint for length change. Starting at the front: slip yoke, universal joint, drive shaft yoke, and drive shaft. At the rear: another yoke and a second U joint connected to the differential pinion flange. Some drivetrains have a drive shaft support bearing. Large vehicles with long shafts often use a double Cardan joint (CV U joint) to reduce vibrations, and may have a center bearing that divides shaft length the ends entering the bearing are slip joints.

Slip Yoke:

The slip yoke is internally splined and fits over the transmission output shaft. It rotates at output speed and slides to change effective drive shaft length with suspension movement.

Drive Shaft and Yokes:

The drive shaft, usually seamless steel tubing, extends the transmission output to the rear axle. Yokes are welded or pressed on to connect shafts. Some vehicles use fiber composite, fiberglass, graphite, or aluminum shafts, offering weight reduction, torsional strength, fatigue resistance, easier balancing and reduced shock interference. Some shafts have a torsional damper. The shaft has a natural vibration frequency, critical speed depends on tube diameter and length. Diameters are as large as possible and shafts as short as possible to keep critical speed above driving range. The shaft turns three to four times faster than tires, so proper balance is essential.

Operation of U Joints:

The U joint allows two shafts to operate at a slight angle. Cardan developed the original Spicer adapted it for automotive use in 1902. The U joint has two Y‑shaped yokes and a cross. The cross arms fit with bearings in yoke ends. The input yoke rotates the cross, which rotates the output yoke. U joints do not rotate at constant velocity. The driven yoke speeds up and slows down twice per revolution. The speed fluctuation depends on operating angle i.e. difference between transmission angle and drive shaft angle. The output yoke’s path appears elliptical, the input rotates at constant speed but the output varies. Over four 90° quadrants, the output alternates shorter/longer travel, causing acceleration/deceleration and torsional vibrations. Steeper angles increase fluctuations, smaller angles reduce them.

Phasing of Universal Joints:

Speed fluctuations are passed to the next U joint. To cancel vibrations, drive shafts need at least two U joints with equal operating angles. The driven yoke must be in the same plane as the driving yoke. On two piece shafts, the center yoke is splined, if not indexed, it can be out of phase. Manufacturers use aligning arrows or a wider master spline. If no marks, index before disassembly.

Canceling Angles:

Vibrations are reduced by canceling angles: the front U joint’s acceleration is offset by the rear’s deceleration, damping vibrations.

Types of U Joints:

Three common designs: single U joints (inside/outside snap ring), coupled U joints, and U joints held by U bolts or lock plates.

Single Universal Joints:

The cross has four trunnions with needle bearings in cups. Cups fit snugly in yokes; movement occurs between trunnions and bearings. Retaining methods vary: Spicer style uses snaprings in yoke grooves, Mechanics/Detroit style uses external snaprings in bearing cup grooves near the seal, nylon injected joints require removing all plastic before replacement. Cleveland style combines both, accepting either clip.

Double Cardan Universal Joint:

Used with split shafts has two Cardan joints connected by a centering ball socket and center yoke. The ball/socket splits the angle equally between joints, canceling fluctuations. It is classified as a CV U joint, often used in front engine RWD luxury vehicles.

Differentials and Drive Axles:

The differential is a geared mechanism between driving axles. It rotates axles at different speeds when cornering and at same speed straight ahead. The drive axle assembly directs torque to wheels; ring/pinion gear ratio multiplies torque. The differential balances forces and allows wheel speed differences on turns. On FWD, the differential is integral with the transaxle design depends on engine orientation i.e. transverse crankshaft and drive axle same plane, longitudinal power flow changes 90°. On RWD, the differential is in the rear axle housing. Four wheel drive vehicles have differentials on both axles. Final drive gears provide gear reduction (torque multiplication). Low‑ratio axles give fast acceleration, high ratios allow slower engine speeds for fuel economy.

Components of Final Drives and Differentials:

Common design includes pinion/ring gears and a pinion shaft. RWD uses hypoid gears FWD uses planetary or spiral bevel. The pinion shaft is in the carrier, supported by two or three bearings. An overhung pinion uses two tapered bearings, a straddle mounted pinion uses two tapered front bearings and one roller bearing on a rear extension. The pinion meshes with the ring gear. When pinion rotates, ring gear turns the case and axle shafts. Most cars have two pinion gears on a straight shaft heavy trucks have four on a cross‑shaped spider, secured by lock bolts or retaining rings. Differentials also have two side (axle) gears, splined to axle ends in constant mesh with pinion gears.

Hypoid Gear:

Hypoid gears have multiple teeth in contact with sliding motion, providing quiet operation. The pinion is placed below the ring gear centerline. Sliding action wipes lubricant from gear faces, requiring extreme‑pressure (EP) lubricants with additives to withstand wiping.

Types of Final Drive Gear Sets:

Gear sets are classified by tooth counts: hunting, nonhunting, or partial nonhunting.

Hunting:

One pinion tooth contacts every ring gear tooth over several revolutions. Example: 9 pinion teeth, 37 ring teeth, ratio 4.11:1.

Nonhunting:

One pinion tooth contacts only certain ring teeth. Example: 10 pinion, 30 ring, ratio 3.00:1; each pinion tooth contacts the same three teeth each revolution. Timing marks must be aligned.

Partial Nonhunting:

One pinion tooth contacts six teeth, three on first rev, three different on second, then repeats. Example: 10 pinion, 35 ring, ratio 3.50:1. Timing marks also required.

Rear Axle Housing and Casing:

Two housing types: removable carrier (banjo housing) differential can be removed as a unit, used on trucks; integral carrier cast iron carrier with pressed steel tubes, rear cover removable for access, differential components serviced separately may require case spreader. Housing also mounts suspension components. Some have ABS speed sensors.

Differential Operation:

Power is split as percentages. Straight ahead: each wheel gets 100% of case speed. Turning: inside wheel may get 90%, outside 110%. Power flows from companion flange accepts U‑joint torque to drive pinion, then ring gear, differential case, pinion shaft, pinion gears, side gears, and axle shafts. When both wheels have equal traction, pinions do not rotate on shaft; they turn end over end with case. On a turn, outer wheel must travel farther, so it speeds up; inner slows down. Pinions rotate on shaft to allow speed difference, producing a reverse walking effect on the slower side.

Limited Slip Differentials:

Conventional differentials divide torque evenly, if one wheel loses traction, it spins and torque to the other drops. Prolonged spinout overheats gears, breaks lubricant film and can damage parts. Sudden traction recovery can shock the axle. To address this, limited slip differentials (LSD) are used under names like sure grip, no spin, positraction or equal lock. Some use viscous clutches, mainly in 4WD.

Clutch Based Units:

Many LSDs use friction material to transfer torque. Clutch pack units have two sets of clutch plates and friction discs. Friction discs (steel with abrasive coating) fit over side gear hubs steel plates (no friction material) fit into case splines. Pressure from S‑shaped or coil springs keeps packs engaged. When locked, side gears are fixed to the case, preventing one wheel from spinning faster. Cone type units use cone‑shaped parts splined to side gears, with friction coating on exterior, coil springs preload cones against the case. Clutches and cones slip at predetermined torque to allow differential action on turns.

Gear Based Units:

Gear based LSDs (torque bias or Torsen) use parallel axis helical gears. They multiply torque from a slipping wheel and send it to the wheel with better traction, responding quickly to traction changes. They do not bind in turns and do not wear like clutch units.

Axle Shafts:

Axle shafts transfer torque from differential to drive wheels. Dead axles support only live/drive axles transmit torque. Three driving axle types: semifloating, three quarter floating and full‑floating.

Semifloating Axle Shafts:

Semifloating axles support vehicle weight. Most RWD vehicles use these. Bearings in the axle housing support the shaft inner ends spline to side gears. They transmit only driving torque, thus floating. The wheel bolts to the outer end bearing is pressed into housing. Retained by bearing retainer bolted to flange or by C shaped washer in differential. If a shaft breaks the wheel may come out of the housing.

Three Quarter Floating Axle:

Wheel bearing is outside the housing; wheel hub bolted to axle end and supported by bearing. Shaft supports only 25% of vehicle weight rest transfers through hub/bearing to housing. Found on older vehicles and trucks.

Full Floating Axle Shafts:

Used on medium/heavy vehicles. Two bearings support the wheel hub outside the housing, carrying all stresses. Wheel hub bolts to a flange on the axle outer end. Shaft transmits only driving torque. If a shaft breaks, it can be removed without affecting the wheel, allowing towing.

Independently Suspended Axles:

Driving axles are open, not enclosed. DeDion axle resembles a drive shaft with U joints at each end and a slip joint at the inner end; outboard joint connects to wheel hub. Swing axle uses a ball and socket system allowing axle to pivot up/down, common in FWD.

Axle Shaft Bearings:

Bearings support weight and reduce friction. Radial loads (90° to axis) are always present; thrust loads (parallel) occur during turns.
Three bearing designs for semifloating axles:

Ball type:

Grease packed at factory, with inner seal (keeps gear oil out) and outer seal (protects brakes). Pressed on/off shaft. Retainer ring is soft metal never use torch drill or notch to remove. Heat can weaken shaft.

Straight roller:

Lubricated by axle gear oil only a brake seal is used. Bearings are pressed into housing, not onto shaft. Remove axle, pull bearing. Inspect shaft contact area; replace if pitted/scored.

Tapered roller:

Sealed and grease‑packed pressed on/off with a press. After packing, install axle and check end play with dial indicator adjust shim size per service manual.

Replacement of axle shaft seals is recommended whenever axles are removed. Some seals are side‑specific check markings or color coding.