The axle shafts connect the transaxle in front-wheel drive cars and minivans to the drive wheels. There are always two (one for each side), and on many applications the one on the right side is much longer than the one on the left. This is because the vehicle has "unequal" length axle shafts due to offset position of the transaxle with a transverse (sideways) mounted engine.
The same type of axle shafts are also found on the front of all-wheel drive (AWD) cars and minivans as well as many four-wheel drive (4WD) trucks. You’ll even find some of these axle shafts on some rear-wheel drive applications such as Porsche and ancient Volkswagen Beetles.
Technically speaking, FWD axle shafts are called "halfshafts" though most people just call them "shafts."
Some shafts are solid steel while others are tubular steel. Why the difference? Tubular steel is stronger and lighter, and is often used for a longer shafts.
CV JOINTS:
On each end of the shaft is a ribbed rubber boot that covers a constant velocity joint (CV joint). The CV joint is the flexible coupling that allows the outer wheel to steer, and the shaft to follow the up and down motions of the suspension as the vehicle travels down the road.
The outboard CV joint (the one that connects to the drive wheel) typically has a much wider operating angle than the inboard CV joint that connects to the transaxle. This is because the outer joint may have to turn up to 50 degrees off center when the front wheels are steered. The inboard joint, by comparison, rarely sees an operating angle of more than about 20 degrees. Consequently, different types of joint designs may be used for the inner and outer joints.
Outboard joints that do not plunge in and out are called "fixed" joints.
RZEPPA JOINTS
The most common type of outboard joint is the "Rzeppa" style.
This type of joint was invented way back in 1920 by a Dana engineer named Alfred H. Rzeppa. His design allowed power to be transmitted through six spherical balls located between an inner and outer race. In this design, the balls are held in position by small windows in a cage assembly that fits between the inner and outer races. The design of the joint is such that the position of the balls always bisects (cuts in half) the operating angle of the joint. It’s a slick design that works something like a bevel gear. But instead of gear teeth transmitting the torque across the joint, the balls push against their respective tracks in the inner and outer housings.
DOUBLE OFFSET & CROSS GROOVE JOINTS
A couple of variations on this design are the "double offset" (DOJ) and "Plunging Disk Type Cross Groove" CV joints. Both designs also have the six ball arrangement, but the joints are used only on the inboard end of the shaft. Both designs allow the inner race (which is mounted on the end of the shaft) to "plunge" in and out so the shaft can change length. Why is this necessary? Because the shaft is usually longer than the control arms on the suspension. The difference in length would create interference problems every time the suspension moved up or down.
So the plunging action of these joints allows the shaft to slide in and out slightly to compensate for the difference.
TRIPOD JOINTS
The other type of CV joint you’ll see is the "tripod" style joint. Tripod joints do not have balls but instead use needle bearing rollers mounted on a three-legged spider. The rollers are mounted at 120 degrees to one another and slide back and forth in tracks in an outer "tulip" housing.
Tripod style joints are used for the inner joints on most domestic and Asian FWD models from 1983 to present. This type of joint is less expensive to manufacture than a ball type joint, and is well-suited to the limited operating angles of the inner joint location. The joint is designed to plunge in and out the same as other inboard joints to allow changes in shaft length as the suspension moves.
A fixed version of the tripod joint is used on a few applications including the former AMC Alliance and some Asian applications (Toyota Tercel and Nissan Sentra).
WHY A CV JOINT AND NOT A U-JOINT?
The answer: U-joints produce cyclic vibrations when operated at the kind of steering angles found in a FWD application.
Ordinary Cardan type universal joints cause a change in speed between the driving and driven shafts whenever the joint operates at an angle. As the operating angle of the joint increases, the speed (velocity) of the driven shaft varies more and more during each revolution of the shafts. The greater the operating angle, the greater the variation in speed of the driven shaft and the greater the vibration it produces.
The driven shaft still turns at the same number of revolutions per minute as the shaft that’s driving it, but because of the geometry of a universal joint the speed of the driven shaft alternately increases (accelerates) and decreases (decelerates) four times every revolution—and this is what causes the vibrations we’re talking about.
To visualize how this occurs, look down the length of a RWD driveshaft at the U-joint in the transmission yoke. If the tail end of the driveshaft were unbolted from the differential and lowered to the floor, it would create a severe angle in the forward U-joint. If the shaft were then turned by hand, you’d then be able to see that the two bearing caps on the U-joint center cross attached to the driveshaft rotate in one plane while the two attached to the transmission yoke rotate in a different plane. All the while the center cross is swiveling back and forth with each revolution.
If you drew what’s happening on paper, the two bearing caps in the transmission yoke would appear to be traveling in an elliptical (oval) shaped path as viewed down the length of the driveshaft. Or, from the other point of view, the two bearing caps on the driveshaft would appear to be traveling the elliptical path if viewed from the transmission.
It’s this difference in geometry that causes the driven shaft to change speed with respect to the driving shaft.
If the transmission output shaft were turning at a consistent 1000 rpm, the driveshaft would likewise turn at 1000 rpm regardless of the joint angle. But the driveshaft would also change speed alternately twice during each half revolution because of the difference in geometry between the two opposing bearing caps in the U-joint.
The speed changes are not great when the angle is less than a few degrees, but as the operating angle of the joint increases so do the cyclic vibrations of the driven shaft as well as the back and forth motion in the joint itself.
The changing rotational velocity of the driven shaft can be offset by installing a second U-joint on the opposite end of the driven shaft "phased" 90 degrees with respect to the first joint.
In theory the second joint cancels out the changes in output velocity caused by the other joint—but only so long as both joints operate at identical angles.
This setup works fine for RWD driveshafts where front and rear joint angles are usually identical and the changes in operating angles are relatively small, but with front-wheel drive the situation is entirely different. The inner and outer joints often operate at different angles. Whenever the wheels are turned the outer joint runs at a much higher angle than the inner joint. This upsets the offsetting relationship between inner and outer joint angles that’s a necessary requirement for ordinary U-joints. What’s more, the front wheels are required to steer at angles of up to 45 degrees—which puts too much strain on a U-joint.
A CV joint, by comparison, always splits the operating angle in half so the driven shaft turns at the exact same speed as the input shaft. So no matter what the joint angle, there are no changes in speed -- thus the name "constant velocity."
JOINT WEAR
If you haven’t noticed already, there’s one thing almost all FWD shafts have in common: the outer CV joints almost always wear out long before the inner ones. The outer joints wear at a faster rate because of the increased operating angles as the wheels steer. Consequently, the inner CV joints on many exchange shafts are in good condition and can be safely reused.
An interesting point worth noting about all the ball type CV joints is that the balls, cages and tracks are a precision fit assembly with close tolerances. So if a joint is disassembled for cleaning and inspection, the balls should be returned to their same original positions as before. Play between the balls and cage windows can produce a popping or clicking sound when the joint is operated at an angle (as when turning a corner). Insufficient clearance in the joint can cause binding and galling that can damage the balls and tracks as well as crack and/or chip the cage windows.
When this type of CV joint is rebuilt, slightly oversized balls and cages with oversized windows may be installed to compensate for wear. But this approach may only provide a short term fix because it does not restore the wear that may have occurred on the tracks in the inner and outer races. Because of this, many joint rebuilders now regrind the tracks on the inner race and outer housing using precision computer controlled numeric (CNC) equipment that allows the original contours of the tracks to be restored for oversized balls.
Some aftermarket rebuilders regrind every joint as a matter of course while others only grind on an "as needed" basis. The proponents of grinding say that machining the joint restores the proper tolerances that were originally designed into it, and allows many joints that would otherwise have to be discarded because of excessive wear to be salvaged.
The tracks in the races are case hardened when a joint is originally manufactured to make the joint wear longer. Grinding obviously removes some of this hardened layer, which may create durability concerns if too much metal is removed. The depth of the case hardening on CV joints varies depending on the application and process used (induction hardening or carburizing), but typically ranges from .048 inch to .100 inch or more. So limiting the amount of material removed to .010 inch to accommodate a .020 inch oversized ball is said to have virtually no significant effect on joint life.
The key to reconditioning a CV joint by grinding, say those who do it, is to exactly duplicate the original arc in the housing and race—a process which requires precision machining equipment and cannot be duplicated by hand grinding with a simple die grinder.
JOINT LUBRICATION
CV joints require a special type of high temperature, high pressure grease. Ordinary chassis grease or multipurpose grease should never be used in a CV joint.
The condition of the grease as well as the amount of grease in the joint will determine how long the joint lasts. One of the purposes of the boot that surrounds the CV joint is to keep dirt and moisture out. The other is to keep the grease in. If a boot is torn, cracked, punctured or comes loose, dirt and water can contaminate the grease in the joint causing accelerated wear which will eventually lead to joint failure. Loss of grease can also occur which will further shorten the life of the joint. So the boots must be in good condition to protect the joint.
Boots often become brittle with age and exposure to heat. Boots can also be damaged by road hazards and the careless use of j-hooks by a tow truck operator. Old boots should never be reused if a CV joint is being replaced. That’s why most new CV joints come with a new boot.
A CV joint that’s failing usually produces various symptoms:
A popping or clicking noise when turning: This almost always indicates a worn or damaged outer CV joint. A quick way to verify this condition is to put the car in reverse, crank the steering wheel to one side and drive the vehicle backwards in a circle (check the rearview mirror first!). If the noise gets louder, it confirms the diagnosis and the need for a new joint or replacement shaft assembly.
A "clunk" when accelerating, decelerating or when putting the transaxle into drive: This kind of noise can come from excessive play in the inner joint on FWD applications, either inner or outer joints in a RWD independent suspension, or from the driveshaft CV joints or U-joint in a RWD or 4WD powertrain. Be warned, though, that the same kind of noise can also be produced by excessive backlash in the differential gears. A quick way to verify the diagnosis here is to back the vehicle up, alternately accelerating and decelerating while in reverse. If the clunk or shudder is more pronounced, it confirms a bad inner joint.
A humming or growling noise: Sometimes due to inadequate lubrication in either the inner or outer CV joint, this symptom is more often due to worn or damaged wheel bearings, a bad intermediate shaft bearing on equal length halfshaft transaxles, or due to worn shaft bearings within the transmission.
A shudder or vibration when accelerating: Excessive play in either the inboard or outboard joints can sometimes cause this, but the most likely cause is a worn inboard plunge joint. These kinds of vibrations can also be caused by a bad intermediate shaft bearing on transaxles with equal length halfshafts. On FWD vehicles with transverse-mounted engines, this kind of vibration can also be caused by loose or deteriorated engine/transaxle mounts. For this reason, the rubber bushings in the upper "torque strap" or "dog bone" on these engines should also be inspected to rule out this possibility.
A vibration that increases with speed: This symptom is rarely caused by a failing CV joint or by FWD halfshaft imbalance. An out-of-balance tire or wheel, an out-of-round tire or wheel, or a bent rim are the more likely causes.
JOINT INSPECTION & BOOT REPLACEMENT
The first things that should always be checked are the boots around all four CV joints on a FWD car or minivan. The boots should be inspected for splits, cracks, tears, punctures, abrasion damage and loose or missing clamps. Damaged boots should be replaced as soon as possible because continuing to drive with a damaged boot is asking for trouble. If the grease inside hasn’t been lost or contaminated yet, it soon will be— which will quickly ruin the CV joint if it hasn’t already suffered damage.
Replacing a damaged boot with a one-piece OEM style or aftermarket boot is a job because the halfshaft must be removed from the vehicle. Axle removal is necessary because the CV joint must first be pulled off the shaft so the new boot can be installed. An alternative here is a split-boot, which appeals to many do-it-yourselfers because it is easier to install.
SPLIT-BOOTS
A split-boot doesn’t require axle removal and can be installed with the shaft and joints in place. You can find Motormite Speedi-Boots and Dorman Bolted Split-Boots in most auto parts stores. Another brand is the E-Z Boot sold by L&S Automotive (405-236-5501). Split-boots are a quick fix approach that saves a lot of labor and time. But the boot seam must be carefully glued to assure a leak-free bond, and the boot must not be disturbed while the adhesive cures (typically 30 to 60 minutes or more depending on the temperature and application).
There is another type of split-boot on the market with a zip-lock type seam. This boot is called CV Easy and is sold by GKN Drivetech (800-458-0123). This type of boot requires no glue and is held together by the locking action of the seam. The only drawback is that the seam is rather difficult to squeeze together. But once joined, it holds very tightly and won’t come apart.
BOOT FAILURES
When a boot has failed, there’s always some uncertainly about the condition of the CV joint and whether or not it has become contaminated with dirt and/or suffered wear or damage. If the joint is making noise, the joint is obviously history and needs to be replaced. But what if it isn’t making noise? Is it safe to install a split-boot or to replace the original boot and reuse the joint?
Most boot failures are not discovered until long after the failure has occurred. The bad boot may not be notice until the vehicle is being serviced or until somebody is investigating the cause of a noise or vibration problem. Consequently, by the time the bad boot is found, the joint may already be dry and/or contaminated by dirt.
If the grease from inside a damaged boot feels gritty when rubbed between the fingers, the grease and joint are contaminated. The joint needs to be cleaned and inspected, and the grease needs to be replaced. Cleaning a joint while it is still in the vehicle is difficult. There are aerosol solvents and similar products for this purpose, but the most thorough way to clean a CV joint is to remove and disassemble it. Once the joint has been taken apart, it can be inspected for wear or damage that can’t be seen from the outside.
CV JOINT DISASSEMBLY & INSPECTION
The first trick is getting the CV joint off the end of the axle shaft. Some joints just pull off while others are held with a snap ring or lock ring which must first be released.
Once the joint is off, it can be disassembled by tilting the inner race to one side. This is done by inserting a dowel or similar tool into the splines of the inner shaft and tilting the race as far as it will go to one side. This will expose one of the balls which can be popped out of its cage window with a small screwdriver. The inner race is then tilted to opposite side so the next ball can be removed, and so on until all the balls have been removed from their cage windows. The cage can now be rotated sideways allowing it and the inner race to be removed from the housing.
Look for nicks, gouges, cracks, spalling, roughness, flaking, etc. on the surface of the balls or tracks in the inner and outer races. The cage windows should also be inspected for dimples, wear or cracks. Each ball should fit snugly in its respective cage window because looseness here is what often causes the clicking or popping noises associated with a worn CV joint.
CV joints are precision fit assemblies, so the balls should be kept in order so they can be reassembled in their same respective positions as before. Each ball and track develop a unique wear pattern as the joint ages, so mixing up the balls may change tolerances and create problems that didn’t exist before the joint was disassembled.
If the CV joint shows no wear or damage, it should be okay to reuse. If it doesn’t pass inspection or is obviously defective, it needs to be replaced. Either way, before the CV joint goes back on the shaft it should be packed with CV joint grease (never ordinary chassis grease!). Special grease is usually provided with the replacement boot along with instructions on how to pack the joint. About a third of the grease is typically packed into the joint, and the rest is put inside the boot before it goes on to serve as a reservoir for the joint. There is some debate as to how much the grease actually moves around inside the joint and boot as the vehicle is being driven, but it’s there for a purpose so it should be used.
BOOT INSTALLATION
When the new boot is installed over the joint, it must be properly positioned on both joint and housing. Most boots have a lip on the inside diameter of both ends that fits into a groove on both parts to assure a tight seal. Installed boots must not be crimped, twisted or collapsed. If the boot is not in its normal shape, loosen a clamp and "burp" it by carefully sliding a screwdriver between the boot lip and driveshaft or joint housing. This should allow enough air into the boot to return it to its normal shape.
Boot clamps must be installed according to the vehicle manufacturer’s instructions or the instructions provided with the replacement boot or joint. Some types of clamps require special tightening/crimping tools while others do not.
Other items that should also be inspected:
On FWD transaxles with equal length halfshafts, the intermediate shaft U-joint, bearing and support bracket should be inspected for looseness by rocking the wheel back and forth and watching for any movement. A bad bearing would call for bearing or shaft replacement.
Transaxle oil leaks around the inner CV joints. This would indicate a faulty transaxle shaft seal. To replace the seal, the halfshaft must be removed.
CV JOINT REPLACEMENT
When a CV joint has failed and needs to be replaced, there are a variety of replacement options: new joints, remanufactured joints and complete replacement shaft assemblies with new and/or remanufactured joints. Many professional installers prefer complete shaft assemblies because they eliminate the mess of changing individual joints and save installation time. Many shafts also have a lifetime warranty, which eliminates worries about comebacks (or at least covers the replacement cost & labor).
The degree of difficulty to remove a halfshaft from a FWD vehicle varies considerably from application to another. Some applications require special tools. Separating the lower ball joint from the steering knuckle may require a ball joint puller or fork. Pushing the outer CV joint stub shaft back through the wheel hub often takes a special puller. On some Japanese applications, the shaft must be pressed out which often damages the wheel bearings. On some cars, the inner CV joint stub shafts are retained in the transaxle by a circlip and may not pull out without the help of an axle puller. Some older Chrysler applications even require removing a cover on the transaxle so a retaining clip inside the differential can be released before the shafts can be pulled out. Some applications (older Ford Escorts, for example) also require the use of a pilot tool (or removing only one shaft at a time) to prevent the differential gears from slipping out of position.
The general procedure goes as follows:
Loosen the axle hub nut with the wheel on the ground using a breaker bar or torque wrench. An impact wrench should not be used because the hammering can damage an otherwise good CV joint as well as the differential gears in the transaxle. The brakes should be applied to keep the car from rocking. On vehicles where the hub nut has been staked to hold it in position, DO NOT attempt to unstake the nut. Just turn it off.
NOTE: Most vehicle manufacturers recommend replacing the axle hub nut. Once used, the nut loses its ability to retain torque. So the old nut should be discarded and replaced with a new one.
Raise the vehicle and support it under the chassis crossmember or side rails so the front suspension can hang free. The wheel can now be removed.
What comes next depends on how the inner CV joint is attached to the transaxle:
If the joint is bolted to a stub shaft (such as the cross groove Rzeppa joint found on many imports), disconnect the inner CV joint first and then separate the outer joint from the steering knuckle hub. On many GM applications, the inboard CV joint stub shaft is retained in the differential gear by a circlip (except on the left hand side of automatics). To pull the shaft out, a special slide hammer axle puller tool is needed.
If the inner joint "plugs into" the transaxle and the stub shaft is part of the joint housing (tripod plunge joint and double-offset Rzeppa joints) the outer joint must be disconnected from the steering knuckle before the inner joint can be pulled out of the transaxle.
To separate the outer CV joint from the knuckle, one end of the knuckle (the lower ball joint or the MacPherson strut) must be disconnected so there will be enough clearance to push the outer CV stub shaft back through the knuckle.
On those applications where it is easiest to disconnect the knuckle from the strut (GM for example) it will be necessary to disconnect the brake hose clip and to remove the brake caliper from its mount.
On applications where disconnecting the ball joint is the path of least resistance (Ford and Chrysler for example) it may also be necessary to disconnect the sway bar from the lower control arm and/or the tie rod from the steering knuckle.
Once the knuckle is free to swing out, the end of the halfshaft can be pushed back through the wheel hub (a puller may be needed for this step). DO NOT pound on the end of the shaft as doing so may damage the wheel bearings, the splines in the hub, the differential gears or either CV joint.
As soon as the end of the shaft is free, DO NOT let it hang. Support it with a piece of wire until the inboard joint can be removed or unbolted from the transaxle. Allowing the halfshaft to hang or pulling on it will pull the inboard joint apart.
Installing the shaft is just the reverse. Once everything is back together and the wheel is in place, the lug nuts and hub nut should be installed and tightened to specifications using a torque wrench (not an impact gun unless torque limiting sockets are used.)
Drivetrain