This brief article is intended as a “primer,” regarding CV (constant velocity) driveline joints. A CV joint is a torque/drive mechanical coupling in which the rotational speed of the output shaft (inner joint) matches that of the input shaft (outer joint), regardless of the shaft angle. This allows torque output from the transaxle to the driven wheels to remain unchanged from the transaxle to the wheels even as the suspension (and the CV shaft) changes angles during up/down travel and during turns. CV joints don’t cause restrictions during angle changes, as compared to universal (U) joints.
What are the differences between CV and U joints? A U-type joint is applicable when there is a very limited shaft angle variation. However, a U-type joint will cause the driven shaft to slightly change length and will create a speed (rotation rate) change between the input and output side of the joint when the shaft must change its angle by more than a couple of degrees. Given the angle movements required for both suspension travel and steering angle changes, a U-type joint will create a vibration as the two sides (input and output) begin to “argue” during shaft angle movement. This is why a CV (constant velocity) joint is required for front-wheel drive (FWD) and independent rear-wheel drive (RWD) systems.
A CV joint design permits freedom of movement without bind or differences in output/input, since the inboard CV joint features a “plunging” movement that allows the shaft to move in/out during suspension travel without restricting suspension travel.
OUTBOARD CV JOINTS
Outboard (wheel side) CV joints are usually the “fixed” type, also often referred to as a Rzeppa joint. This type of axle joint features a bearing race with slightly arced and offset longitudinal grooves. A series of ball bearings run along these grooves. The balls are “trapped” in a cage that keeps the balls aligned to their grooves. The offset groove/caged ball design allows the joint to articulate to follow suspension travel and steering angles. Depending on the specific manufacturer, a typical outboard CV joint will allow a maximum articulation angle of about 47 to 50 degrees (naturally, a higher maximum angle will allow a smaller turning circle).
Due to the greater operating angles and their greater exposure to road hazards such as rocks, dirt, moisture, etc., and greater potential for CV joint boot damage, outboard CV joints tend to wear sooner than inboard CV joints.
INBOARD CV JOINTS
An inboard joint is designed to allow in/out or “plunging” movement in addition to articulating for suspension travel angles. This plunging movement allows the shaft assembly to slightly change its length during suspension travel, compensating for control arm up-down/angles during suspension travel, and preventing lower control arm movement. Several styles of inboard CV joints are in use. Tripod style CV joints use a three-legged design of three equally-spaced roller bearings (instead of balls) that glide along track grooves inside a “tulip” style housing. A DO (double offset) is also a plunging type joint but features a series of ball bearings.
Typical inboard CV joints provide a plunge movement of about 50mm and a maximum articulation angle of about 22 to 31 degrees (depending on make and model).
TIPS ON DIAGNOSING CV JOINT PROBLEMS
NOISE DURING TURNS
A clicking or popping noise is indicative of a worn or damaged outer CV joint. One way to confirm this is to drive the vehicle in reverse (in a circle). If the noise is more pronounced, this confirms the need to replace the joint. If you’re driving backward with the wheel cranked full right and the noise is louder, suspect the right outer joint. If driving backward with the steering wheel cranked full left, suspect the left outer joint. Be aware that these noises may also be caused by worn or damaged rack-and-pinion steering inner tie rod ends.
CLUNKING NOISE
If you hear/feel a clunking noise when the transmission is placed into drive gear, or during acceleration or deceleration, this may point to a worn or damaged CV joint. Keep in mind that the same type of noise/feel can result from excessive backlash in the differential gears.
If the vehicle features FWD, suspect an inner CV joint. If the vehicle features RWD with an independent rear suspension that features CV joints, suspect either the inner or outer joints. Naturally, the same condition may be caused by primary driveshaft CV or U joints, worn/damaged inner tie rods or other worn suspension parts. If you suspect inner CV joints on the drive axle (FWD or RWD), try driving the vehicle in reverse while accelerating and decelerating. If the problem becomes more noticeable, the cause is likely one or both inner joints.
ACCELERATION VIBRATIONS
If a vibration is felt during vehicle acceleration, this may indicate a worn or damaged inboard plunge joint. Other possible causes include excessive play in either or both inboard or outboard CV joints. Other potential causes can include worn/failed engine and/or transmission mounts or torque strap bushings, so be sure to rule these out before blaming the CV joints.
Note: Always consider the basics before deciding to replace the CV joint. If the vibration begins to occur at a specific vehicle speed, and increases at higher speeds, the problem is most likely not CV-joint related or CV-joint shaft imbalance. More likely causes include wheel imbalance, tire radial runout, bent wheel, etc.
GROWLING
Again, not all unusual noises are caused by bad CV joints. If the problem involves a constant growl or hum while driving, the most likely cause is worn/dry/damaged wheel bearings. Granted, a CV joint can cause a constant noise if the boot has been damaged and the joint has lost its lubrication. Anytime you find a damaged CV joint boot (tear, hole, loose clamp), definitely examine the joint. If lube has been spit out, it’s very likely that road contamination has entered the joint, which is cause for replacement.
On the subject of CV joint boots, this is the most common cause of CV joint problems and failures. Make a point to always inspect CV joint boot conditions. Note that even if a boot is not already torn, it may be subjected to high heat levels, in which case the boot material may become heat-hardened and brittle. When the boot reaches the point of being brittle, it’s only a matter of time before the boot cracks and splits. As already noted, whenever a boot is compromised, the joint will eventually fail. It’s only a matter of time until failure will occur.
If any doubt exists, or especially if the joint (with the bad boot) is making any abnormal noise, don’t even think about boot replacement. Instead, replace the joint. As we all know, it’s far quicker and easier to simply replace the entire CV shaft assembly as opposed to disassembly, rebuilding and reassembly.
Considering the labor time involved in performing a boot replacement, it makes sense that the customer should consider a replacement of the entire CV shaft assembly. Yes, this involves a higher product expense, but shaft assembly replacement will eliminate the time needed to remove/reinstall the joint as needed for boot replacement. Replacing the entire shaft assembly reduces the customer’s labor cost and eliminates concerns for a potentially compromised joint.
An alternative to replacing the joint (or joint/shaft assembly) is to remove the bad boot and replace it with a “split boot.” This involves cutting the old boot off, cleaning the shaft and inspecting/cleaning/greasing the joint as needed, and installing the two-piece split boot. However, it is critical to properly seal the boot halves together using the supplied adhesive and allowing adequate curing time (some split boots feature small screws that secure the halves together instead of requiring adhesive). Basically, you should only consider using a split boot if time, budget (or both) are critical factors. A split-type boot, regardless of how well it was designed, will not provide the durability of a one-piece boot. But, it’s a handy alternative, depending on the individual situation. As with various aftermarket quick fixes, they may not be ideal, but split boots have their place.
CV SHAFT REMOVAL
As required when servicing any vehicle system, if you’re not already familiar with the vehicle at hand, always refer to the appropriate service manual for CV shaft removal. Some inner CV shaft ends will pop out by simply applying leverage between the transaxle housing and a dimple/recess on the inner joint housing, while others are secured with a retaining clip (which may require removal of a transaxle cover plate for access). Never pull on the shaft in order to dislodge the inner joint spline stub axle from the transaxle. This can result in pulling the inner joint apart. Apply pulling force only at the inboard joint housing where it meets the transaxle. Also, once the outer CV joint has been disconnected from the hub assembly, be sure to support the shaft assembly prior to and during inner joint disconnection. Allowing the shaft to hang down can also result in pulling the inner joint apart.
While it may be tempting, especially if you’re in a hurry, never pound the end of the outer CV shaft’s stub with a hammer in order to dislodge it from the hub. Even if you temporarily install the old nut to protect the stub splines, you can cause damage to the inner joint and/or the outer joint, the differential gears and the wheel bearing. If the stub axle is stuck in the hub and won’t push out by hand, use a proper puller tool to separate the stub from the hub.
When it’s time to replace a bad CV joint, while you certainly can disassemble the shaft and rebuild or replace an individual CV joint, since today’s aftermarket offers ready-to-install complete CV axle assemblies, it just makes sense to replace the assembly rather than attempt to rebuild. You’re able to get the job done much faster (good news for the customer), freeing up shop space for increased vehicle service turnaround.
