Tech Stuff

CV joint tech -- A primer on constant velocity drive joints and diagnostic tips

I realize that much of this introductory text will seem very basic, but let’s start by explaining CV joint technology. A CV (constant velocity) 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-type) joints.

A U-type joint works fine when there is a very limited shaft angle variation (primary driveshaft on a RWD vehicle, for example). 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. For this reason, a CV joint is required for FWD and independent RWD systems.

One factor of CV joint design that permits this freedom of movement without bind or differences in output/input is that the inboard CV joint features a “plunging” movement which allows the shaft to move in/out during suspension travel without restricting suspension travel.

Outboard joints

Outboard (wheel side) CV joints are usually the “fixed” type, also often referred to as a Rzeppa joint.  This 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. These are also referred to as AC (angular contact) joints. 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).

NOTE: Because of the greater operating angles and their greater exposure to road hazards (rocks, dirt, moisture, etc.), outboard CV joints tend to wear sooner than inboard CV joints.

Inboard CV joints

In order to accommodate suspension travel, the 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 change its length during suspension travel, compensating for control arm up-down/angles and to prevent restricting lower control arm movement.

Several styles of inboard CV joints are in use (although they all accomplish the same task). 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).

Diagnosing CV joint problems

Popping/clicking 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 backwards with the wheel cranked full right and the noise is louder, suspect the right outer joint.

If driving backwards with the steering wheel cranked full left, suspect the left outer joint. NOTE: Clicking/popping can also be caused by worn or damaged rack & pinion steering inner tie rod ends.

Shudder/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 (front wheel drive), suspect an inner CV joint. If the vehicle features RWD (rear wheel drive) 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.

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