- CV Joints

The drivetrain requirements of front-wheel-drive (FWD) cars pose some design challenges that seem daunting, at first glance.  The power from the engine must be transmitted, without interruption or fluctuation, to front wheel hubs that can simultaneously roll forward or back, turn in and out to accommodate steering motion, and bounce up and down and change camber and caster angle as the suspension adjusts for road surface variations.

A miracle of modern technology, you say?  Nothing for the design geniuses of the '80s and '90s, you think?  Perhaps true sentiments, but they're historically inaccurate.  The mechanism that allows successful front-wheel-drive design --the constant-velocity joint-- was patented originally in the late 1920's (there are references alternately pegging the date at 1926 or 1928) by a gentleman named Alfred Rzeppa.

The Rzeppa CV joint is a monument to simplicity and clever design.  Considering the geometry requirements, it contains relatively few moving parts, and the ones it does contain can be made robust enough to handle almost any power transfer requirements.

The heart of the joint is a cage with slotted openings that constrain six balls within the confines of the inner race.  In the drawings below, you can see that the shape of the race, the cage, and the housing allow tilting and rotation of the inner race and cage, to
accommodate steering motion and rotation of the wheel hubs to which the housing is attached.

In the lower drawing, you can also see that the housing is elongated to allow for the plunging (in toward the vehicle centerline and back out) caused by movement of the wheel and suspension up and down.

The outer CV joint, attached to the front hubs, is typically a fixed joint.  The inner CV joint, at the inboard end ot the halfshaft, is usually the plunging joint.

Some designs replace the inner plunging Rzeppa joint with a plunging tripod joint.  In this design, the ball, cage, and inner race assembly is replace by a three-pointed star, called a "tulip."  At the end of each point of the star is a bearing assembly that rides in the elongated grooves of the outer housing.  Thus, the tulip can move in and out within the housing as it rotates.

Failure Modes
A CV joint can fail for one of several reasons.  Of course, normal wear and tear will eventually take their toll, since the joint is filled with moving parts that carry heavy loads.  They can also fail from mechanical damage caused by such things as halfshafts bent by the careless placement of a towing j-hook, or the impact of a collision.

Premature failure, though, is usually caused by a lack of lubrication, or the presence of harmful contaminants like dust, pebbles, and other road debris.  Such premature wear almost invariably begins with a failure of a CV joint boot.

The boots are rubber bellows type covers installed over the inner and outer joints to protect them from the hazards we just noted.  While inner boots and outer boots may be shaped somewhat differently to accommodate the difference in joint geometrics, they're much alike in construction and function.

Each end of the boot is held in place with a boot clamp.  The boot material itself is sturdy but flexible, and a well-sealed, intact boot will protect the joint from nearly all common hazards.  If a rock should be kicked up into the boot, though, and cause a small tear or hole, or the boot should be stressed hard when the weather is very cold and the boot's flexibility compromised, that's when progressive failure begins.

If found early, a damaged boot can simply be replaced, along with any lost lubricant, and the joint again protected.  On the other hand, if the boot is not inspected for an extended period of time, even a very small hole in a boot will tend to enlarge because of the constant flexing and pulling inherent in the joint's operation.

Ultimately, the lubricant will dry out, road grime, moisture, and small pebbles will find their way into the joint, and the serious deterioration of the joint surfaces will begin.  The inevitable outcome is a failed joint, and the need for replacement.

A failed CV joint is usually detectable audibly.  When the front wheels are turned hard to the lock, and the car driven slowly, a definite clicking sound can be heard coming from the affected side.  Inspection on a lift will generally confirm the damage to the boot (or mechanical damage to a halfshaft or the like), but the joint will have to be removed and inspected before the extent of the damage can be determined.

Naturally, the cure is replacement of the joint, or worn components, along with the installation of a new boot, and new clamps, over a carefully and thoroughly lubricated joint.

Remember, CV joint removal requires removal of the front wheel hubs, so new hub nuts will be requred.  Also, the boots should always be replaced when a joint is replaced, even if the boot was intact at the time of disassembly.