Constant-Velocity Joint

Why not just use a universal joint?

A simple Cardan (universal) joint transmits torque through any angle but introduces a sinusoidal velocity error: the output shaft speeds up and slows down twice per revolution, with the magnitude of the error growing with articulation angle. At 5° the error is barely measurable; at 30° the output velocity oscillates by ±15% — a pulsing torque that beats the drivetrain to death and is felt as vibration through the steering wheel.

The fix is geometric: arrange the connecting elements so the rotational axes always bisect the articulation angle. If both shafts contact the coupling at points that lie on the bisecting plane, no rotational position favours one shaft over the other. This is the principle behind every CV joint design — they differ only in how they hold the coupling in the bisecting plane.

The Rzeppa joint (most common)

Alfred H. Rzeppa patented the design in 1926. Six steel balls ride in curved grooves machined into a spherical inner race and a matching outer race, held in plane by a slotted cage. As the joint articulates, the cage geometry forces the balls to remain in the plane that bisects the angle between the two shafts.

          Outer race (cup, attached to wheel hub)
        ╱  curved grooves ╲
       ╱                   ╲
      │  ●  ●  ●  ●  ●  ●   │   <- 6 balls
       ╲   in slotted cage ╱
        ╲   ╲   ╱   ╱
         Inner race (star, on shaft)
              │
           Shaft (axle)

Six balls is the standard count, though some heavy-duty designs use eight for higher torque capacity. The cage is the most highly loaded part — when it fails, the balls escape and the joint locks up.

CV joint variants

	Rzeppa	Tripod (Tripode)	Double-cardan	Weiss	Thompson coupling	Disc / flex coupling
Geometry	6 balls in curved grooves	3 needle rollers in axial grooves	2 U-joints + centring yoke	4 balls in straight grooves + retainer	Two yoked sets, mechanically synced	Flexible disc / spider
Max angle	~47° (fixed); ~22° (plunging)	~26°	~30°	~30°	~50°	~5°
Plunge	Limited	Excellent (up to 50 mm)	None natively	Limited	None	Some via flex
Vibration at high angle	None	Slight axial pulse (GAF)	None when proper geometry	None	None	Damped, not perfect CV
Common use	FWD outer half-shafts	FWD inner half-shafts	4WD/RWD driveshafts	Pre-1960 Jeeps	Aerospace, marine	Industrial pump shafts
Cost	$$	$	$$	Obsolete	$$$$	$
Failure mode	Click on turn	Shudder on acceleration	U-joint wear, vibration	Ball wear	Sync linkage wear	Disc fatigue

Plunge: the inner joint's job

As the suspension compresses and rebounds, the distance between the transaxle and the wheel hub changes — a typical FWD strut suspension demands 30–50 mm of axial travel through the half-shaft. The outer joint can't plunge (the wheel hub is constrained), so the inner joint must.

The tripod joint solves this elegantly: three needle-roller-equipped trunnions on a tulip-shaped tripod ride in three straight axial grooves machined into the housing. The tripod can slide axially within the housing while still transmitting torque through the rollers. Plunge of ±25 mm is routine; ±40 mm is achievable in long-travel applications.

Tripods do produce a small axial force ("generated axial force", GAF) that pulses at three times rotational frequency — under hard acceleration this can manifest as a low-frequency shudder felt in the cabin. Modern tripod designs use spherical rollers and refined groove profiles to minimize GAF below perceptible levels.

Worked example: steering articulation

A typical FWD compact car has an outer Rzeppa joint that must operate from 0° (straight ahead) to about 40° at full steering lock. With wheel torque of 1500 N·m at low speed, the load on each of six balls in the joint at full articulation is roughly:

• Effective ball pitch radius: 35 mm
• Tangential force per ball at 0°: 1500 / (6 × 0.035) ≈ 7,140 N
• At 40° articulation, ball loading becomes uneven — peak load on the most-loaded ball ≈ 1.6× nominal, so ~11,400 N
• Hertz contact stress in the raceway approaches 3,500 MPa at peak load

This is why Rzeppa balls are made of through-hardened 52100 bearing steel and the raceways are induction-hardened to HRC 60+. A long, sustained full-lock manoeuvre under heavy throttle (donut, J-turn) is the worst case and is what eventually pits the raceways.

Common failure modes

• Boot tear → grease loss → wear. The dominant failure cascade. Inspect boots at every oil change; a 30-second visual check saves a $400 half-shaft.
• Click on turn (Rzeppa). Pitted raceways from worn or contaminated grease; grit acts as lapping compound. Audible only at large angles under load.
• Shudder on acceleration (tripod). Worn rollers or galled grooves cause rough plunge motion under torque; inner joint replacement.
• Cage fracture. Rare but catastrophic — overloading or impact damage cracks the cage and balls escape, locking the joint and shearing the half-shaft.
• Spline wear. The shaft splines into the joint can fret if the retaining circlip allows axial play; symptoms include a loud knock on torque reversal.
• Heat from sustained high-angle drive. Off-roaders who steer at full lock for long crawls can cook the boot grease past 130 °C, breaking down the moly carrier.

Picking a CV joint

• FWD outer (high articulation, no plunge): Rzeppa, fixed-length.
• FWD inner (moderate angle, high plunge): tripod.
• Independent rear suspension: Rzeppa or tripod, depending on travel and torque.
• Truck driveshaft (low angle, very high torque): single Cardan plus slip yoke, or double-cardan if angles exceed 6°.
• 4WD front driveshaft on solid axle: double-cardan, because angles exceed what a Cardan handles smoothly.
• Industrial / agricultural PTO: wide-angle Cardan or double-cardan, oversized boots, easy field service.