Rick's Free Auto Repair Advice

CV Joint — What is a CV Joint?

What is a CV Joint and how does it fail?

What is a CV Joint?

A constant velocity, or CV oint is a flexible joint used on a drive axle to allow for up and down and left and right turning movements while still providing turning power to the wheels. CV joints are often used in place of universal joints on both front and rear wheel drive vehicles because they offer a greater range of flex.

What is a half shaft?

A half shaft assembly consists of an inner plunging tripod joint, a solid driveshaft and an outer fixed ball CV joint. When CV joints fail, most mechanics replace the entire half shaft with a rebuilt unit, rather than disassemble the old half shaft and replace the individual components. The decision to replace the half shaft as a unit versus replacing components comes down to a matter of economics. The labor time required to remove and reinstall the half shaft in the vehicle is the same in both cases. But the shop labor to disassemble and replace individual CV joint components is usually higher than the cost of a rebuilt assembly.

What is a CV joint

Half shaft with inner CV joint, driveshaft and outer CV joint

How a CV joint is different from a universal joint?

Universal joints (U-joint) don’t turn at an even speed. The greater the angle, the greater the speed or velocity changes as the driveshaft turns. As the U-joint rotates around the outside of its angle, it slows down. Then, when traveling through the inside of the angle, it speeds up. The greater the operating angle, the greater the speed difference is.

To compensate for the speed difference, it’s important to have the U-joint at the same,  but  opposite operating angle so the joints cancel out this speed difference and keep noise harmonics at a minimum.

Although U-joint designs differ somewhat, most are limited to a maximum turn angle of 28° to 37°. That’s usually not enough for front wheel drive vehicles.

U joint

Maximum U-joint turn angles

CV joints have a larger turn angle

Fixed ball, or Rzeppa style CV joints have a

CV joint

Fixed ball Rzeppa style “outer” CV joint

much larger turn angle of up to 48°, making them the perfect choice for installation at each front wheel. These are referred to as the “outer” CV joints. Since the inner CV joint only moves up and down and in and out, most car makers use a  tripod plunging joint at that location.

inner cv joint

“Inner” Tripod plunging CV joint

What is a CV boot?

As you can see, CV joints contain many small rotating parts. Those parts must be

cv boot

CV boot

constantly lubricated and protected from road debris. So both the inner and outer joints are protected by a rubber boot.

How does a CV boot fail?

Over time, CV boots can be damaged by road debris or wear and tear. Once the boot tears, the grease leaks out and road debris can enter the joint. Once dirt enters the CV joint, it grinds away at all internal parts.

Once dirt enters the joint, replacing the boot alone will not solve the problem. The fix is a rebuilt half shaft.

torn CV boot

Torn inner CV boot. Notice the grease has leaked out

Repair cost for CV boot

The labor time to remove and replace a half shaft is usually less than one hour and many rebuilt half shafts cost around $150, so figure roughly $300 per side.

 

Can you prevent CV joint failure?

Yes. If you catch boot failure early, before dirt enters the joint, you can often get by with a boot replacement procedure

leaking CV boot

This CV boot has just started to leak grease. Replacing it now can save the joint

Symptoms of a failing CV joint

• Broken CV boot
• Grease leaking out of the boot
• Clicking or popping noise especially in tight turns.
• Vibration especially in turns

How long can you drive after a CV boot fails?

That depends on how much road grit has entered the CV joint. However, once dirt enters, you’ve got a fairly short window of time to get it replaced. If you don’t the CV joint will grind itself up and the driveshaft will separate from the joint, causing catastrophic damage to steering components, engine mounts, pulleys and any other component in its path. In other words, trying to save $300 on a CV joint repair can wind up costing you thousands.

Posted on by Rick Muscoplat

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