Constant Velocity (CV) Joints & Boots

Automobiles are comprised of many relatively small components that may not get much attention but are vital to the safe operation of the vehicle. One such part is a Constant Velocity (CV) joint and the one little bit of protection it has called a CV boot. What exactly is a CV joint and boot, and how do you know if yours is in trouble? Let's find out.

All front-wheel drive vehicles have CV joints on both ends of the axle drive shafts, also known as half shafts. Inner CV joints connect the axle drive shafts to the transmission or differential, while the outer CV joints connect the axle drive shafts to the wheels. Depending on the type of suspension used, rear-wheel drive, all-wheel drive, and four-wheel drive vehicles may also have CV joints and boots.

CV joints are needed to transfer the torque from the transmission and differential system to the drive wheels at a constant speed, while accommodating an up-and-down motion of the suspension. They allow for all types of movement and any odd angles required as the vehicle passes over the road. A CV joint basically keeps the axle from breaking every time the vehicle hits a bump.

The two most commonly used types of CV joints are the ball-type and tripod-type. In front-wheel drive vehicles, ball-type CV joints are used on the outer side of the drive shafts (outer CV joints), while the tripod-type CV joints are mostly used on the inner side (inner CV joints).

A CV joint is packed with special grease, usually molybdenum disulfide grease, and sealed tight with a flexible and durable rubber or plastic CV boot held in place with two clamps. The boot typically looks like an accordion in the shape of a funnel. A CV joint doesn't need any maintenance and can last a long time as long as the protective CV joint boot is not damaged, and adequate grease is being held in the joint and protected by the sealed boot. CV boots serve a simple but important purpose and allow the CV axles and joints to enjoy a long service life.

The most common problem with CV joints is when the protective boot cracks, tears, or becomes damaged. Once this happens, the grease leaks out and moisture and dirt get in, causing the CV joint to wear faster and eventually fail due to loss of lubrication, contamination, and corrosion. Usually outer CV joint boots break first, as they must endure more movement than inner ones.

Grease leaking from a small crack or tear is an early sign that a CV joint boot is failing. If the damage is more severe, you may see grease splattered on the inside of the wheel rim and around the inside of the drive wheel area. Often times the grease can also be flung onto the chassis or other parts on the underside of the vehicle as the CV axle turns.

If a vehicle is driven with a damaged CV joint boot, the CV joint will wear out and eventually fail. The most common symptom of a badly-worn outer CV joint is a clicking or popping noise when turning.

Usually the noise gets louder when accelerating during turns. In worst cases, a badly-worn outer CV joint will break apart while driving. This will cause your vehicle to be undriveable and may potentially leave you stranded.

Inner CV joint failures are rare. One of the symptoms of a failed inner CV joint is a shudder or side-to-side shaking during acceleration. A worn out inner CV joint may also cause clunking when shifting from drive to reverse.

A used vehicle inspection should always include a visual inspection of all the inner and outer CV joints and boots by turning the wheels while watching the boots. Any evidence of cracks, tears, rips, or damage, or of grease leaking or flinging from the boots will be obvious during the inspection. Boot clamps should be inspected to make sure they are present and secure. CV axles can become bent during collisions. This damage may not always be visible but can be detected in the vehicle's handling. A test drive should be performed, during which several turns should be made in both directions along with accelerating while turning. This will generally reveal unusual clicking noises, vibrations, and other problems with CV joints.