About Constant Velocity Boots

Constant velocity boots are used in cars with front-wheel drive and all-wheel drive. Constant velocity boots protect the constant velocity joint, which transmits power for movement from the transmission to the wheels. If these boots are damaged, wear and tear on the joint can result, leading to mechanical failure. Regular inspection and early detection and replacement of the constant velocity boots when damaged can prevent the much costlier repair or replacement of the joint itself.

Function

Constant velocity boots are rubber coverings that surround the constant velocity joint with a waterproof seal. These boots keep the grease that the constant velocity joints are packed with in place, while keeping out dust and water from the road. Constant velocity boots prevent the joints from excessive wear and premature failure.

Types

There are two basic types of constant velocity boots. Each constant velocity joint running from the transmission to the wheel has two boots, known as an inner boot and an outer boot. This is because there are two joints along each half shaft. Some vehicles also use a constant velocity joint with rear-wheel drive, providing joints and boots along the length of the drive shaft. These are known as front and rear boots. Standard factory boots have clamps on each end to seal the boot onto the drive shaft. After-market universal replacement boots are available that can seal a failing boot without removal of the tire assembly. These after-market universal boots use a zipper type configuration with clamps to allow the boot to be installed with less specialized work.

Identification

Constant velocity boots can be identified by following the drive train (or drive shaft, also known as a trans axle) from behind the drive wheels to the transmission. The drive wheels are the wheels that push or pull the vehicle when in motion. On front-wheel drive vehicles, the outer constant velocity boot will be located just inside the wheel, behind the rotor and spindle assembly. The inner constant velocity boot will be located right up against the exterior of the transmission.

Considerations

Regular inspection of the constant velocity boots should be part of every automobile owner's maintenance schedule. An inspection of the boots can reveal early failure, indicated by cracks in the boot (see photo). In addition, ripped or torn boots will be cause for immediate inspection of the constant velocity joint and replacement of the boot and joint if warranted. Another indication of a boot problem is the appearance of grease on the inside of the tire rim. If the outer boot becomes ripped, the centrifugal force will cause the grease inside of the joint to seep out. All of these are indications that the boot needs to be replaced, at the bare minimum.

Prevention/Solution

There are several common factors that can lead to premature constant velocity boot failure or damage. One of the most common and the most easily prevented results from improper towing of vehicles with front-wheel drive. Most front-wheel drive vehicles provide towing attachment points on the vehicle frame itself; tow chains are often incorrectly applied to the constant velocity half-shafts, which causes boot damage during towing of the vehicle. Rough terrain and objects in the road can also damage boots. Inspections should be performed any time a vehicle encounters and makes contact with an object or obstacle in the road or off-road terrain. Simple preventative measures like these will go a long way to providing long life to this critical component of the modern automotive drive train.