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A constant velocity boot, sometimes also called a CV boot, is a protective cover that fits over certain joints in the wheels of a car or truck to protect them from accumulating debris and getting damaged or bent with excessive movement. Their main role is protection, and they’re most common in the CV joints of front-wheel drive cars where the front axle carries a lot of torque to the drive wheels. Some rear-wheel drive cars have CV joints and boots, too, particularly if they have independent rear suspension. Boots are usually found directly behind the wheels, though they aren’t always easy to see; they’re usually deep in the wheel bed, which also makes them somewhat difficult to access. They’re built to be very sturdy since they need to withstand a great deal of heat, road debris, and wear. Over time, though, even the best-quality boots will tear and crack, and quick replacement is usually essential in these cases to avoid more serious damage to the drive shaft or the wheel joints themselves.
The main purpose of any CV boot is protection. It’s usually made or rubber, plastic, or some other flexible synthetic, and in most instances it’s made to fit over a car’s joints exactly. Car manufacturers typically encase the CV joints during basic assembly. Ideally there’s enough room in the boot for the joint to move freely, but not enough exposure to the engine elements or road air to allow in debris or moisture.
CV joints are really important to most vehicles. They are how the main drive train, which is the energy-bearing shaft that connects to the engine, delivers power to the wheels. Covering these joints is one of the best ways to ensure smooth driving. In most cases the boots also contain a certain amount of lubrication in their interior, which can keep the joints moving without allowing for scraping or wear.
In most cases, the boot requires maintenance or replacement before the joints because rubber and plastic compounds have a tendency to degrade with age, especially when exposed to heat. The material that makes up the boot may rip or tear over time, too, which reduces its ability to protect the CV joint. Boots that are ripped or torn cannot retain their lubrication, and the joint is more prone to breakdown in these circumstances.
A damaged boot cannot protect the CV joint, and this can lead to bigger problems down the line. Owners probably won’t notice a problem with the boot at first. Many of the earliest cars didn’t have this protective sheath, and they still worked just fine — they were just much more prone to problems. Depending on where the boot is damaged, it can sometimes be really difficult to tell how long the problem has been going on. If there’s a lot of wear on the joint underneath this can give some clues, but if the crack or tear is relatively recent the joint might not have been affected yet.
Mechanics usually diagnose a boot problem through visual inspection. This part of the car can be difficult to access, though, which can make even just the labor involved in inspecting it rather expensive; the effort of replacement is often very costly, too. Although it is possible for self-trained or “backyard” mechanics to replace a boot, the job usually is best left to professionals. The task requires the vehicle to be safely lifted and supported, and also requires the front tires, as well as the wheel hubs and brake calipers, to be removed. An amateur attempting such a repair may accidentally inflict bodily harm on himself, and may cause further damage to the vehicle, too.
Some mechanics recommend replacing the entire drive shaft when CV boot damage is discovered. This is because the cost for replacing the entire drive shaft isn’t always that much more than replacing the boot alone. The cost of the shaft has to be factored in, of course, but the labor is almost the same in most cases. Additionally, it's often hard to determine how much damage has occurred because of the faulty boots, at least at first. It’s often safe to assume that drive shaft will have to be replaced in the near future anyway, so doing it all at once usually works out to be the most economical option.
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