Universal and constant-velocity (CV) joints connect your transmission to your driveshaft and allow rotational power to be sent to your axles. Where they differ is how they’re able to transmit this energy. Though most modern vehicles use CV joints, there are still many people who prefer U-joints. Both styles have strengths and weaknesses.
Universal joints, more commonly called U-joints, have been in use since the 1600s and continue to be used today. The beauty of the U-joint is its simple design.
A U-joint looks like a cross, with four cylindrical points. These points connect via a yoke with bearing caps. The U-joint is then held in place with a c-clip, internal snap ring, or full-circle snap ring. Despite only having one design, U-joints have several names, including Spicer joint and Cardan joint.
U-joints are used in several locations. They connect the transfer case to the front and rear differential in a 4WD vehicle. They’re also used in vehicles with a solid front axle to connect the inner and outer axle shafts. As an example, a 4WD vehicle with a solid axle (like the TJ Wrangler), has a total of 7 U-joint locations.
The reason your driveline needs a joint is that your driveline is frequently not level. U-joints act as a pivot point, so they can continue transmitting power even when at an angle. U-joints have a very limited range of motion.
On a standard 2-joint driveshaft, the operating angle is determined by the respective angles of the U-joint on either end. These angles should be within 1 degree of each other when the vehicle is not in motion. To have the best performance and operation, the U-joints should remain within 3 degrees of each other. Each individual U-joint should stay at approximately 22 degrees for smooth operation.
If you need a greater range of motion, a constant-velocity joint, referred to as a CV joint, is recommended. CV joints are especially useful on lifted vehicles.
Unlike U-joints, which lose velocity at certain angles, CV joints are designed to keep velocity constant regardless of their operational angle. There are still limits on CV joints’ total operational angle. This varies based on what type of CV joint is being used.
Unlike U-joints, which are all the same basic cross pattern, CV joints come in a lot of different forms. Most vehicles that use CV joints use a combination of styles. The three CV joint styles most popular in modern vehicles are Rzeppa, tripod, and double Cardan joints.
Out of all the CV joint styles, Rzeppa joints get the best articulation. Most Rzeppa joints average between 45-48 degrees of articulation, but some claim even greater degrees of flex.
Rzeppa joints have a center gear that accepts the input shaft. Around that gear is a circular cage with six grooves, each with a ball. The output shaft is attached via a cup to the outside of these six balls. This makes the input shaft look a little like a person with the output shaft sitting like a hat. In between the two are the six balls which provide the movement.
Rzeppa joints are sometimes referred to as ball-type joints. Since 2007, the Jeep Wrangler has used Rzeppa-style CV joints on the driveshaft.
Tripod joints have less range of movement than many other CV styles. What they lack in range though they make up for with a simple, cost-effective design.
In a tripod design the input shaft is attached to what is called the spider, or tripod. This looks like a circle with three circles mounted on roller bearings surrounding it. The differential will have a cup of some kind that accepts the tripod (these can be open or closed). The rollers allow the tripod to slide back and forth.
This design accounts for the reduced range of motion. In many ways, a tripod looks the most like a universal joint. But, unlike a universal joint, the tripod joint can accept a greater range of travel. While the max angular articulation of a tripod joint is 26 degrees (still a lot more than a U-joint), it can travel 50 mm.
Double Cardan Joint
A double Cardan joint is exactly what it sounds like, two universal joints. They’re joined together by a central yoke. A centering ball helps to make sure that each universal joint is bearing half the load.
One area that double Cardan joints really excel in is torque load. This makes them a great choice for off-roading applications. They also have a great range of joint articulation.
Unfortunately, double Cardan joints do get some vibration at higher speeds. This is rarely a concern for off-roading though.
Double Cardan joints are a great way to get the extreme angles of a CV joint while maintaining the simple design of a U-joint.
Is a CV Joint or a U-Joint Better?
Depending on how and where you drive, you may be better suited to using CV or U-joints. While this isn’t an easy modification, it’s worth the time if it’ll improve your experience. You shouldn’t be using something that isn’t meeting your needs.
These joints, if properly maintained, can last a long time. Some vehicles manage to hit 300,000 miles with their original CV joints. While U-joints don’t always last as long, if you’re a careful driver, you’ll still get a lot of miles out of them. That’s a long time to drive with something you dislike.
U vs CV Joint Pros and Cons
||Easy to replace
|Less movement available
||Allows for higher lifts
|Difficult to replace
Can cause other issues when/if they break
U-Joint Pros and Cons
While changing out a U-joint may not be your favorite thing to do, most people can do it. Even though it’s easier with assistance, most people can do it on their own.
A U-joint is an especially good pick for people whose off-roading would result in any type of joint snapping. In particular, U-joints are easier to maintain for people who go mudding a lot.
The overall replacement cost for U-joints is also significantly less. This is the number one reason people decide to use U-joints rather than CVs.
CV Joint Pros and Cons
It’s well-known that a lifted Jeep can take on challenges that a stock Jeep can’t, but in order to make the most of the height, a CV joint will be necessary. Replacing the driveshaft is a standard upgrade if you’re lifting your Jeep more than 3 inches.
While a CV joint and U-joint are roughly the same at 0 degrees, the CV joint is able to maintain velocity through a larger range of angles. It’s also able to handle a greater torque load. This makes the CV joint stronger and less susceptible to issues like vibration and friction.
This means that CV joints break less. Yet you probably hear about them breaking more often. While U-joint failures are pretty common and expected, CV joint failures tend to be more catastrophic. CV joints are significantly more likely to cause other issues when they break. That makes it harder to install a CV joint on the go.
Some breakage issues can be avoided simply by maintaining the CV joint. CV joints are protected by a rubber boot, which can unfortunately tear. If the boot tears, then the joint can be exposed to the elements. Water and dirt can easily destroy a CV joint. It’s important to check your joints after each off-roading expedition.
What to Know Before Upgrading Your U or CV Joints
While some people need to change to a different joint, many can simply get a hardier U-joint or more flexible CV joint. Regardless of your reason for changing your joints, there are a few factors to keep in mind.
Don’t Make an Indestructible U-Joint
You can purchase U-joints that are made for much more powerful vehicles. This makes them nearly indestructible, which is the last thing you want in a U-joint.
The U-joint’s primary strengths are that it’s easier to replace and inexpensive. If anything is going to break in your driveline, you really, really want it to be your U-joint. Try to keep your U-joint well-matched to your other driveline components.
Greasable vs Non-Greasable Joints
Greasable joints aren’t just capable of being greased, they need to be greased regularly in order to maintain proper functionality. Non-greasable U-joints still rely on grease, but they keep it sealed.
Since most U-joint failure is due to a lack of grease, many people prefer the greasable U-joints. This forces them to check the grease regularly and replace it. Typically, greasing will need to be done every 10,000 miles.
Non-greasable U-joints are actually stronger due to the method of manufacturing. Instead of having a zerk hole for feeding grease into, they’re properly sealed. In general, they’re stronger and require less work. These are a better fit for all but the most hardcore off-roaders.
CV joints are all non-greasable, but that doesn’t mean they’re maintenance free. If the CV joint’s rubber boot tears, debris could contaminate the packed grease. Be sure to check your rubber boot for tears frequently.
For most applications, a CV joint makes more sense than a U-joint. Even at high angles, CV joints don’t lose strength or velocity. But if you want a simple joint that you can work on yourself, U-joints are practical and affordable.