Underneath your Wrangler is a collection of various bolts, joints, axles. Two of the more common joints that you will find on your Wrangler (and many other vehicles for that matter) are CV and U joints.
“CV” and “U” stand for constant velocity and universal, respectively. These joints are the gatekeepers of your vehicle’s powertrain. Any power produced by the motor has to pass through either a CV joint or a U-joint on its journey to the wheels.
Although they serve similar functions, they have different applications, and more so, they aren’t interchangeable.
Understanding U and CV joints are essential for off-roading vehicles. They are the main way any significant ground clearance can be achieved on a vehicle since they allow for greater driveline angles than any other form of torque transference.
There are several places you can find U or CV joints in your vehicle’s driveline, but the most prominent and important is on the driveshaft itself. The driveshaft connects the transmission to the axle. Without the use of joints, the driveshaft would have to be completely flat between the two parts.
This would make the vehicle rigid and fragile, and also make packaging all of the necessary parts difficult.
With a name like “universal” joint, it’s surprising that these aren’t used on absolutely every part of the car. U-joint is one of this style of joint’s many names.
It is commonly called a cardan joint or sometimes a Hardy-Spicer joint. The design of a U-joint is quite simple; it’s a plus-shaped piece of metal that fits into slots in another shaft.
Part of the way a U-joint works is that it is able to transmit torque at an angle. The effective maximum angle for U-joints is three degrees. They can technically function above three degrees, but they tend to cause vibrations that can harm other parts of the car.
While that is a relatively small angle, being able to have the transmission above the differential is critical to driving a lifted vehicle. The only other way to accomplish something like this would involve a portal axle, which is considerably more complex than U-joints.
Since U-joints are tasked with transferring torque to other parts of the drivetrain, they can break if they are pushed above their limits. If your engine is pumping out more power than your vehicle is prepared for, then the more fragile components are liable to break.
That being said, installing heavier U-joints is not necessarily a good idea. Since U-joints (or any other joint for that matter) are one of the weakest points in your vehicle’s drivetrain, installing a larger U-joint could disrupt that balance and make a more expensive and difficult to replace drivetrain component liable to break instead.
Basically, it’s better to fix a U-joint than it is an axle or a differential, so only upgrade your U-joints if they are constantly snapping on you (and the rest of your drivetrain is up to the task).
Part of the issue with U-joints is that they can’t work at all angles. A U-joint can transmit torque at impressive angles, but due to the way they are designed, they will wreak havoc on your drivetrain if pushed too far.
U-joints don’t move at a constant velocity, and while this effect is relatively minor at low driveline angles, it becomes pronounced the more the angle increases. This means that a U-joint pushed beyond its comfort zone would aggressively vibrate. For these aggressive angles, however, there is, fortunately, an alternative.
To solve the uneven velocity potentially caused by U-joints, CV joints should be used. As their name implies, they rotate at a constant velocity, effectively quelling any additional vibrations that could happen in the driveline. As opposed to U-joints, CV joints can work up to around 50 degrees.
One thing that should be pointed out is that there isn’t one specific type of CV joint. There are plenty of ways to accomplish torque transfer while maintaining a constant velocity. In the automotive context, there are a few examples that you will likely come across.
First off, there is a type of CV joint that is two U-joints coupled together, more often referred to as a double-cardan joint.
Double-cardan CV joints are some of the simplest CV joints. There are several other styles of CV joints that aren’t double-cardans. On some suspensions, you might find a more complicated system like a Rzeppa joint, which uses a series of balls that fit into grooves on a larger ball.
This type of joint is more complex, but the gist of it is that it can transfer torque at a constant velocity over a larger series of angles. Many of these other forms of joints need to be covered in a rubber boot, or some type of protective sheath in order to keep them working. The more complicated a joint is, the more contaminants can cause damage.
You are likely to find CV joints in the driveline of many FWD cars. Since the angle from the transaxle to the wheels is pretty sharp, CV joints are a good way to transfer torque efficiently.
CV VS U-Joints
The only time that you will really have to make a decision between a CV joint and a U-joint is if you are completely remodeling your vehicle’s driveline or suspension. The biggest issue specifically with U-joints is finding an equilibrium between the angles on both sides of the driveshaft.
The rule of thumb is that U-joints are fine to use if the angle between the transmission and the differential is the same. If they aren’t, then a CV joint is necessary.
As mentioned previously, there’s no reason to swap a CV and U-joint all willy nilly. In fact, U-joints are the stronger and cheaper of the two, so there is no reason to use a CV joint unless it is required by the constraints of your vehicle’s driveline.
This distinction isn’t one that you will frequently have to make, but it’s good to know nonetheless.
Joints are one of the more breakage-prone parts of a vehicle, especially a 4X4 that puts a ton of stress on its driveline. In the future, if you hear a snapping sound erupting from underneath your Wrangler, be sure to check your joints.