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# Constant-Velocity Joints: Smooth Operators

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What was wrong with universal joints anyhow? After all, they worked fine for years, and they still do! This is true, and U-joints can handle a lot of torque, but they do have a downside in the nature of their operating characteristics.

Here are the basics: U-joints are located on the ends of a driveshaft, mounted between the driveshaft and a front and rear yoke. The front yoke attaches to the transmission and the rear yoke attaches to the rear differential. As the engine moves from the effects of torque and as the suspension of a vehicle travels up and down, the angle of the driveshaft changes.

A U-joint does two things. First, it transfers the motion between the yoke(s) and driveshaft; and second, it does this at different angles, allowing for driveline movement. Here’s where the fun begins. When a yoke and the driveshaft are in perfect alignment, the velocity from one is transferred to the other at the same rate. However, when there’s an angle between the two, the velocity of the driven member fluctuates continuously during rotation.

It can be hard to visualize, but the reason this happens is that as the angle of the U-joint changes, the two halves of the U-joint cross are forced to rotate on a different axis. The drive axis remains at a constant velocity, and both ends of the U-joint cross rotate in the same consistent circular path.

The driven axis, however, rotates in a path that causes the distance of travel at the outer ends of the U-joint cross to increase or decrease in relation to the consistent points of the drive axis.

This effect results in the continuous fluctuation of velocity between the input and output sides. While the input remains at a consistent speed, the output speeds up and slows down as the points of the driven axis continuously alter between a long and short path of travel.

So, why don’t we feel that on a vehicle with a traditional driveshaft? Because there are two U-joints, and the fluctuation on each end balances out, effectively allowing the driveshaft to provide a consistent output speed to the rear differential. The angle of the two joints must be the same, however, and it doesn’t take much wear in one for the angles to differ, and subsequently cause a vibration.

U-joints are known for their propensity to cause vibration, and the other disadvantage they have is the greater the angle of the U-joint, the greater the fluctuation in velocity. Anything over 30 degrees and the fluctuation dramatically increases. Have you ever noticed how jittery an old four-wheel-drive truck feels in the front when the hubs are locked and you turn a corner? Now you know why.

You may have heard of a Double-Cardan U-joint. It’s basically two joints side-by-side with a common link-yoke in between. This is one of the original concepts for a true constant-velocity (CV) joint, and they’re often referred to as this. The advantage they have is they offer smoother operation at greater angles, and they’re common on four-wheel-drive trucks, and a common upgrade for lifted trucks where the driveshaft angle is altered considerably.

The drawback to a Double-Cardan joint is they’re bulky, and they still can suffer from limitations due to operating angle. True CV joints as we know them today have been around since the early 20th century, but the popularity of the front-wheel-drive (FWD) vehicle is what made them a household name.

Today’s CV joints are a radical departure from anything resembling a U-joint, and not only do CV joints transfer power without speed fluctuation, but they also can operate at angles up to and exceeding 50 degrees, depending on the joint. Since the drive wheels on a front-wheel-drive vehicle also steer, the ability for this increased operating angle is what makes the CV joint so beneficial for FWD.

A front-wheel-drive vehicle has two CV shafts – one on each side – and each shaft features an outboard and inboard joint. The outboard joints are considered fixed joints, meaning they don’t offer in and out movement. It’s their ability to operate at the increased angles for steering that’s important. The inboard joints are considered plunge joints, meaning they offer a wide range of inner and outer directional movement in order to make up for length differences as the suspension travels up and down.

## Types of CV Joints

You’ll see two types of CV joints. One is the Rzeppa design, which features steel balls trapped in a cage and riding on an inner and outer race. The tri-pod design is the second, which features three roller bearings that ride in a race or cage, sometimes referred to as a tulip assembly. Both types of joints can be found in either a fixed or plunging design for outboard or inboard use, but the Rzeppa design has proven more popular as an outboard joint. The Rzeppa works well as an inboard joint too, but the tri-pod design gets the nod for the most effective operation as a plunge joint.

The CV shafts themselves can differ in length from side to side, and in early FWD development, torque steer – the vehicle pulling in one direction or the other during acceleration – was sometimes a result of this difference. Different diameter shafts as well as hollow versus solid became part of the design aspects to combat this problem. Drivetrain mounting and torque control have advanced considerably since the early days of FWD, and torque steer rarely is a problem.

Even though the FWD vehicle put the true CV joint on the map, due to their overall advantages, CV shafts now are utilized front and rear, and it’s not uncommon to see driveshafts that feature CV joints instead of U-joints. U-joints aren’t forgotten, however, due to their ability to handle high torque, and they work well in abusive environments that may not be so friendly to the boot on a CV joint (such as the exposed location of a driveshaft under a truck).

CV joints are packed with a specially formulated grease, and a rubber boot is sealed to both the CV shaft and the joint, to keep the grease in place. When a boot is torn or begins to leak, the grease goes away, and dirt gets inside. CV joints typically need no service until this happens.

There was a time when the most common service for a bad boot was to remove the CV joint, take it apart, clean it, repack it and install a new boot. Generally, this was routine. However, from time to time you could experience a nightmare. Much of the reason we replaced the boots and serviced the joints in this manner was due to the high cost of a replacement joint or a complete shaft. Even with the additional labor, it was far more cost-effective to replace just the boot.

Over time, with advancements in manufacturing and the availability of supplies, the cost of complete CV shafts went down, and it simply made more sense to replace them as a complete unit, not to mention it makes things easier for technicians.

## Selling Tips

The most important part of selling a new CV shaft is making sure it’s the correct one. You should compare shaft length, the size of the CV joints themselves, and make sure it has an ABS tone ring installed if the vehicle is equipped with an anti-lock braking system. Some early CV joints had the tone ring cast into them, but that design was quickly abandoned for a press-fit tone ring. If your customer doesn’t yet have the original shaft out, be sure and make these recommendations to them so they ensure the shaft is correct prior to installation.

Installing a CV shaft is routine for professional technicians, but DIYers likely will have questions. One of the most important factors is torque of the fastener that secures the outer CV joint in the hub. If they don’t adhere to the factory procedure and don’t follow the correct torque specification, damage can and will occur to the wheel bearing.

Some CV-shaft applications come with an ABS tone ring installed, regardless of whether or not the vehicle is equipped with ABS. If not, in most cases, the ring has no consequence. However, in the rare situation where it rubs or contacts something, the rings can be removed easily.

The tricky part of CV-shaft service is there are some you can have out in a few minutes without even removing a wheel, and others that may take an hour or longer. The majority of them require some portion of the suspension to be separated, so the outer joint can be pulled out of the wheel bearing, then the entire shaft pulled outward as it’s removed from the transaxle. It’s fair warning that this is not always easy and might require specialized tools. As long as you convey that to the customer, they can’t say you didn’t warn them.

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• ### Similar Topics

• When ball joints are on the counter, what’s the No. 1 upsell? Shop rags, latex gloves and floor cleaner, because it’s going to be a messy job!
Seriously, though, it depends on the suspension. For light cars and trucks with strut-type suspension, most ball joints aren’t greaseable, and most of them only have one lower control arm on each side, resulting in only one lower ball joint on each side (multi-link suspensions are another story). Most of these are a cakewalk to replace, and you barely get your hands dirty. But, heavier trucks and older cars with upper-lower A-arm suspension and greaseable joints are a different story. That’s when it gets real.
But before we start wiping up grease, let’s look at two of the stickier aspects of ball joints: inspection and installation. Ball joints have wear specifications, and the maximum allowable play as well as proper inspection procedures can vary considerably between different applications. When checking a ball joint for wear, technically speaking, not only should we confirm the recommended procedure and specifications, but we also are always supposed to check them using a dial indicator. In the real world, that rarely happens.
Most technicians understand that a little bit of play in a ball joint is normal and acceptable. But at the same time, there’s a common misconception that any play in a ball joint means it’s wearing out. The root of the problem goes deeper than this article can cover, but let’s face it: Time is money, and taking the time to look up specs, set up a dial indicator (if you have one) and recording the readings just isn’t realistic, especially when a shop manager is breathing down your neck for a diagnosis.
Another part of the problem is when you check a ball joint for wear, you always can feel even the slightest amount of play – so again, rather than performing the correct procedure, it’s easier to estimate the free play in your mind based on your familiarity with these types of measurements.
You can’t always see the movement when it’s minimal, but the worse it gets, the easier it is to see. Experienced technicians are good at recognizing when the amount of play is still “acceptable,” or when a ball joint is – as we like to call them when the vehicle isn’t safe to drive – “wasted.” When a ball joint exhibits wear but still is acceptable and safe for use, that’s how we represent it to the customer, and we’ll just recommend checking them again at the next service. “Let’s keep an eye on those ball joints,” we might say.
This all might sound like I’m criticizing technicians, but that’s far from the case. I’ve been a tech my whole life and it can be tough to wear our shoes. There’s a lot we need to know – we’re all human – and we do make mistakes. When it comes to parts, we rely on the knowledge of a counterperson more often than you realize. One of the strongest traits of a good technician is understanding that you don’t know everything, and not being afraid to ask questions or accept advice. In the case of ball joints, they usually don’t come with specifications, and there’s rarely any information with them aside from installing the grease fitting. And when they do come with information, does it always get read? You probably can guess the answer. This is the real world of automotive repair.
As crazy as it sounds, when you’re deep into a suspension repair with parts and tools all over the place, it can seem like it takes an eternity to unbox a bunch of parts and remove them from their plastic bags, etc. – so again, it’s no surprise that details are missed should they happen to be included. It’s worth its weight in gold when we learn something we don’t know about any particular part, and we’re always eager to learn.
If the line between misconception and mistake isn’t blurry enough, there’s an extra kicker with ball joints. Some vehicles utilize telescoping ball joints. What this means is that the ball-joint stud telescopes a small amount to compensate for manufacturing tolerances, primarily related to the ears of a steering knuckle.
When you install one of these joints, it may appear as if the stud is too long or too short, potentially causing a technician to think it’s the incorrect joint. Also, since the stud is engineered to slide in and out of the housing, they can exhibit as much as .060” (sixty-thousandths of an inch) of free play. For comparison, .060” is about the thickness of a penny, and while this amount of play rarely would be represented as unsafe, it could easily be misdiagnosed as a worn joint.
This may turn out to be more important in the case of a warranty concern. I’m sure it wouldn’t be the first time you had a part returned as defective and you were surprised by it. This is when your knowledge can save time and money for your company as well as for a technician, shop and the end customer. Information like this often doesn’t make it to a technician level, and it’s a great opportunity for you to educate and build rapport with your customers at the same time.
Replacing Control Arms
Where do control arms come into the picture? Independent suspension, be it front or rear, has been around for a long time. There are many different types, of which upper-lower A-arm, MacPherson strut and multi-link are the most common variations we deal with today. One thing they all have in common is some type of control arm.
To put a simple spin on it, any control arm is nothing more than a link between the fixed frame of a vehicle and the steering knuckle – the component that in turn provides a mounting point for the brakes, wheel bearings and wheels. Control arms move freely up and down in response to suspension movement and not only offer mounting points for springs and sway bars, but they also are integral to suspension design, affecting the alignment angles and suspension travel.
The control arm also carries another distinction: It offers a provision for mounting a ball joint to provide articulation between the arm and steering knuckle. Ball joints are either bolt-on or press-in, and in many cases on newer vehicles, the ball joint is an integral part of the control arm. If you have to replace the ball joint, you have to replace the entire arm.
Control arms are either steel, cast-iron or aluminum, and the most important factor when replacing a press-in style of ball joint is making sure the hole in the control arm isn’t worn. Generally, if there’s no visible damage or corrosion to the control arm and the old ball joint requires considerable force to remove, as long as the new joint requires a similar force to install, the control arm will be OK.
However, since we don’t have the technology (in our shops) to measure the roundness and integrity of the hole, many manufacturers recommend to only replace a press-in style ball joint one time. The next time, the whole arm should be replaced, and this is good advice to pass along to your customer.
As with any type of suspension work, any torque-to-yield fasteners should be replaced, torque specifications always should be utilized, and in the case of control-arm replacement, fasteners should be torqued with the vehicle at ride height.
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