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In this blog, we would like to talk about the difference between the regular, ball-type CV-joints used in most passenger cars and the tripod joints that we use in our driveshafts. First of all, there is a general misconception about the term CV-joint, as the tripod joint is in fact a CV-joint as well. The CV abbreviation stands for constant velocity.
The function of a constant velocity joint is to transmit power, while the angle between the input and output can vary. The constant velocity part means that the rotational speed of the assembly does not change regardless of the angle of the joint. This is the main difference when compared to the universal joints used before the CV joint was invented.
The universal joint was compact, but the output did not have the same constant velocity as the input. This difference in velocity gets larger when the angle gets greater. The joints generally allow some axial movement, as the total length of the driveshaft will change when the suspension goes up and down in its motion.
Figure 1: A traditional universal joint
The most commonly known CV joint is the Rzeppa joint. Most streetcars use this joint on both ends or one end of the driveshafts. These joints consist of four parts: the outer race, (usually) six balls with a cage around them and the inner race. The outer race is usually integrated in the output shaft or flange, while the inner race usually has a spline on the inside and can be mounted to an axle or a shaft. Both the inner and outer race have grooves that have the same diameter of the balls, while allowing the balls to move axially in relation to the shaft. These grooves are not straight, but curved so the joint can operate at different angles. The balls are contained in the groove by the cage.
Figure 2: Parts in a Rzeppa joint
Figure 3: Rzeppa joint assembly
Figure 4: Parts in a tripod joint
The tripod joint does not work with six balls to transmit the rotation from the input to the output. Instead, it uses three rollers on a spider. This spider replaces the inner race. The rollers have needle bearings inside and therefore rotate very freely.
Figure 5: A Tripod Joint Assembly
There are a few benefits to each style of CV joint. The main benefit of the Rzeppa joint is the larger articulation angle they provide. For this reason, most cars use this type of joint on the wheel side of the driveshaft. This is especially important in front wheel drive cars, where the CV joints copes with both the steering and suspension movement. The tripod style joint allows for less angle, but more axial movement. Quite often, a tripod style joint or a plunging Rzeppa style joint is used on the inboard side of a car. This means the driveshaft as a system has a big allowance for both changes in angle and length. In case two Rzeppa joints or two tripod joints are used, one end is usually axially fixed while the other end allows for axial movement. This can differ between different designs.
Street and rally cars
In street cars, the tripod joint is mostly used because they have plunging movement, are easier to make than a Rzeppa joint and most of the time have sufficient range of angle on the inboard side. The required angle determines whether a tripod joint can be used. This is also dependant on the suspension geometry and the suspension travel. A car used in rally racing could for instance have to much suspension travel, making the angle on the joint too big. Consequently the amount of camber in race cars also plays a big role in the required angle on the joint. These angles and the geometry can be measured using one of our measurement shafts.
For race cars, there are other benefits to the tripod joint. The tripod joint is more compact than the Rzeppa joint. The result is that a tripod joint of the same size as a Rzeppa joint is stronger or a tripod joint with the same strength as the Rzeppa joint is smaller and lighter. Sometimes, when space is at a premium, switching to a tripod joint means a stronger driveshaft can be made without using more space.
As the suspension of the car is constantly moving, the joint is constantly moving as well. The tripod joint creates less friction during movement, as the rollers have needle bearings inside them. This also means they create less heat. If a car is raced, this could make the difference between the grease inside the joint being functional or not. This helps a lot with the longevity of the joint.
Another benefit of the tripod style joint is serviceability. The joint can be completely taken apart and serviced, whereas the Rzeppa joint is usually a replacement item. This also means that the tripod joint will last longer and that it’s condition of can be examined in between races, which reduces the chance to drop out of a race due to driveshaft failure.
Regardless of what type of joint is chosen, they all have to be sealed. This prevents moisture or debris coming in from the outside and it keeps the grease inside the joint. This is usually done with a rubber or silicone boot.
In conclusion, both types of CV joints mentioned here have their own benefits. As race cars are mostly rear wheel drive and have limited suspension travel, the tripod style joint can be used. This provides many benefits over the usual Rzeppa joint, most importantly that they are stronger and easily serviceable.
Pro’s and con’s of a Tripod joint?
Pro’s and con’s of a Rzeppa joint?
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