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Setting Pinion Angle Correctly
Pinion angle is one of the more important measurements on a lifted vehicle. The wrong angle can lead to vibration and premature failure of U-joints, drive shafts, pinion bearings and even transfer case output bearings.
Types of drive shafts
There are two common types of drive shafts used in 4wd vehicles, standard with one U-joint at each end and CV drive shafts. CV stands for Constant Velocity. CV drive shafts are so named because they have a constant velocity joint at one end, the other end has a single U-joint.
There are two types of constant velocity joints, the double cardan and the caged ball type. The double cardan type is the CV joint with the two U-joints at one end that is so popular with 4 wheelers. The caged ball type is similar to a CV joint in a front wheel drive car or a Birfield from a Samuri or Toyota front axle. The caged ball type CV joint is notoriously weak and is it fortunate that these drive shafts can be found in only a few four wheel vehicles including some Bronco II's and a few early XJ Cherokees. Most who find caged ball type CV drive shafts on their vehicles quickly upgrade to a standard double cardan type CV drive shaft.
What happens when the pinion angle is off?
As mentioned above, improperly set pinion angle causes vibration which can lead to early parts failure. The vibration results from unequal velocities at the opposite ends of the drive shaft. When a U-joint is run at an angle the drive shaft will actually speed up and slow down twice during each revolution, due to the changing angular velocity of the U-joint ends.
Take a look at figures at right to help understand why the drive shaft speeds up and slows down. The U-joint caps connected to the pinion yoke spin around the pinion without moving forwards or backwards, when viewed from the side. The U-joint caps on the drive shaft move forwards and back quite a bit when viewed from the side, causing a change in the speed that the drive shaft rotates.
There will be no vibration if the angles are the same at both ends of the drive shaft because both ends will speed up and slow down at the same time and in the same amounts. If the angles are off the U-joints will still cause the speed to change at the same time (unless the drive shaft is twisted) but in different amounts, causing vibration. Keeping the pinion parallel with the output of the transfer case will keep the angles the same, avoiding vibrations.
What about CV drive shafts?
As the name implies, a CV joint keeps the velocity of the drive shaft constant throughout a revolution. In a double cardan type constant velocity joint each joint in the CV assembly will speed up and slow down, just like on a regular drive shaft. The double cardan assembly has a centering mechanism that keeps the angle between the U-joints correct, allowing them to completely cancel out the speeding up and slowing down and spin the drive shaft at a constant speed.
In a perfect world the pinion could be pointed parallel with the drive shaft when using a CV joint so that all four caps on the lower U-joint will spin in the same circle to prevent the single lower U-joint from speeding up and slowing down as it rotates, which would cause a vibration. Unfortunately, our world isn't perfect and the needle bearings inside a U-joint need to move to stay lubricated. The pinion angle must be 2 degrees below the drive shaft in to cause the needles to move and be lubricated.
Adjusting the Angle
The correct way to adjust pinion angle will vary from vehicle to vehicle. Leaf spring equipped vehicles can use wedge shaped shims for quick adjustments, but a more permanent fix is to remove the spring perches and weld them on again at the correct angle. The adjustment is normally even easier on coil sprung, solid axle equipped vehicles. For these most of the time the control arms just need to be adjusted.
If you have any questions about correcting your pinion angle please post them to the 4x4Wire TrailTalk Forum.