The Cure To What Shakes You
Have you recently had your 4x4 lifted and are suffering from a harmonic oscillation that can both be felt and heard? Have you rebalanced your tires or swapped in newly balanced driveshafts only to discover the vibration still persists? If so, it is probably time to take a look at your pinion and driveline angles, or the relationship of your driveshaft to your transfer case and differentials. We’ve all been there and we can tell you that one of the biggest frustrations of 4x4 ownership is to have a persistent vibration you can’t chase down on a setup that cost you a wad of hard-earned cash.
The key to eliminating driveline vibration is good geometry. You know, that subject in math class you swore to your parents you’d never use in real life. We won’t tell them our little secret, but a little lesson in geometry could extend the life of your driveshafts, increase the comfort of your 4x4, and make you fall in love with your rig all over again.
So how does one go about measuring driveline angles? While it is true that you can go to the local hardware store and pick up an angle finder or use a modified protractor, there is an even easier way with a tool you probably already own—a smartphone. With sophisticated smartphones, all it takes to figure out your driveline angles is a free app you can download. Once you figure out what your operating angles are, you’ll have a better idea of how much your pinion needs to be adjusted to minimize vibration.
With your tool of choice in hand, it is time to start measuring. You’ll need to know the slope of your transfer case output, driveshaft, and pinion as they relate to level ground. Once you know the slope, you can determine the net operating angle at the joint (either the transfer case side or pinion side of the driveshaft). If you are running a conventional two-U-joint-style rear driveshaft, the pinion slope should be about the same as the transfer case slope, as the rotations of the joints at opposite ends of the driveshaft will cancel each other out, delivering smooth power. If these angles are off just a few degrees, the U-joints will not be in phase and you are setting yourself up for vibration.
On rear axles with a double cardan-style ’shaft, the pinion and driveshaft should ideally be at a 0-degree operating angle at ride height. It is important to allow the double cardan joint at the transfer case to take up the difference in slope between T-case output and the driveshaft. Now with that said, there are some exceptions in the real world. On a vehicle with soft springs or springs that are susceptible to axlewrap (such as on a spring-over), it is acceptable to keep the pinion down a degree or two to make sure the angles are correct on acceleration. Again, every vehicle is different and this should be discussed with your driveline builder.
Adjusting the rear pinion angle on a leaf-spring-equipped vehicle is as easy as adding some pinion-correction wedges under your leaf pack. Six degrees is about the most angle you want to run with a wedge. Beyond that, the spring perches should be cut off and re-welded to the correct angle. Coil-sprung vehicles can be more difficult to adjust because the control-arm mounts will sometimes have to be removed and rewelded, unless you are running eccentric bolts or have adjustable arms. It all depends on the vehicle and amount of angle needed.
Another thing to do if your axle has been clocked for steep pinion angles is to check for proper fluid levels in the diff housing, as you may have to overfill the housing to ensure that the pinion bearing is not starved for oil. However, be sure the added oil doesn’t lead to less-effective lubrication from foaming of the gear oil.
If your rear checks out, it is time to focus on the front. Front axles with poor pinion angles are often the cause of vibration. To understand pinion angle in the front, one also has to understand caster. Caster is the angle of your steering pivot (imagine a line drawn through your upper and lower ball joints), measured in degrees, and how it relates to the vertical centerline of the axle when viewed from the side. If the top of the pivot is leaning toward the rear of the car, you have positive caster; if it is leaning toward the front, you have negative caster. Too much caster will lower the pinion, increasing the operating angle and bringing about driveline vibration, while too little caster will give you a great pinion operating angle but will cause the vehicle to be squirrely or flighty at highway speeds.
The easiest way to adjust caster on the front of a leaf-sprung vehicle is to use pinion-correction wedges, similar to what is offered for rear axles. If you are adding longer shackles on the front, be sure to adjust for the difference in caster they cause. On coil-sprung vehicles, options include eccentric bolts and adjustable control arms.
Setting front caster/pinion angle is a delicate balance, but one that can be achieved. It is important to remember that setups running oversized tires won’t require as much caster as a stock wheel and tire package, and sometimes you can back off of the factory specs and still be fine on caster, while improving your pinion angle and possibly eliminating front driveline vibration.
So what if you have done all of the above and your vehicle requires more caster, but less pinion angle? If you fall in to this category, there are a few options out there. One of these options is to have your steering knuckles cut off and reclocked, allowing the relationship between your rig’s steering and pinion to be altered permanently. If that seems too radical, you can always go with a new aftermarket axle, such as a Dynatrac ProRock, which can be built to have additional caster. Lastly, you might consider a multi-double cardan driveshaft, such as those offered by Tom Wood’s Custom Drive Shafts. With a double cardan on each end, the shaft can better absorb a greater difference in operating angles, leading to a heavy, complex, but smooth-running ’shaft.
While none of these options are what we consider inexpensive, they could be your last hope in eliminating the bad vibes that plague your chassis. With proper driveline geometry, there is no reason a lifted rig can’t run as smooth as a stock one.