If you seat a group of gearheads at a table, the topic of basic driveshaft angles can usually get responses ranging from “it’s no big deal” to practices of surgical precision. However, paying attention to vertical and side-by-side alignment — and even driveshaft phasing — can prevent calamity with every bit of additional horsepower you push to the ground. It is not about just the driveshaft, but also the alignment of your entire drivetrain.
Chasing a vibration most commonly points to a driveshaft problem. Many call it an “out of balance” driveshaft, but the culprit can have many sources. In a more performance and racing-oriented application, such problems as chatter and wheel hop can result. In extreme horsepower and torque situations, a pulsing transfer of power is a source for radical suspension and tire shake.
Transmission And Pinion Angle Alignment
The U-joint angles at the front and rear of the driveshaft are the most common sources for alignment problems. When a hot rod or race car undergoes any suspension modifications, too much misalignment between these points can occur.
Our starting point is to get the transmission and rearend yokes matched as closely in height with each other as possible from the floor surface. With our heights close, we begin adjusting the rear pinion angle to match the transmission. As a general rule, the angles at each U-joint should not exceed 5-degrees. An even tighter tolerance is highly recommended as your horsepower increases.
Measure, Adjust, Measure Again
Though we have elevated our project car evenly off our shop floor, it is most critical to begin measuring with your rear suspension sitting as if it is ready for the road or track.
Your front suspension positioning is not as critical, because you are beginning the measuring process from the engine/transmission centerline, whatever that is. You simply measure the current angle as it sits, and deduct that number from your following yoke and U-joint measurements.
For example, our engine/transmission is at a 1.85-degree upward angle. Our pinion is at a 5-degree downward angle as a starting measure. That means our current driveshaft angle is heavily misaligned at 6.85-degrees. With that, we adjust our pinion angle upward to match the 1.85-degree transmission angle while continuing to adjust, measure, adjust, and measure. We continue to measure the angle of each U-joint face. Note that if your transmission and pinion angles are genuinely parallel from step one, each U-joint angle will be identical.
Changing the rear pinion-angle adjustment requires the most effort. A leaf spring or truck-arm-style suspension can use axle shims or wedges at the mounting perch to alter the pinion angle. With many three-link, four-link, or ladder bar suspensions, a threaded adjustment coupler is incorporated into the suspension components.
Our project Camaro’s case is a prime example of needing adjustments; with large diameter slicks and ladder bar suspension, our geometry is drastically different from stock. Additionally, with factory-style engine mounts still in place, we had no idea if the previous owners had our suspension angles appropriately adjusted, and it was not correct.
In our case, the pinion angle is moved by an adjusting nut on the ladder bars. It is also an exercise in detail to occasionally reaffirm your entire rearend remains square in the chassis during these continued adjustments. By adjusting and re-adjusting the suspension height and re-establishing our proper pinion angle, we were able to get our U-joint alignment down to within less than 2-degrees for our application.
Many rodders and racers with factory or modified factory chassis assume that the left-to-right (offset alignment) of the driveshaft angle is already correct. From 1950’s-era cars to today, various manufacturers offset the entire engine differently, based on the engine size and torque characteristics, even within the same model car. As an example, the Camaro has different engine offsets (towards the passenger side) as much as 1 1/2-inches, depending on whether a six-cylinder, small block, or big block was under the hood.
These offsets were engineered into a bone-stock drivetrain to counteract any torque-based drivability for a novice driver. The factory differential is factory offset by 1/2-inch to the right of center for the same reasons.
These angles may not be detrimental to an all-factory car, but when you start adding suspension modifications along with performance engines and transmissions, these angle attributes could be as critical as any other misalignment.
Our Camaro has spent most of its life as a Super/Stocker and has been equipped with a plethora of small block, big block, and varied manual and automatic transmission combinations over the years. Considering that history, we thought it was necessary to be certain that our side-to-side alignment is proper.
Most race car, pro street, and autocross chassis builders will fabricate a chassis with a drivetrain centered within a vehicle. This allows for overall balancing on the four corners by using wheel scales. Any chassis adjustments then take place with shock and suspension-link tuning. With a wealth of plumb bob markings on our shop floor, we matched OEM chassis holes on our front crossmember to establish a dead centerline throughout the car’s length. We now know our engine, transmission, and differential are currently centered within the chassis.
Driveshaft phasing is the process of “clocking” the driveshaft as it relates to the front and rear yokes. The common automotive driveshaft is built with each end in exact relation to each other. This phasing has each U-joint mount at each shaft end in the same phased position.
Though precise phasing is the duty of a good driveshaft shop, many driveshaft kits rely on the enthusiast to get this adjustment correct. When you look down your driveshaft from the end (such as looking at a pool cue), if the front and rear yoke are even slightly out of phase by a couple of degrees, vibration at speed is almost inevitable.
Master The Art Of Alignment
When you understand how a misaligned driveshaft can physically speed up and slow down as it transfers power from the front to the rear, you will begin to understand its source as a vibration point or worse, as a point of failure.
We have all heard the story of the incurable vibration in someone’s hot rod. Scrutinizing the various U-joint angles and overall alignment from the engine to the rear yoke may be a time-consuming effort, but it’s one that is well worth the effort when it comes to a vibration-free outcome.