VOLUME XIII,  ISSUE  - JAN - FEB,  2018

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Tech Stuff

The Accurate Measurement of Your Driveshaft

 

Working the Angles

 

Text and Photos by Chris Holley

Photo 1:  To provide a smooth, efficient, and vibration-free ride, there are a few procedures that must be performed to ensure proper operation of the driveline components. The working angles between the driveshaft’s weld-yokes and the slip-yoke (transmission) and pinion-yoke (differential) are critical and can be fairly easily adjusted as necessary.

 

Can a chassis vibration be caused by something as simple as an incorrect driveshaft angle? Many vehicle owners are of the belief that the factory simply suspended a driveshaft between the output shaft of the transmission and the pinion gear of the differential to pass the torque from the front of the vehicle to the rear. When a vehicle was purchased, the owner drove the vehicle, and there was an expectation that the ride would be vibration free. This occurred because the factory engineers designed the drivetrain to get the transmission, differential, and driveshaft working in specific planes that allowed the smoothest operation and minimized component wear.

 

The factory did all the work to provide a vibration-free vehicle, but we have all witnessed the modifications owners can perform, and those alterations may result in vibrations. What kinds of owner modifications can lead to these unforeseen vibration concerns? The addition of lengthened shackles, Super Stock springs, aftermarket mono leaf or split-mono leaf springs, lowering blocks, moving the rear end forward or moving the engine lower or higher in the chassis, and moving the engine forward or rearward in the engine bay will all change the driveshaft’s working angles. Major surgery to the rear of the car for the addition of a ladder-bar or 4-link suspension will certainly require readjustments to the driveshaft angle.

An inclinometer is the tool necessary to measure the working angles of the driveshaft, pinion-yoke, and the slip-yoke. The inclinometer consists of two plates attached at the bottom by a fastener. The two plates can move independently of each other. One plate has a magnet at the end opposite of the fastener. The same plate has a graduated scale on the face of the plate. The second plate has a spirit level on it and an indicator pointer that lines up with the scale on the first plate. To make the adjustments, wedge-shaped steel shims are used to adjust the pinion up or down by shimming at the rear axle housing mounting pads.

 

While the techniques and tools discussed in this article can be used to correct the driveline angles on greatly modified vehicles, we will focus on leaf-spring suspended rear differentials found in all the 60s and 70s Mopar vehicles. To verify our driveshaft angles, we will measure three different Dodges with similar drivetrain setups. All three vehicles are Mopar A-bodies, which we will adjust as necessary and use to provide information and clarity about driveline angles.

 

The Cars

The testing will be done on three different Dodge Darts. The first Dart is a 1967 with a 225 slant six, 904 Torqueflite, and a 7 ¼” rear end. The rear springs are the 50-year-old originals. To make the measurements required, the Dart was lifted onto a car lift, and then a pair of jack stands were placed under the rear axle. The Dart was lowered onto the jack stands, so all the rear vehicle weight was on the rear axle and the suspension. It is important to jounce the rear of the car to guarantee the rear suspension has settled into its normal trim height.

The second test vehicle is a 1975 Dodge Dart with a 225 slant six, 904 Torqueflite, and a 7 ¼” rear end. The rear springs were recently replaced by a pair of 002/003 Super Stock rear springs, and the owner did not measure the driveline for any changes to the angles. If you do not have a lift, a pair of jack stands under the rear axle will do the job. The work area to make all the checks and measurements is greatly decreased, but the rear of the Dart is up enough to work.

The third test vehicle is a 1969 Dodge Dart with a 340, 904 Torqueflite, and an 8 ¾” rear end. The axle rides on a pair of split-mono leaf rear springs and Caltrac bars. There are extended rear shocks and an aluminum driveshaft on this Dart. If additional clearance is needed to measure the driveline angles and there are enough jack stands available, the entire Dart can be placed on jack stands at the unibody frame. The rear axle can be jacked up and placed on another pair of jack stands. The weight will now be distributed over the front two jack stands and the pair under the rear axle. The rear jack stands under the unibody do not support any weight, but they are there for additional safety.

 

The first A-body to test is a 1967 Dodge Dart GT equipped with a 225 slant six, a factory 2.94:1 geared 7¼” axle on a pair of factory springs, a pair of cheapo factory-replacement hydraulic shocks, and a factory driveshaft. The second A-body is 1975 Dodge Dart Swinger, again, with a slant six, a factory 2.73:1 rear geared 7¼” axle seated on a pair of 002/003 Super Stock springs with aftermarket extra length shocks, and coupled to the factory driveshaft. The Super Stock springs have been recently installed and no checks were made to ensure the proper driveline angles for a vibration free ride. The last A-body to measure is an 11-second 1969 Dodge Dart 340 with an 8¾” 4.10:1 geared differential on a pair of split-mono leaf springs and Caltrac bars with extra length shocks and an aftermarket aluminum driveshaft. This Dart’s drivetrain has been massaged to the point of near perfection, so, hopefully, this Dart will be just a review of all the driveline measuring procedures.

 

Overall Driveline Component Condition

 

Before we get into the driveshaft angles, a few checks must be performed to ensure proper operation of the drivetrain components. If a vibration exists, first, ensure the proper balance of the wheel and tire assembly. Second, have the driveshaft checked for minimum runout and if a concern exists about the driveshaft, have the driveshaft check by a professional and have it high speed balanced at the same time. The driveshaft can be checked with a dial indicator to measure the radial runout. If the runout exceeds a few thousandths of an inch, the driveshaft will need to be replaced or repaired. Third, check for looseness of drivetrain components. Any looseness due to wear-and-tear or physical damaged needs to be addressed before any further testing can be performed. Fourth, measure the pinion-yoke runout. If runout is discovered, the pinion-yoke can be removed and re-indexed on the pinion’s splines at which point it can be rechecked for runout. If the runout remains, continue rotating the pinion-yoke one spline on the pinion at a time until the runout is reduced to a few thousandths of an inch. If the runout cannot be removed, a new pinion and pinion-yoke may be necessary. All four of these procedures will be further explained in the photos and captions. Once all four of the above tests are successfully completed, the evaluation of the driveshaft angles can begin.

 

Before we can start our angle measurements, the driveshaft runout is measured at several points on the driveshaft. If the runout is more than a few thousandths of an inch at any point, check for play in the U-joints. If the U-joints are in acceptable condition, the driveshaft needs to professionally repaired or replaced. If there is any doubt about the balance of the driveshaft, it should be high-speed balanced and again the runout should be checked.

The pinion-yoke runout is checked by measuring the pinion at the flat pad where the U-joint bearing cap attaches. Measure one position, then rotate the yoke 180°, and measure again. If there are more than a few thousandths of an inch runout, remove the pinion nut, and re-index the pinion-yoke to the next spline and re-torque the pinion nut. Measure again, and repeat the indexing until the runout is minimal. If the runout cannot be removed, the pinion-yoke or pinion shaft will need to be replaced. When doing this nut removal procedure, torque the pinion nut properly. Do not overtighten as it could lead to an over compressed crush sleeve if equipped in your vehicle’s rear end. All three of the Darts passed the two runout tests, and a quick overview of the driveline components showed no loose or damaged parts.

 

Explaining Driveshaft Angles

 

Additional, vibrations caused by the drivetrain can be narrowed down to improper yoke alignment (twist) of the driveshaft or incorrect working angles of the driveshaft. To measure improper yoke alignment or working angles of the driveshaft, an inclinometer is used. An inclinometer is comprised of two separate plates. One plate has a magnet attached on one end (top) that is used to affix the inclinometer to the U-joint bearing cap. Also on the plate face is a scale measured in degrees ranging from 0° to 30°. The second plate has a spirit level on one end of the plate and an indicator point that lines up with the first plate’s degree scale. At the bottom of both plates is a bolt that attaches the two together, but it still allows the plates to move separately of each other. To use the inclinometer, the magnet is attached to the bearing cap being measured, and the second plate with the spirit level is adjusted until the spirit bubble is level. The pointer on the second plate positions an indicator line on the scale of the first plate. The number is the angle in degrees of the component attached to the bearing cap. Some technicians will attach a magnet to an angle finder or a protractor to acquire the driveline angles, but with 25+ years of experience working with drivelines, I have found the inclinometer to be the most efficient tool to measure U-joint type drivelines.

The inclinometer is set to 15° and attached to the weld-yoke U-joint at the transmission end of the driveshaft. The inclinometer scale was facing the rear of the vehicle. The driveshaft is rotated until the bubble on the level is centered. This provides the true 6:00 o’clock position of the U-joint. Refer to illustrations 1 and 2 for additional information about the components and their orientation in the driveline.

Without moving the driveshaft or disturbing the inclinometer indicator, move the inclinometer to the rear weld-yoke U-joint at the differential end of the driveshaft. In a perfect world the bubble in the level would center without any manipulation of the inclinometer. If the bubble does not center, the second plate (the one with the level) is moved until the bubble centers. The indicator points to the angle of the U-joint. This reading should be 15° +/- 0.5°. In this case, the reading is just a touch over 15° but less than 15.5°. If the reading is above 0.5°, the driveshaft is twisted and professional intervention will be necessary to repair the driveshaft.

The inclinometer is turned 90°, so it faces the driver’s (or passenger’s) side of the vehicle. The driveshaft remains in the same position it was in when the driveshaft twist measurements were made. The inclinometer’s spirit is centered on the level. Once the bubble settled down, the measurement was 14°. This is the driveshaft angle at the rear differential.

The inclinometer is moved to the front weld-yoke U-joint bearing cap. It is installed in the same direction as on the rear, which in this case is towards the driver’s side. The bubble centered, and the indicator provided a reading of 13.5°. If difference between the front and rear U-joints is greater than 0.5°, this may indicate a bent or damaged pinion-yoke. This, again, requires a repair procedure to correct. With 14° at the rear and 13.5° at the front, the driveshaft is in acceptable working condition.

 

With an idea about how the tool operates, we can now cover the procedures required to measure the working angles. First, the vehicle must be in neutral, and the rear end must be supported on jack stands. The angle of the vehicle is not a concern because the measurements are taken between the transmission slip-yoke, driveshaft weld-yokes, and the differential pinion-yoke, and they are independent of the angle of the vehicle. With the weight of the vehicle on the rear suspension, the vehicle must be jounced to ensure the rear springs are settled. The driveshaft is now rotated until a front U-joint bearing cap attached to the driveshaft weld-yoke is pointing toward the ground (6:00 o’clock). The inclinometer is set to 15° on the scale. The inclinometer’s magnet is affixed to the driveshaft U-joint bearing cap at the transmission end of the driveshaft with the scale and spirit level facing the rear of the car.

The driveshaft is rotated 90°, so a slip-yoke U-joint is pointing toward the floor. The inclinometer is set to 15° and attached to the U-joint bearing cap. The driveshaft is gently moved until the bubble is centered. This establishes the 6:00 o’clock position of the U-joint.

 

The driveshaft is gently rotated until the bubble in the spirit level centers itself. Without moving the driveshaft or bumping the 15° setting on the inclinometer, the inclinometer is moved from the front driveshaft U-joint to the rear driveshaft U-joint (again the inclinometer is facing rearward). In a perfect world, the bubble in the spirit level would center at the 15° setting. If the bubble does not line up, a gentle repositioning of the second plate (the one with the spirit level) to center the bubble is necessary. Once the bubble centers, the degree measurement is noted. The variance should be no more than 0.5° (0° is perfect) between the front and rear U-joints. If there is a measurement of 0.5° or greater, there is a problem with the driveshaft (twisted) that will require the driveshaft to be professionally repaired (or replaced).

 

Without turning the driveshaft, the inclinometer is rotated 90° on the rear U-joint, so the inclinometer can be viewed from the passenger’s (or driver’s) side of the vehicle. Just as previously performed, the spirit level is centered by gently moving the second plate until the bubble is centered. Note the indicator pointer against the scale. Move the inclinometer to the front U-joint without moving the driveshaft. With the inclinometer fixed to the front U-joint (facing the same side (passenger/driver) that was measured at the rear), adjust the bubble until it is centered, and note the degrees. The front and rear driveshaft weld-yoke U-joint degrees should be the same. These numbers represent the driveshaft angle. If the difference between the front and rear U-joint is greater than 0.5°, this may indicate a bent or damaged pinion-yoke. This, again, requires a repair procedure to correct.

The inclinometer is rotated 90°, so it faces the driver’s side of the vehicle. The bubble is centered in the level and the indicator shows a slip-yoke angle of 14.5°. The difference between the slip-yoke angle and the driveshaft weld-yoke angle (13.5°) is 1.0°. The 1.0° is the front working angle. It is greater than 0.5° and less than 3.5°, which means the working angle is acceptable.

The inclinometer is moved to the pinion-yoke bearing cap, and the angle is measured. The measurement is 13.5°. This provides a rear working angle of 0.5° (13.5° pinion-yoke subtracted from 14.0° rear weld-yoke). The 0.5° working angle is with specifications. The front-to-rear working angle is 0.5° (0.5° rear working angle subtracted from 1.0° front working angle). The front-to-rear working angle should be 0° +/- 0.5°.  These measurements were performed on the 1967 Dart, and this driveline is within specifications.

 

With the checks of the driveshaft for twist and weld-yoke damage completed, the driveshaft is rotated 90°, so a bearing cap in the transmission slip-yoke is pointing toward the floor (6:00 o’clock). Place the inclinometer magnet on the bearing cap with the degree scale facing the rear of the car and the spirit level indicator set to 15°. Once the driveshaft is rotated to bring the spirit level bubble to the center, turn the inclinometer 90°, so the degree scale can be seen from the passenger’s side or driver’s side (which ever was used in the previous steps) of the car (parallel with driveshaft). Adjust the spirit level until the bubble is centered, and note the indicator’s position on the scale. This is the transmission’s slip-yoke angle. Move the inclinometer to the differential pinion-yoke bearing cap. Again, have the inclinometer at a parallel position with the driveshaft. Adjust the spirit level until the bubble is centered. This measurement is the differential’s installed angle. To determine the front U-joint working angle, subtract the driveshaft angle from the transmission’s slip-yoke angle. Note this number.

 

To find the working angle of the rear U-joint, subtract the driveshaft angle from the differential pinion-yoke bearing cap. Note this number. The working angles of the front or rear U-joints should not be less than 0.5° and it should not exceed 3.5°. As the vehicle’s performance is increased, the working angles should be minimized. The working angles should be less than 2.0° but still no less than 0.5°. Once the front and rear working angles are calculated, it will be noted that they are nearly equal but opposite from each other. By subtracting one working angle from the other, the overall driveshaft working angle can be determined. The difference of angle between the front and rear U-joint working angles should not exceed 0.5° (0° is perfect). If not, the differential may need to be rotated (dropping or raising the differential pinion height) or the transmission may need to raised or lowered to achieve proper working angles.

 

Taking the Knowledge to the Cars

This illustration shows how the driveshaft is set up to measure the driveshaft angles with the inclinometer. The illustration also points out key names of components in the drivetrain.

This illustration shows how the slip-yoke (transmission) angle and the pinion-yoke (differential) angle are measured with an inclinometer.

 

Now let’s take this information and apply it to our Dodges. The first Dodge was the 1967 Dart. The factory 14” wheels and tires were balanced. The driveshaft had a factory balance, and it had minimum runout. The pinion-yoke runout was less than a few thousands of an inch, and while the drivetrain components were old, they were tight. With all the pre-tests passing specifications, we proceeded to the driveshaft angles. The rear leaf springs are the originals installed in December 1966, and although the chassis has only 23,000 miles on it, the springs are a bit soft allowing a little sag in the rear. Even with the worn springs, the Dart has rolled over 1500 miles the chassis dyno without any noticeable drivetrain vibrations. With the Dart’s rear end supported and the transmission in neutral, the driveshaft was rotated until a front driveshaft-bearing cap was in the 6 o’clock position.

 

With the inclinometer set to 15° on the scale, the inclinometer was attached to the bearing cap with the scale facing the rear of the Dart. The driveshaft was lightly rotated until the spirit bubble centered. The inclinometer was transferred to the rear driveshaft bearing cap and the bubble did not center perfectly, so a slight touch of the spirit’s plate centered the bubble. The result was just under 15.5°, which was within the 0.5° tolerance for driveshaft twist. The next measurement was to turn the inclinometer 90° on the rear driveshaft bearing cap and center the bubble in the spirit level. The measurement was 14°. A quick transfer of the inclinometer to the front bearing cap and a centering of the bubble provided a reading of 13.5°. Again, this was within the tolerance of 0.5° or less.

 

Now the driveshaft was rotated 90°, so the transmission yoke bearing cap was in the 6 o’clock position. The inclinometer was set to 15° and affixed to the bearing cap with the scale facing the rear of the Dart. The driveshaft was manipulated until the bubble centered in the spirit, and the inclinometer was rotated 90° in the same direction as it was turned in the previous tests. The spirit level bubble was moved until the bubble centered. The angle was noted, and the inclinometer was transferred to the rear pinion-yoke bearing cap. The bubble was centered again and the angle noted. The angle for the transmission slip-yoke bearing cap was 14.5° and the pinion-yoke bearing cap was 13.5°. With all the measurements done, the smaller number is subtracted from the larger number at each U-joint even though the numbers are actually opposite of each other (one U-joint is positive and the other should be equal but opposite). This works out like the following:  the working angle for the front U-joint was 13.5° (driveshaft weld-yoke angle) subtracted from 14.5° (transmission slip-yoke bearing cap) for a result of 1.0°. The rear working angle was 13.5° (pinion-yoke bearing cap) subtracted from 14.0° (driveshaft weld-yoke angle) for a result of 0.5°. In both cases, the U-joints working angles were greater than 0.5° and less than 3.5° (1.0° front and 0.5° rear), so another test was passed. Lastly, the rear working angle of 0.5° subtracted from the front working angle of 1.0° provided a result of 0.5°, which was within the driveline specifications. Chrysler nailed it 50 years ago on this car, and even with the age and slight sagging of the suspension the results still fall within specifications.

The 1975 Dart with the new Super Stock leaf springs is way out of the tolerance window. The pinion-yoke angle is 23° (top left photo), which provides a rear working angle of 5.5° and a front-to-rear working angle of 5.0°. To correct the problem, a 6° shim (one per side) with the wider end facing the rear of the Dart needs to be installed between the spring and the spring pad on the axle housing (top right and bottom left). The pinion-yoke angle dropped to 16.5° with the addition of the shims. The rear working angle dropped to 1.0° and the front-to-rear working angle is now an acceptable 0.5°.

 

The second Dodge to check was a 1975 Dart. The factory steel 14” wheels and tires were properly balanced. The driveshaft had been treated to a high-speed balance and had a runout of a few thousandths of an inch. The pinion-yoke runout measured zero with a dial indicator, and the drivetrain components were tight. This Dart passed all the pre-tests. The Dart’s owner had recently installed Super Stock springs replacing the factory springs and did not measure for any driveshaft angle changes. There were no complaints about a vibration, but as explained earlier, any changes to the suspension can cause driveline pulsations even if they are not noticed by the owner. With the inclinometer, we checked the driveshaft for twist, and there was none. The driveshaft weld-yoke angle at the rear U-joint was 17.5°, and the driveshaft weld-yoke angle at the front U-joint was 17.5°. That was perfect. The measurement of the slip-yoke was 17.0° for a front working angle of 0.5°. The measurement of the pinion-yoke was a jaw-dropping 23.0°, and the rear working angle was 5.5°. The front-to-rear working angle was 5.0°, both of these angles are way out of the allowable specifications.

 

The transmission angle seemed realistic, but the differential angle was way off, so it was time to shim the rear differential to adjust the rear working angle. With a driveshaft angle of 17.5°, it was determined that a 5.0° wedged shaped shim would get our rear working angle and overall working angle into compliance. The only problem was there was not a 5.0° shim available. The choice was a 4.0° or a 6.0° shim. The 6.0° shim was selected, which should have placed the pinion-yoke angle somewhere around 17°, but the shims are never perfect, and it becomes a trial and error process to establish the proper shim. The axle housing U-bolts were all loosened so the shims (one per side) could be slipped into place. The shims have a 9/16” hole to allow the leaf spring alignment pin to center the shim under the axle pad. With both shims installed (narrow end of wedge facing forward), the U-bolts were reinstalled and torqued to specifications. The driveshaft was measured again, and now the pinion-yoke angle was 16.5°. The rear working angle was 1.0° (16.5° pinion-yoke angle subtracted from 17.5° driveshaft weld-yoke angle). The front-to-rear working angle was now 0.5°, which was within tolerances. The Dart now drives much better. The owner did not notice that there were minor vibration(s) until they were removed with the proper adjustments of the driveshaft working angles.

The 1969 Dart had been highly refined before our measurements began. The driveshaft had zero runout, there is zero pinion yoke runout, and front and rear working angles are 0.5° with a front-to-rear working angle of 0°.

 

The last vehicle to evaluate was a 1969 Dodge Dart. The Dart runs in the low 11-second range in the quarter-mile. It is powered by a 340, and it uses split-mono leaf springs and Caltrac bars. The driveshaft is aluminum, and it was manufactured by Denny’s driveshaft. The measurements revealed a driveshaft runout of zero. The driveline components were all tight, and the pinion-yoke runout was zero. The drivetrain in this Dart was very well sorted out, and there had been a 1.0° shim (narrow end of shim pointed to the front of vehicle) placed on top of each leaf spring and underneath each spring pad on the axle housing. A quick check with the inclinometer ensured that the driveshaft was not twisted. The driveshaft angle was 16.5° at the front weld-yoke, and the angle was 16.5° at the rear weld-yoke, so there was no damage resulting in twisting or bending. The slip-yoke angle was 16° for a front working angle of 0.5°. The pinion-yoke angle was also 16°, which provided a rear working angle of 0.5°. The front-to-rear working angle was 0.0° (0.5° front working angle subtracted from 0.5° rear working angle). This was as close to perfect as possible as the working angles at each end of the driveshaft are minimal, and the overall working angle (front-to-rear) is zero degrees. There is a minimal amount of parasitic loss with these working angles.

 

Wrapping It Up

The 1969 Dart had a single 1° shim on both leaf spring axle pads to bring the driveline into nearly perfect compliance.

The 1967 Dart required no adjustments to the driveshaft working angles. The rear working angle of 0.5° and the front working angle of 1.0° were within specifications. The front-to-rear working angle of 0.5° was also within specifications. It looks like Dodge nailed the driveline angles, and 50 years later, the components have maintained their integrity and their proper angles.

The owner of the 1975 Dart did not notice any serious vibrations until the vibrations were removed with proper adjustments to the driveline angles. The Dart ended up with a rear working angle of 1.0° and a front working angle of 0.5°. Both of these measurements are on the lower end (more desired) of the spectrum and within specifications. The front-to-rear working angle is 0.5°, which is in tolerance as well.

The 1969 Dart was perfect. The rear and front working angles were 0.5°, which are the lowest acceptable working angles for each end of the driveshaft. The front-to-rear working angle was zero, which again is the lowest and most desired front-to-rear working angle. The transmission and the rear differential housing are in the same identical plane.

 

After all the driveline component inspection, measuring, and if necessary adjustments, all the Darts exhibit excellent road manners completely void of any vibrations. The 1967 Dart had a factory driveline that was still within specifications even with 50-year-old parts. The 1969 Dart had been previously adjusted and it was obvious that a great deal of time and thought had gone into manipulating the driveline into a near textbook setup. When it came to the 1975 Dart, the driveshaft angles were way off, and even though the owner did not notice a serious vibration, the adjustments performed to the driveshaft angle brought the Dart into the acceptable range. The owner noticed that the Dart ran much smoother after the addition of the rear axle shims. The owner stated the low speed up and down pulsation (caused by the U-joint speeding up and slowing down due to the sharp angle) at the rear of the Dart that he thought was an out of round tire was gone.

 

When it comes to drivelines, it is important to make sure the tires and wheels are balanced, the driveshaft is balanced and has minimal runout, the pinion-yoke is free from runout and damage, and all the driveline components are properly torqued and free from play. Once these components are checked and deemed acceptable, the inclinometer can be used to measure the driveshaft angles. Shims at the axle or the raising or lowering the transmission can bring the working angles into specifications. Armed with this new information, it is time to go to the garage and start working the angles, so the angles will work for you. 

 

-the Professor

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