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Setting Pinion Angle

Story & Photos By Jim Clark (The Hot Rod MD)

The article “Drive Shaft Harmonics” in Hot Rod MD explains how to set up the pinion angle on a rear end differential. It states in the process that the next step is to measure the rear end pinion angle. “It should be pointing upward towards the front with the vehicle sitting at ride height on a level surface.” A reader asked a good question about why the vehicle needs to be sitting at ride height before setting the pinion angle.

Question: I’m reading your piece in “Hotrod MD” titled “Drive Shaft Harmonics” for info on setting up a chassis with the proper driveline angles. I can understand the basics of measuring and setting the angular relationships between the components via measuring their centerlines. What’s confusing to me is when you introduce “ride height” into the equation. Being from a construction background, I most always start with a “level line” as a reference. So, on the chassis I am setting up (a 57 Chevy) I have the main frame rails setting “level” and I’m taking/comparing my angle readings of the components to that level line.

Once I have everything adjusted to the correct relationship to each other, what difference will it make when I put the assembly on the ground as a unit? I realize the intake plenum might not be “level” but that is another issue I believe. Equally critical issues include front crossmember/oil pan clearance, trans. tunnel clearance, firewall clearance. The MAIN question is “won’t establishing the angles and setting the correct relationships between the components with the frame setting at “level” result in having the proper driveline conditions when the chassis is put onto the ground on its wheels??

 

How: When you build a chassis and position the driveline components, you need to orient them so that the angle of a line drawn through the center of the transmission exiting through the output shaft (A) and a line drawn through the center of the driveshaft (B) is equal to but opposite the angle of a line drawn through the pinion shaft in the rear differential (C) and the line drawn through the center of the driveshaft (B). The accompanying drawing illustrates how the lines drawn through the transmission and pinion shafts (A&C) are parallel to each other, though not in the same plane.

Point in question as it appeared in the article: Next step is to measure the rear end pinion shaft angle. It should be pointing upward towards the front with the vehicle sitting at ride height on a level surface. At least 1-degree and ideally not more than 3-degrees up. You can alter the rear end pinion shaft angle by inserting or removing wedge shaped shims under the rear spring mounts or by adjusting the length of the control arms positioning the rear end.

Answer: Most vehicles have a frame that is kicked up in the rear and may have other portions that are not flat. Old style hot rods and some cars like gassers are raked down in the front or sit high in the front when fully loaded and sitting at normal ride height. If the engine, transmission and rear differential in these vehicles are all mounted in a fixed position like the ones on a Corvette or Jaguar rear-end assembly then the relationship between them will remain the same under any condition.

Corvette IRS under an early GM pickup.

Jaguar rear end assembly in the original metal cage that installed in the original car as a unit.

 

Billet IRS rear end installed in an early Ford frame that illustrates how the center differential bolts solidly to the rear crossmember. However, most early vehicles used in the building of hot rods have a straight-axle rear end that moves up and down as the vehicle is driven down the road. Where it is sitting in relation to the other components when fully loaded is the best starting point for calculating pinion angle. This can be calculated without the vehicle sitting at ride height but the final pinion angle will still have to be adjusted after the car is fully loaded and sitting at rest. This should be as close to zero as possible and not exceed more than three-degrees in either the up or down limits of differential movement.

 

Shown here is a frame table with a ’32 Ford chassis being set up at ride height with the front and rear suspension positioned where they will be located when the vehicle is completed and sitting on the road with a full finished load.

 

Gary Dagel has tacked some tubing between the rear end housing to establish clearance with the frame at ride height. He is verifying the height of the vehicle when the rear wheels and tires are installed. Pinion angle still cannot be set yet until the engine and transmission are installed. Limiting the amount of rear-end movement up and down is important because they don’t move vertically in a direct path. All locating devices have a fixed anchor at one end and actually travel through an arc as the unit rises or lowers in its travel. This is the main cause for the change in pinion angle and why the starting point for setting pinion angle needs to begin at ride height; that is the closest to zero degrees as possible.

 

Making adjustments to pinion angle If adjustments must be made to the system: Install shims between the axle housing and springs to rotate the axle input yoke to change operating angles. Change operating angle on radius rod type suspensions by turning in or out on the adjustable rod ends lengthening or shortening the radius rods. Add shims under the rear of the transmission mount or raise the engine at the front. Important to remember: Keep the centerlines of two components that are connected by a driveshaft parallel in both the top and side views, so the operating angles will ALWAYS be equal.

 

Pinion angle can be adjusted by adding tapered shims between the semi-elliptic springs and the mounts on the axle housing.

 

Pinion angle can be changed on radius rod style suspensions by adjusting their length causing the pinion to rotate up or down.

 

Inserting shims underneath the transmission mount changing the centerline in relation to the rear differential raised the rear of this transmission.

 

This highly modified Jaguar rear end mounted in a ’32 Ford chassis illustrates how this driveshaft has angles to be calculated from both the top and side. Not all differentials and engine packages are centered in relation to each other so both angles have to be calculated and the operating angles must always be equal.

 

Because the center differential is mounted solidly to the frame it was necessary to figure out how many degrees of pinion angle was desired and the crossmember installed to accommodate that. Any final adjustments to pinion angle in this situation must be made by changing the front or rear mounts on the engine / transmission assembly. Square tubing was used in place of the coil-over shocks to establish ride height because the coil-over shocks would not be compressed to ride height on this bare chassis.

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