Setting Pinion Angle
Pinion angle is important due to the forces at work in the rearend and rear suspension of a rear-wheel-drive vehicle. As power is applied to the rearend via the driveshaft, the pinion gear tries to climb the ring gear. This causes the rearend housing (along with the pinion gear) to rotate about the axles' centerline. The optimum relationship between the pinion gear and driveshaft is when they are perfectly aligned. In order to achieve this under power, you much have some angle built into the setup when the vehicle is at rest and not under any power.
The typical leaf spring rear suspension will need 5-7 degrees of negative angle. A ladder bar or 4-link setup is much more rigid and therefore usually only needs 1-3 degrees. You never want the the rearend setup with positive pinion angle. This is when the yoke of the rearend is tilted upward more than the driveshaft. This can lead to binding of the u-joints and will hurt traction (the real reason we race enthusiasts are concerned about this).
To check the pinion angle, the vehicle needs to be sitting with its weight on the tires, as it would be normally. For the best accuracy, place weight in the driver's seat to simulate the driver. An angle finder can be bought from places such as Sears and Home Depot for under $10. Begin by measuring the angle of the driveshaft and writing it down. Then remove the driveshaft from the yoke (no need to totally remove it and spin tranny fluid everywhere) and place the angle finder on the end of the yoke. The angle from vertical will be equal to the angle of the centerline of the pinion gear to the horizontal. If the driveshaft was angled upward (it would be rare to find one that isn't) and the rearend is nose down (not always the case, so be sure you know which way its positioned), just add together the two angles you measured. For instance, if the driveshaft is 2 degrees up from level and the pinion gear is 3 degrees nose down, then you have 5 degrees of pinion angle. If the rearend is nose up and its angle is less than the angle of the driveshaft, subtract the rearend angle from the driveshaft angle. If the angle of the rearend is steeper than that of the driveshaft, subtract the angle of the driveshaft from that of the rearend to see how much positive pinion angle you have. For example, if you have a rearend angle of 5 degrees from vertical nose up and the driveshaft is angled up at 3 degrees, you have 2 degrees of positive pinion angle. If this was a leaf spring car, you'd need to change the angle by 7 degrees to get 5 degrees of negative pinion angle.
If the rearend is so close to level that you can't tell if its nose up or nose down, put the angle finder on the yoke and then see which direction you have to rotate the finder to make it read zero (or 90 depending on how the angle finder is made). When viewed from the passenger side, if you have to rotate it counterclockwise, the rearend is nose down.
Now that you know the pinion angle, you may have to change it. On a leaf spring car, this is done with wedges between the rearend and the springs. Speed shops and sometimes alignment shops keep these around. If you can't find them locally, Summit and Jegs carry them. If you have a factory 4-link car, you'll need some adjustable control arms or revised control arm mounts. Aftermarket ladder bar and 4-link cars are adjusted via the Heim joints. For a graphic description of all this, click here.
Recently, I've begun getting questions from the street rod crowd. They aren't so much concerned about straightline traction as they are a smooth, vibration free ride. So this requires consideration of the front u-joint's working angle, as well as the rear. You want these angles to be equal, but opposite. This can sometimes be hard to do, especially in cars with low stances. You might even have to compromise on the rear u-joint's angle in order to get them equal, so that requires deciding whether dead-smooth operation is more important than straightline performance.
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Copyright © 2009 Bruce Johnson and Craig Watson