NOTE: THIS DRAWING IS AN EXAGGERATION, YOUR PINION ANGLE WILL NOT BE THIS RADICAL.
Pinion angle affects the efficiency of the power transmission from the drive shaft to the rear end. The pinion angle is the spread between the pinion centerline and the drive shaft centerline. For maximum performance, the ideal is to have a slight pinion down angle (min. 2 degrees) under full power.
Obviously, the pinion angle changes as power is applied and the axle housing begins to twist as the pinion tries to drive the ring gear. Rear axle wind-up can take some of the initial energy or "hit" away from the launch. As a general rule, a leaf spring rear suspension should have between 5-7 degrees of static pinion angle (angle down); and a factory four-link should be set at 4-6 degrees for drag racing..
Keep in mind that these pinion angles are for competition. If you primarily drive your car on the street, you shouldn't have more than a 5 degree angle maximum. The U-joints are designed to run at this angle, which allows the cups to rotate avoiding premature wear and failure. This doesn't affect leaf spring suspension since it runs well within this tolerance.
You will need to have the car level on four (4) car ramps or blocks of equal heights so you have room to work under the car when checking the pinion angle. Also, both the front and rear suspension need to be loaded to get accurate readings. The height of the rear end relative to the rest of the driveline will affect the pinion angle. Using an angle finder, which is a tool that combines a bubble level and a protractor, to determine the correct pinion angle for your car. Position the angle finder on the drive shaft and record the measurement. Then place the angle finder on the pinion (the U-joint yoke rotated to vertical position will give an accurate reading) and record the measurement. The pinion angle is a combination of the two measurements.
You can purchase shocks that are 3-position adjustable for both front and rear applications. These shocks are available from your local speed shop or mail order supplier. If you can't find shocks designed for your specific application, you'll have to check in a shock catalog for O.E. applications. You may have to use shocks from another make or model that has the same mounts at each end and the same length, or an inch or two longer. This won't hurt, because it allows for more suspension travel, so you don't hit the end of your shock travel and pull the rear wheels off the ground.
Coil spring cars, use 90/10 front shocks on small block cars, 80/20 or 70/30 on big block cars and high horsepower small blocks. On short wheel base cars like Mustangs and Capris, use the 80/20 or 70/30 settings. Use 50/50 rear shocks along with an air bag (4-8 lbs air) in the right rear coil spring only. This you will have to play with to find the best setting for your car. The air bag is used to counteract the engine torque wanting to lift the left front corner.
The basic information regarding instant center is the same for all cars, but for this discussion, we'll focus on the factory-designed four-link system. The instant center (IC) is an imaginary point defined by extending the line of the upper and lower control arms forward until the two lines intersect. By changing the locating points of either the upper or lower control arms, the IC can be moved longitudinally (fore-aft) as well as vertically. Moving the instant center closer to the rear of the car reduces the leverage on the rear axle, reducing and eventually eliminating the car's tendency to squat. There are two basic ways to change a Chevelle's IC position.
As the instant center is moved toward the rear by altering the upper or lower control-arm mounting points, this has an effect on tire load, or the "hit" on the rear tire. If you are using a sticky rear tire like a wrinkle-wall M/T E.T. Street or a pair of slicks, moving the instant center rearward will apply more leverage to the rear suspension, reduce the squat, and take maximum advantage of the wrinkle-wall tires. If a set of stiffer-sidewall drag radials are used, positioning the instant center back from its stock location--but not as far back as for a wrinkle-wall tire--would be beneficial.
The height of the instant center will also move as you reposition the control arms, and this affects average tire loading on the rear tire. If you draw an imaginary line between the tire contact patch to the car's center of gravity (CG), this is the 100 percent antisquat line. If the instant center is located above this line, antisquat will be more than 100 percent, while an instant center below the line is less than 100 percent antisquat. Theories abound on the proper location of the instant center, and this will change based on power, tire condition, track conditions, and perhaps a dozen other variables. This is just a hint of what you can learn about instant centers and traction. If you're into maximum traction, there's a ton of material to learn about putting the power to the ground.
Note: You need at least 45% of the car's weight on the rear wheels to make a 4 link suspension work properly..
Note: Check your pinion angle before you start, so that you know which way to adjust the new arms. But beware that if the upper and lower control arm bushings are worn out, the axle may shift when the car is driven, making your measurements less accurate.
1. Raise the vehicle and place it on jack stands with the rear axle hanging. (no weight on the rear
axle.) Place a jack under the differential housing and support it lightly.
2. Unbolt and remove one upper rear control arm using a ¾” wrench. Remove only one control arm at a time. This will make it easier to line up the new control arms when they are installed.
3. Adjust the length of the control arms as needed to achieve the desired pinion angle. 3 to 4 degrees nose-down is recommended. Shorten the arms for more nose-down pinion angle. Lengthen the arms to bring the pinion angle up. 1/8 inch adjustment will change the pinion angle approximately 1 to 2 degrees. Adjust both arms at the same time to be sure they are the same length.
SHORTENING THE CONTROL ARMS MORE THAN 1/2" FROM STOCK CAN CREATE
NOISE, VIBRATION AND HANDLING PROBLEMS.
FAILURE TO HEED THESE WARNINGS MAY LEAD TO SERIOUS INJURY.
4. Tighten the jam nuts on the control arms. Loctite thread locking compound is recommended.
5. Grease the sides of the bushings and the thrust washer with a synthetic grease and install the front end of the control arm into the car. Do not tighten the bolt yet.
6. Pull the control arm down over the axle bushing and install the bolt. Raise or lower the front of the differential with a jack to align the boltholes. In some cases it may be necessary to jack under the rear of the differential (under the cover) to tilt the bushings forward.
6. Tighten both bolts and repeat the procedure for the other arm.
7. Check the pinion angle using the Chassis tuning tips link. Because of production variances and the large variety of modified cars, some re-adjustment may be necessary.
To aid in the installation of our rear suspension products, mark the rear shock absorbers
(with a marker) just below the protective skirt. This will be used as a reference later.
Place the transmission in neutral and hoist the vehicle into position. While the vehicle is in the air support the weight of the rear axle, raising it to the marks made on the shock tubes.
1. Remove the control arm with a ¾” wrench.
2. Lube the bushing mating surfaces of the control arm with a synthetic grease.
3. The factory control arm length should be duplicated if using an adjustable lower control arm. To accomplish this place your new control arm
on top of the factory one and align bolt holes.
4. Repeat steps 1-3 for the control arm installation of the second control arm.
5. Instal arm in reverse order.
6. Reinstall the wheels lower the vehicle and road test.
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