This chapter looks at muscle cars with rear-suspension systems using coil springs with two lower control arms and a torque tube. These factory three-link cars include GM 1982 to 2002 Camaros and Firebirds (also referred to as third- and fourth-generation F-Body cars). Since rear-wheel-drive cars with rear coil springs, two lower control arms, and a torque tube did not have upper and lower control arms with opposing angles like the factory four-link cars, it was also necessary to add a track locater (called a Panhard bar).
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General Motors promoted Fire-birds and Camaros as performance cars, and the design engineers at General Motors developed a much better suspension system than the earlier factory four-link. This three-link system worked well for drag racing applications with minor modifications.
The Neutral Line
Extending an imaginary line forward on the same angle as the lower control arms, it crosses the torque tube that is attached to a bracket mounted to the rear-end housing. The front of the torque tube is attached to the tailshaft housing of the transmission. This intersection point determines whether the car lifts or squats in the rear when leaving the line as compared to the neutral line.
The neutral line is a reference point used when considering weight transfer (pitch rotation) in suspended cars. It begins at a point above the front spindle centerline as high off the ground as the center of gravity and continues to the rear tire contact-patch directly below the rear-axle centerline. If the intersection point of the control arms and torque tube is above the neutral line, the rear of the car raises upon launch. The closer the intersection point is to the rear-end housing, the harder the suspension hits the rear tires (just remember you can over-power the tires). If this same intersection point is below the neutral line, tires (just remember you can over-power the tires). If this same intersection point is below the neutral line, the car squats upon launch.
A car that lifts in the rear has to push down harder on the tire to do so, just as you can‘t lift 100 pounds of something without putting 100 pounds of extra pressure on your feet. The Camaros and Firebirds already had an imaginary intersection point above the neutral line from the factory, so with no modification these cars planted the tire hard and without any wheel hop. That fact, along with the car having a short wheel-base (101 inches, compared to a Chevelle’s 112 inches) made these cars an excellent candidate for doing huge wheel stands.
Several modifications were necessary to make the rear suspension of these cars an optimal contender. Since these cars came with only 24-inch-diameter tires, running a large-diameter slick in the stock wheel well was impossible. To overcome this, it was necessary to install a 2-inch-thick aluminum spacer between the top of the spring and the frame. This lifts the body over the rear end enough to allow the installation of a 9×30-inch slick. Once the rear end is lifted with the spring spacers, the Panhard bar (being fixed in length) pulls the rear axle to one side and the wheels are no longer centered (side-to-side) in the wheel-well openings. It is now necessary to install an adjustable Panhard bar. I have TIG-welded chrome-moly (for light weight and more strength) Panhard bars in a fixed length for factory ride-height cars and an adjustable Panhard bar for cars that use a non-factory ride height.
The suspension had no means of preload from the factory. Typically, when the tires of a Camaro or Firebird stick, they launch the car toward the right as it leaves the line. Since these cars had no upper control arms for adjusting preload, the only thing that can be done is to install an air bag in the passenger-side rear coil-spring (or a much heavier and taller passenger-side rear coil-spring). Using an airbag is the easiest way to accomplish preload as the air bag is adjustable. Using a heavier and taller spring requires you to replace the spring every time you want to make an adjustment. Twenty-five pounds of air is usually the normal operating pressure used in an air bag in the passenger-side spring with close to 0 pounds or no air bag at all in the driver-side spring. You may need to adjust air pressure based on the car’s combination. If the car still turns toward the right, add more air in the passenger-side air bag. If it turns to the left, it has too much air in the passenger-side air bag. Make changes in 2-pound increments up or down. Too big a change does not allow you to discover the best-possible pressure for your car’s combination.
The best way to be aware of what is happening is to videotape your hand on the steering wheel as you launch the car. Growing up in the snowy state of Michigan, I have very good instinctive reflexes if sliding around while leaving the line. Some-times your instinctive reactions to correct the cars launch are so natural that you don’t notice them. That is why it is very important to record either hand on the steering wheel and mark the steering wheel with a white or contrasting stripe at 12 o’clock (with the front wheels straight) that shows well in the video. The video shows you if the steering wheel is turned right or left to correct a less than desirable launch. This testing can only be accurate if the car hooks at the starting line. A car with spinning tires naturally slides side-to-side or only one way just because of the spinning tires.
Factory pinion angle (which is near zero for longer U-joint life) needs to be modified. Since there were no upper arms, there’s really no way to set pinion angle for a harder bite. I developed my own torque tube with a built in adjuster for the F-Body cars that allows pinion angle to be set to a more desirable setting. This torque tube is TIG-welded chrome-moly and looks similar to a long ladder bar.
To make pinion angle adjustments, the car needs to be sitting on all four tires with the driver or someone of similar weight sitting in the passenger seat. Using an angle finder, measure the driveshaft angle and then the pinion angle. Once you have the driveshaft angle, you may need to unbolt the driveshaft to get the pinion angle using the differential yoke that the driveshaft bolts up to.
Quite often there is a flat spot on each side of the rear cover where the axle tubes are pressed in on GM rear ends. This should be machined 90 degrees to the pinion. Normally the driveshaft and the pinion angles make the shape of a very flat letter “V.” If so, you need to add the two angles together to get the present pinion angle.
To this point, I’ve said nothing about the transmission or the ground. While both have an importance while building the car, pinion angle is only the difference between the pinion and driveshaft. Nothing else! From the factory, your car probably has next to 0 degrees of pinion angle. Each car needs its own setting. Too little pinion angle and the car doesn’t hook well. Too much pinion angle and the car hooks, but at a loss of horsepower. The pinion and driveshaft, no matter how much pinion angle you are using, want to straighten out under hard acceleration. That creates a lever-age effect that drives the tire into the pavement. However, it robs horse-power to make this leverage effect happen. Therefore, never run more pinion angle than your car’s combination requires.
As mentioned earlier, start with 2 degrees negative pinion angle for cars in the 400-hp range, 4 degrees for cars in the 600-hp range, and up to 7 degrees for cars running 1,000 hp or more. Very seldom does any car need more than 7 degrees of negative pinion angle.
When you measure your drive-shaft angle (e.g., 1.5 degrees down) and the pinion angle (e.g., 3.5 degrees down in the opposite direction), adding the two together represents 5 degrees total negative pinion angle. To change the pinion angle, simply loosen the jamnuts locking the adjuster in place on the torque tube. Turn the adjuster to make the arm longer for more pinion angle or shorter for less pinion angle. Remeasure and repeat if necessary. When you have the total pinion angle you desire, lock the jamnuts in place.
Now you can see why all the top-quality adjusters are double adjustable, meaning no bolts need to be removed to make changes. Single-adjustable adjusters require one end to be removed to make changes, which makes the process more difficult and more lengthy.
The factory soft rubber bushings in the lower control arms were another issue. Even though Camaros and Firebirds were promoted as performance cars, the design engineers had to incorporate a combination of maximum comfort, stability, and traction. Unfortunately for performance enthusiasts, stability and traction gave way to comfort. With soft rubber bushings in each end of the control arms, a quiet suspension with a soft ride was achieved. These arms are what hold the rear-end housing in the car, along with the torque tube. The front of the two lower control arms are attached to the frame rails and the rear to the rear axle housing.
To improve performance, the bushings in the lower control arms should be replaced with polyurethane bushings, such as those made by Energy Suspension. These greatly improve the handling of a car whether it be hard cornering or just aggressive driving. All four bushings in the lower control arms must be replaced. For ease of replacement, remove one arm at a time, complete the polyurethane bushing installation, and then reinstall that arm before moving on to the next arm.
Lower Control Arms
Even with boxed lower control arms and polyurethane bushings the factory control arms are still formed from thin mild steel and not rigid enough for truly heavy-duty drag racing applications. I have developed tubular chrome-moly lower control arms with greaseable Zerk fittings for each bushing to keep them well lubricated. Chrome-moly tubing is lighter in weight and far stronger than mild steel. Chrome-moly tubing is much lighter and less bulky than aluminum. Using chrome-moly tubing builds strength into these arms not found in other materials while still being lighter in weight than aluminum or mild steel.
The front of the torque tube is attached inside a rubber mount within a steel bracket bolted to the transmission tailshaft housing; the rear of the torque tube is bolted solid to the rear-end housing. As explained in Chapter 1, under hard launches, the rear-end housing is attempting to roll the pinion upward and then roll the entire rear end out of the car. This rear-end bracket and torque tube eliminates this condition. However, the stress transferred to the torque tube’s attachment point on the transmission tailshaft housing very often cracks or breaks the transmission tailshaft housing or the bracket the torque tube attaches to at the rear-end housing.
Avid racers routinely searched salvage yards for spare brackets when these cars were newer. By monitoring the bracket for cracks at the race and with a spare bracket in hand, complete bracket failure could be avoided by replacing the bracket at the first sign of a crack. With only four bolts (two in the torque tube and two in the bracket), the part could be changed easily at the track in just a few minutes.
The powdercoated chrome-moly torque tube comes with a beefier rear mount and a much-improved front mount with a 3/4-inch heim joint on the torque tube for strength and maximum freedom of movement when attached to the new front mount. Three-quarter-inch heim joints are used to attach the chrome-moly torque tube to the beefier rear bracket, which attaches to the rear-end housing.
The front mount looks similar to a rear leaf-spring shackle, which moves back and forth as the body rises or lowers. This allows the torque tube to freely get longer or shorter as the body raises or lowers on the chassis, thus removing most of the stress on the tailshaft housing and help prevent broken transmission cases. I have developed several mounts for many different transmission combinations.
shorter as the body raises or lowers on the chassis, thus removing most of the stress on the tailshaft housing and help prevent broken transmission cases. I have developed several mounts for many different transmission combinations.
If you have not four-corner scaled your car (covered in Chapter 10), take it to a commercial scale and follow this procedure:
- Drive the front tires onto the scale as close to the center of the scale as possible without getting the rear tires onto the scale.
- Have the scale read while you are still in the car, giving you the car’s front-end weight.
- Once you have that weight, drive all four tires onto the scale and get the car’s total weight with the driver in place.
- Drive the car forward until the front tires are off the scale and the rear tires as close to the center of the scale as possible. Have that weight recorded, again with the driver in the car.
- To double check the accuracy of the various weights, you should be able to add the “front only” weight to the “rear only” weight and be within a few pounds of the “total car” weight.
Now you should be able to get your GM F-Body car on the track and be competitive in your class. As with any car, there is no single answer as to what works every time for every car. However, with the information in this chapter, you should have the necessary knowledge to make your car the best it can be.
Written by Dick Miller and Posted with Permission of CarTechBooks