Torque and horsepower are wonderful things, especially when you've got a rock 'n' roll big-block bolted under the hood of your '64-'72 Chevelle. But that major dose of power is of little benefit if you can't put the power to the ground. Fat, sticky tires will always improve traction over stock street radials, and that's a quick and easy way to help hook you up. But ultimately, the search for more traction will require digging into the rear suspension if you are at all serious about planting that power to the pavement. That's what this story is all about.
The stock Chevelle rear suspension is a coil-spring, trailing-arm design using two short-upper and two longer-lower trailing arms to position the rear axle in the car. To prevent the axle from moving from side to side, the upper arms are splayed outward to create a triangular-mount effect. As power is applied to the rear axle, the upper control >> arms are put in tension while the lower control arms experience compression. Several things are happening at once when power is applied, and it's important to know how all this affects traction.
As power is applied on the starting line, the left front corner of the car rises and the right rear tends to squat. While the body is compressing the coil springs in the rear, the rear axle is torquing in the opposite direction (see illustration 12). This torque reaction tends to lift the right rear off the ground, reducing traction and allowing the tire to spin.
The beauty of this factory four-link system is that it operates much like a dragstrip four-link. One of the first things that will help one of these cars is lowering the ride height to position the lower control arm as close to parallel with the ground as possible. This improves the instant center (see the Instant Center sidebar) while also almost eliminating wheelhop.
The first step any self-respecting Chevelle owner with even mild small-block power should do is install a pair of rear crossmember braces. Edelbrock, Hotchkis, Year One, OPG, and others sell different versions of this factory piece that connects the front upper and lower control-arm mounting points. This triangulates the rear crossmember and prevents the crossmember from cracking with the torque of violent starting-line launches. It's also a good idea to carefully inspect the rear crossmember for cracks, especially around where the crossmember attaches to the frame.
Next you might want to consider adjusting the rear-axle pinion angle to read around 2 to 3 degrees nose down. Under load, the pinion tries to climb the ring gear, pushing the pinion flange upward. By creating a nose-down angle, the pinion will remain in a neutral position under load. Pinion angle can be adjusted using one of several adjustable-length upper control arms offered by companies like Currie, Edelbrock, Hotchkis, Metco, and several others. These adjustable arms are especially useful if you are installing a 9-inch in a Chevelle where the upper locating points on the 9-inch are in a different location than stock 10- or 12-bolt housings.
A low-buck way to accomplish this same thing is to use a die grinder to elongate the rear hole in the upper arms to increase the length slightly. We got this tip from the guys at Just Suspension. They recommend some experimentation to establish your 2 to 3-degree down pinion angle with these slots. Then they supply a set of hardened steel washers with a 1/2-inch center hole that can be welded to these upper arms to positively locate the upper position. If you were willing to experiment, you could even make up a second set of arms with a different pinion angle and do back-to-back testing.
The original-equipment rear control arms for Chevelles are amazingly spindly stamped-steel components for the job they have to perform. The upper control arms are designed to be able to twist, because in major body-roll >> conditions, the arms are intended to deflect slightly to prevent the rear suspension from going into a complete bind. The lower arms also need to deflect slightly, but this is over a greater distance since the arms are longer than the uppers. This deflection, unfortunately, also means the rear axle has a tendency to move around underneath the body. For these original cars with fist-sized clearance between the body and the tires, this was no problem. But when you begin stuffing monster tires and wheels under stock wheelwells, deflection becomes a significant dilemma.
The solution is a lower control arm that does not deflect. Some factory lower arms came boxed to accommodate mounts for a rear sway bar. The next step was tubular lower control arms much like those from Global West, Hotchkis, Art Morrison, and others. The most interesting one is the Global West tubular arm equipped with a spherical bearing in the front pivot point. This allows a certain amount of movement without binding. For straight-line applications, polyurethane bushings do a good job, but add body roll into the equation and it's possible to create bind in the rear suspension. The beauty of the tubular lower control arms is that you can now run a much larger rear tire with less clearance to the body without worry of a tire rubbing, because the rear-axle deflection is significantly reduced.
There are several other variables that we only have room to touch on here including rear spring rates and shock absorbers. Shocks can be a significant tuning tool if you are willing to experiment and step up to a set of adjustable shock absorbers. Competition Engineering, Koni, and QA1 offer adjustable shock absorbers. The QA1 shock is interesting because it offers 12 positions of adjustment that can allow you to really fine-tune your application. Of course, these shock ideas also apply to the front as well. While that is a whole different story, adjusting the front suspension rise to work with the rear suspension is the ultimate approach to the search for better traction.
We've offered some very targeted information on Chevelle rear suspension treatments that will work for both drag-race and street-performance applications. The key is to make the rear suspension work with the tires to create more traction so you can use all those ponies you worked so hard to create. After that, it's just a matter of hammering the throttle and letting the tires do the work.
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. The original Lakewood "No Hop" bars relocate the rear-axle mounting point of the upper control arms higher, which shortens the IC. Another popular approach is to drop the lower rear mounting point of the bottom trailing arm. This is the approach taken by the SSM Auto Lift Bars or Metco's Instant Center Modification brackets included with each set of Metco's lower control arms.
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.