Stop the Hop
Bernard Quinlan; Oahu, HI: I have a 1979 Chevy Malibu classic 350 four-speed with a 3.73:1 locker rear with 225/60R15 tires. My problem is I cannot keep the rear wheels on the ground. The wheelhop is so bad I have to shut down or I will break something. I have a stock rear suspension with heavy-duty rear springs and air shocks. Can you please tell me what I have to do or get to make the car stay on the ground? The 350 has the power to lift the front.
Jeff Smith: I have some personal experience with my Chevelle and air shocks that dates back to my high-school days. While that was a long time ago, the physics haven't changed. I learned very quickly that jacking the back of my big-block, four-speed 1966 Chevelle with a set of air shocks resulted in violent wheelhop. Conversely, lowering the car back to nearly the stock ride height eliminated the wheelhop. I will assume that you are using the air shocks and stiff rear springs to raise the car, perhaps to clear the rear tires. But since a 225/60R15 tire isn't that big, it's possible that the backspacing on the wheels is the reason for tire-clearance issues. If this is the case, the solution is to perform some careful measurements to determine the amount of room between the inside edge of the current tire and wheel package and the closest inner fender obstruction. Let's assume you have 11⁄2 inches of clearance to the inside and an 8-inch-wide wheel with 4 inches of backspacing. We define backspacing as the distance between the wheel mounting pad surface and the inboard edge of the wheel. You can measure this easily by laying a straightedge across the back side of the wheel (not the sidewall of the tire) and measuring straight down from the straightedge to the wheel-mounting surface.
By changing to a set of wheels that increase the backspace from the original 4 inches to 5 inches, you will move the wheel inboard, allowing the outboard edge of the tire to clear the inside of the fender lip. This will then allow you to lower the ride height in the rear. With the car at rest, shoot for a ride height that will place the lower rear control arm roughly parallel with the pavement. This will change the location of the rear suspension's instant center (IC), thus eliminating the violent wheelhop.
It might also be a good idea to replace the air shocks with a quality shock absorber. For the street, you don't need expensive adjustable shocks. A quality shock like the Bilstein (PN F4BE5E250M0, rear, $62.97 each, Summit Racing) is an excellent choice that will offer great ride quality as well as improved handling. The best idea is to change the front shocks, as well (PN 24-009492, $69.97 each, Summit Racing). I would also suggest checking the upper and lower control arm bushings in both the front and rear suspensions just to make sure they are in decent shape. It's possible that after 35 years of use, these bushings have never been replaced and may be cracked, broken, or even non-existent.
If the lower rear control arm bushings are in need of replacing, consider upgrading to a pair of aftermarket control arms. Global West builds an excellent tubular lower control arm with Del-A-Lum bushings (TBC-2, $279.60, Global West) at one end and a high-quality spherical bearing in the front that allows the arm to articulate but not bind. Even with the spherical bearing, minimal road noise is transmitted through the car. The advantage is the arm will accurately locate the rear axle under the car, which will eliminate lateral movement of the body over the rear axle. This is a common problem with stock, stamped lower control arms that flex horribly and allow the tires to rub on the body. My 1965 road-race/autocross Chevelle uses an 18x9.5-inch rear wheel with a 275/35ZR18 tire with barely 1⁄2-inch of clearance to the wheelwell on the outside. Yet even under hard, 1-g cornering, my tires never rub using the Global West arms. Other companies like Hotchkis and Currie sell a similar lower control arm, although they do not use a spherical bearing. Above all, however, beware of cheap Internet rip-offs of these quality arms. There are many unscrupulous vendors out there selling fake arms as the real thing. If the price for a brand-new part looks too good to be true, it probably is. Buy Global West, Hotchkis, or Currie parts from the vendors themselves or a known source such as Jegs or Summit Racing to be sure that you're getting the right parts. I recently spoke to Doug Norrdin, owner of Global West, and he told me that these off-shore knock-off parts are a huge problem because customers often unknowingly buy these cheap parts only to have them fail. So be smart and avoid the rip-off parts.
Currie Enterprises; 714/528-6957
Global West Suspension Components; 909/890-0759; GlobalWest.net
Hotchkis Performance; 562/907-7757; HotchkisPerformance.com