Volume III, Issue 9, Page 22

Make Your Chevy Work

There’s plenty of hype out there on how to make a coil spring car (a late model Malibu and an early Chevelle or a Monte Carlo) work down the straight and narrow.  Some of the stuff we’ve all seen includes airbags (either one or two), so called “lift” bars, “no hop” bars, and ladder bars along with a whole bunch of questionable bolt-on paraphenalia.  Much of this stuff works a wee bit or doesn’t work at all.  We’ve lived with name brand no- lift bars (complete with revised pickup points).  They might have worked, but the geometry was so bad you could see where the bolt holes (drilled into the rear end housing brackets) were becoming egg-shaped.  Simply stated, those bars were trying to rip the axle right out of the car.  Nice.

Big power will usually smoke the tires in an early A-Body (Chevelle) or a late model G-Body (Malibu).  As far as the back end of these cars is concerned, there are three major ingredients to the righteous hook: shock absorbers, an anti-roll bar, and adjustable trailing arms. Early and late body styles pretty much use the same hardware layout, although the pieces will not interchange.

Without question, the grist of the back suspension is the upper and lower trailing arms.  GM used U-shaped stampings, and while you can use boxed-style replacements, the preferred set-up is adjustable tubular pieces. What’s so important about adjustment? One big issue is to establish a working pinion angle. The core is to understand that the coil suspension found on an A- or G-body is nothing more than a four-link with angled trailing arms. Laying out the suspension with angled bars meant GM didn’t have to worry about a means to control lateral movement in the rear end.  Instead of using some form of track locator or Watts linkage, the trailing arm triangluation keeps the rear end in the car.  With this arrangement the bean counters were happy (no added parts meant less cost), but so were the engineers (no added parts meant less unsprung weight, which equates to better performance).

However, the “instant center” location on these cars is way out in front, in some cases close to the engine. What’s instant center? I/C is an imaginary point about which the chassis or a suspension member rotates in a given (instant) position. It’s found by projecting lines along suspension members to a point of intersection, which is called the instant center. The I/C acts as a pickup point for the back suspension, even though the points are imaginary, and in many cases, the I/C is rather high (due to the angles of the bars it can be somewhere above the front wheel spindle). 

The problem with many of the no-hop bars or any of the traction devices for A- and G-bodies is that they tend to create a very short instant center (by changing the angle of the bars).  That might work under modest power but as you feed more grunt to it, the car becomes violent.  Worse, some of the top-mount bars (that raise the upper trailing arm location on the rear end housing) have a tendency to hammer the trunk floor when you get on the throttle. 

GM’s stock I/C location isn’t really that bad.  Believe it or not, many of today’s very successful Pro Stock drag cars have instant center locations that intersect way ahead of the vehicle.  In fact, many torque arm suspension combinations (e.g., late model Camaro) that are known to hook pretty much on grass have a long I/C location.  Add big power to you’re A- or G-body and you’ll likely find that an I/C location near stock is actually close to optimum, provided that you don’t jack the body way up.

If you have adjustable bars, then you can fine tune the suspension and set the static pinion angle. The static pinion angle is a very big deal. In order to keep the driveshaft and U-joints operating in a (more or less) straight line, the pinion angle must be correct.  Typically, this angle is measured between the pinion gear flange and the driveshaft.  As the suspension “wraps up” under power, the pinion is driven upward and out of whack.  To ensure that the pinion is in the correct location under stress, it is typically set nose- down static. In cars equipped with spherical bearings (rod ends) or solid back suspension bushings, you strive for a pinion angle of between -1 to -2 degrees (negative angle, or pointing down).  If the car has OEM style rubber suspension bushings, a pinion angle of -3 to -4 degrees is more appropriate.  

Preload is another major tuning tool, but with a triangulated four-link, there is a big caveat:  In a high-horsepower car, the engine torque tends to roll rotate the chassis, which means the car has more bite in the right rear.  In turn, that bite tends to move the car toward the left.  In a lower-powered car, the forces that rotate the rear end (torque rotation) tend to provide more bite to the left tire.  In either case, you can counteract all this with preload.  In a conventional, non-triangulated four-link (such as a dedicated drag race car set-up), you can shorten the upper right bar to increase the preload on the right rear tire.  If you can lengthen the same bar, then more load is placed on the left rear tire.

Sounds good, but here’s the rub: on cars equipped with triangulated upper control arms setting preload isn’t possible with the upper bars. When you adjust only one upper control arm then you actually shift the rear end housing left or right, depending upon which bar you adjust. The upper bars on a triangulated 4-link are used to center the rear end housing from side to side (by adjusting one upper).  The upper bars are also used to set the pinion angle (by adjusting both uppers the same direction and same amount). Adjustable lower bars are used to center the tires front-to-back in the wheel well and to make minute wheelbase dimensional changes. To preload the suspension in a Chevelle or Malibu, an anti-roll bar (ARB) is most often used.  From there, you use tune the adjustable shock absorbers (front and rear), front suspension limiters and to a lesser degree, specific spring rates to get the car to hook, but as you can see, the trailing arms are major traction players.