Details, details. When it comes to squeezing the utmost performance out of a given combination of parts, it's often little things that count the most. This month's installment in our Project SuperNova series proves that, as we dig deeper into the science of chassis setup.
Last month we bolted a set of slapper bars onto our '71 Chevy Nova and were stumped when they didn't prove to be a magic solution to the car's traction problems. We were already running slicks (Mickey Thompson 28x9.0-15 ET Drags) and 4.10 gears, but the car's 60-foot times were no better than before we installed them. We scratched our heads and called Competition Engineering for some advice; we even sent before and after photos showing how the car looked when it launched. Our ensuing conversation with Competition Engineering's Mike Kugler revealed some interesting background on how a leaf-spring rear suspension works, and how it reacts to slapper-style traction bars.
In any stock leaf-spring car, the front half of the spring basically acts like a control arm to locate the rearend housing and control its motion. The rear half of the spring from the spring perch back to the shackle, controls ride height and ride quality, which can be altered by changing the number of leaves in the spring pack and the free arch of the spring. Back in the days when Stockers and Super Stockers ruled the dragstrips, Mopar was famous for its Super Stock springs, which combined a relatively short front half with a stiff spring pack that essentially acted like a built-in traction bar and kept the rearend from wrapping up under hard acceleration.
Monoleaf springs like the ones on our Nova do a good job of providing ride height and quality. In fact, after 30-plus years of service, the springs on SuperNova had sagged less than 11/48 inch when we checked the free arch. But although monoleaf springs are popular in drag racing, they work best when combined with a traction device to prevent the rear axle from twisting or wrapping up under acceleration. The cheapest and simplest solution is a set of slapper bars, like the Competition Engineering traction bars we installed last month. A slapper bar prevents the rearend from rotating too far upward, which allows the pinion yoke and driveshaft to overextend and unload the tires. The result is that the body of the car should lift and separate from the tires, planting them harder to the ground and increasing traction.
Suffice it to say, after we bolted on the traction bars, SuperNova was lifting and separating, but not enough. We had the pinion angle checked and found it to be at zero degrees, which meant that the yoke and driveshaft were actually rotating into a positive orientation under launch, which is bad, as the pinion has a natural tendency to climb the ring gear under power, and this unloads the rear tires. To counteract this, pinion angle in a drag car running a leaf-spring suspension generally needs to be set to a negative angle when the car is at rest. How much pinion angle is necessary can only be determined through on-track testing, but Competition Engineering suggests starting at 5 degrees negative angle for a slapper-bar setup like ours.
We took SuperNova to a local shop, Hotrods to Hell, where owner Steve McClenon set up our Dana 60 on his chassis jig with 5 degrees of negative pinion angle. We also had him remove the stock-type Camaro/Nova spring perches and install conventional perches with U-bolts, which makes adding or subtracting pinion shims to tune the pinion angle much easier.