Calvert Racing and Competition Engineering both offer an excellent traction-bar alternative that places an adjustable link below the leaf spring, placing the leaf spring in tension rather than in compression. The link pivots from underneath the front spring eye and is adjustable for preload to compensate for the rear axle's tendency to lift the right (passenger-side) rear under acceleration. Either of these bars is superior to standard traction bars but are more expensive. Calvert and Landrum Spring also offer drag-tuned monoleaf springs that eliminate the leaf-to-leaf friction of multileaf springs.
One significant variable with leaf springs is that as load is applied and the rear suspension either squats or rises, this forces the rear spring shackle to move. According to AFCO Performance, this shackle movement also changes the effective spring rate, which varies the load applied to the rear tires. Ideally, if the shackle never moved, this would maintain the same amount of load on the tire. The original Mopar Performance books used to preach keeping the rear shackle pivot point on the body located ahead of the rear spring eye, which would increase load with body rise. There is some great tech information about leaf springs on Afco's Web site (afcoracing.com) that explains this more fully. The solution for this shackle movement is a slider assembly. A slider assembly is a steel roller that travels in a slot designed to fit between the rear axle and the spring to allow the rear to move fore and aft under load. AFCO sells these sliders, designed mainly for Mopar-width springs in drag racing-only applications.
Tach Tuning If you're really serious about chassis tuning on the dragstrip, it makes sense to invest in a data logger to record each run and compare them to see exactly how the car used the power on each pass. Several NMCA racers we spoke with use the Auto Meter Ultimate II Playback Tach and matching software to log engine and driveshaft rpm to evaluate torque-converter efficiency as well as traction. The data is recorded by the tach and then downloaded to a computer that uses the Auto Meter Tach Facts software to plot the rpm curve of each pass. The Ultimate II tach logs up to four runs with a rate of 100 samples per second.
Most racers use the data to study driveshaft rpm in search of tire spin. This information can then be used to make chassis changes to improve 60- or 330-foot times. For example, we had a Chevelle that would initially launch and then unload the rear tires and spin roughly 30 feet off the line. Slowing the rate of front suspension rise changed the point at which the front suspension topped out and the car went quicker. The key to using any data-logging device is to accurately evaluate what the car is doing and how to use that information to improve e.t. Of course, there are dozens of other data-logging options. We did a story called "The Racepak" on an affordable data recording in Nov. '04.