1967 AMC Rambler - Get Your Leaf-Spring Car To Launch

Moser sells replacement axles for the AMC Model 20 rearend thanks to the Jeep market. The company also sells axles with any spline count for any application and can turn the order around in two days. These babies will withstand twice what our 360 can produce.

Last month we ran a corrected 12.36 with a stock 2.71:1 rear gear and no limited slip. Our math called for a mild 3.54:1 gear to avoid overrevving the engine through the traps with a 26-inch-tall tire. We drove over the finish line at 5,400 rpm and bagged a corrected 12.06 at 112.28 mph. With nitrous we will be in the lights at 6,200.

Because the Rambler is a Car Craft test car, we wanted to use a limited slip that would not wear out after repeated dragstrip burnouts. Auburn offered to let us test its new electronically controlled traction-enhancing differential (ECTED), which locks like a spool with the flip of a switch. Look for the complete flog in the We Test it! section of Speed Shop.

The traction bar completely replaces the shock mount bracket with a baseplate and can be bolted to the spring with either the stock U-bolts, or if you have a GM with T-bolts, with the J-bolts provided. The bracket in the front keeps the bar from digging into the ground when the spring unloads.

Before we did anything, we took the Rambler to Icon Engineering in Hawthorne, California, and had the shop press in a new set of spring-eye bushings. In addition to all the other things they do, leaf springs keep the housing square in the car, preventing roll steer.

Preload is the amount of pressure applied to the spring eye while the car is sitting on the ground at rest. The recommended preload is 11/42 inch between the pinion snubber and the leaf eye. Adding more gap adds more inertia and therefore more smack on the tire. Ours worked best with zero gap, which can be adjusted by lowering the bar with a shim between the spring and the bar at the baseplate.

The traction bars allow the axlehousing to rotate between 2 and 4 degrees, which forces the pinion toward the body of the car. Our pinion angle measured -2 degrees, giving us a zero angle on the launch. Most chassis shops offer wedge plates in 2-degree increments that fit between the spring and the pad to adjust pinion angle, and most hot rod shops will have an inclinometer so you won't have to buy one.

With both tires spinning, the axle began to wrap and sent the car sideways. As you can see from the illustration, the front of the spring is forced up and the rear is forced down, making the tire unload and hop. The counterclockwise rotation of the housing as seen from the rear of the car is also uncontrolled, causing the car to dart toward the guardrail.

The slapper bar forces the housing to stay in place, transferring the power straight down.