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

This is one of the last photos you will see from LACR. The track is scheduled to close in July 2007.

We've been told that for every 0.10 second that can be shaved off of the 60-foot time, you get 0.20 second off of the e.t. in the quarter-mile. For a car that flounders for traction at the green, the promise means a full second at the top end if you can get the car to shave 0.50 on the line.

In the Mar. '07 issue, we used some John "Bugsy" Lawlor math from the Auto Math Handbook and predicted that our '67 CC/Rambler needed about 500 RWHP to get the car to 130 mph in the quarter-mile and into the 10s. We also predicted that with the roughly 385 hp it is currently making at the wheels (480 - 20 percent for drivetrain losses) and a 3,000-pound race weight, it should run a corrected 11.50 at 119 mph with addition of a 3.55:1 rear gear ratio. The calculation came from Comp Cams Desktop Drag, the same program that accurately predicted the corrected 12.36 e.t. for our baseline run. With that number in mind, we trailered the Rambler to Los Angeles County Raceway and ran a string of corrected 12.06-12.10s after the gear swap. So what gives? What happened to the 11.50? If you haven't guessed by now, the answer is a 2.00-second 60-foot time that refused to improve with tire-pressure adjustments or track temperature. The Desktop Drag program had predicted a 1.70 60-foot and assumed we could hook the car up. Instead the AMC would squat hard and unload the passenger rear tire, then follow with some scary fishtailing, before it would get traction and start building speed. So going back to the theory, if we can cure the traction problem and clip 0.29 off of the 60-foot time, we should also reduce the overall e.t. by 0.58, putting us right around the predicted mid-11s mark.

Note the black stripes that veer right. As the passenger tire unloads, the driver-side tire begins to steer the car from the rear. When you see a street racer with the passenger side of the car jacked up in the rear, it is done to avoid this by preloading the tire.

How It Works
When a leaf-spring car launches, the axlehousing rotates counterclockwise as seen from the rear of the car. At the same time, the axlehousing is rotating up toward the body, and because it is bolted to the leaf spring, it is also pushing the rear of the spring down and the front half up, forming an S shape. This is called axlewrap. This action misaligns the driveshaft and, as the spring unloads, rotates the housing down. The repeated oscillation of the spring and driveshaft housing causes what is known as wheelhop. The twisting force also drives the passenger-side tire up into the wheeltub and unloads it, resulting in tire spin.

In a perfect world, when the car launches, the axlehousing rotates from 1 or 2 degrees of negative pinion angle to zero pinion angle, transferring the maximum amount of horsepower and torque to the differential. At the same time, both ends of the housing are forced down, planting the tires on the dragstrip as the body is lifted and separated from the axlehousing, making the car launch straight. This concept applies to any leaf-spring car. Including yours.