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.

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

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.

Note the black stripes that veer right. As the passenger tire unloads, the driver-side tir

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.

Some Solutions
The oldest and likely cheapest solution is to heat metal strips and bend them around the front leaves to keep them from separating and distorting. The front half of the spring from the axlehousing forward is used like a control arm to locate the housing in the car. If you can prevent the front of the spring from distorting under acceleration, that energy will be transferred to the axle centerline, forcing it and the tire down onto the track surface.

Before we added the Competition Engineering traction bars, the Rambler would squat and unload the passenger-side tire, causing wheelspin.

Before we added the Competition Engineering traction bars, the Rambler would squat and unl

The second solution is the traction bar (or slapper bar) trick that we used on the Rambler. A set costs about $160 from companies like Competition Engineering. As the pinion rotates toward the body and the axle begins to wrap, the snubber on the end of the traction bar contacts the spring eye and transfers the energy directly to the centerline of the axle, driving the tires down like a big lever. At the same time, the spring is forced down, lifting the body into the air. The distance between the snubber and the leaf eye at rest determines how hard the system strikes the tires. It's about as simple as a sledgehammer and works about the same.

The third solution is the adjustable traction bar. There is a point on your car, the center of gravity (CG), where the vehicle weight is equal front to rear. If you put a jackstand under that point, the car would be balanced in the air. Now imagine that you walk to the front of the car and lift up on the bumper. The car will rise in the front and dive in the rear. The same is true for lifting the rear bumper as the car rotates or pitches around the CG. CalTracs or Slide-A-Links have the ability to move the point of lift, or instant center (I/C), if needed closer to the center of gravity. This is helpful as you make more power and try to go faster. As the I/C is moved forward, the load on the tire is greater and less violent. As the I/C is moved rearward, there is less load delivered to the tire with more violence. The benefit of an adjustable instant center is increasing or decreasing the speed of load transfer as you begin to build more torque, or to compensate for track conditions. Since we are making only about 400 lb-ft of engine torque, the current rearward I/C doesn't produce a hit on the tire so violent that it smashes the tire down causing it to later unload and spin. As we build more power using a larger engine or nitrous oxide, we are going to play with the CalTracs bars.