The angle of the leaf spring...
The angle of the leaf spring to the shackle should be very nearly 90 degrees when the car is at ride height with all of the weight in it. This helps eliminate several effects that alter the spring rate that the car feels.
Leaf springs are not easy to install properly. The pinion angle is not easily adjustable and the mounting pad on the rear axle tube must either be repositioned to adjust for pinion angle or wedge-shaped blocks must be used between the spring and the housing pad. There is no adjustment for the amount of axle wrap-up dampening either. What the spring gives you is all that you get.
The use of rubber or other compliant material as bushings in the spring eyes will help cushion acceleration and braking forces, but overall, compared to the pull bar and lift bar systems used with the three- and four-link system, there isn’t much in the way of dampening or adjustment for what is there.
During installation, we must be careful to make sure the leaf spring is not inadvertently bent or bound up when we bolt it into the car. Longitudinal twisting (looking forward or rearward) can add spring rate to the leaf.
If the spring pads on the rearend axle tubes are not parallel from a side view, then when we bolt the leaf springs up, the springs on each side will be bound in opposite directions. It’s best to bolt up one side and then check the other side to see if the pad is flat to the spring. If not, angled spacers must be used to relieve any bending tendencies.
As the angle of the spring...
As the angle of the spring shackle changes from 90 degrees, a series of events that will load and unload that corner of the car take place. We need to understand these tendencies so that we can properly design our cars.
The angle of the spring shackles to the leaf is important too. The shackle needs to be at 90 degrees to a line drawn from the center of the rear mounting bolt to the center of the axle when the car is at ride height. That way there will be less change in the spring rate due to mechanical binding that would take place if the shackle were angled more or less than 90 degrees.
The pinion angle is to be taken into consideration during installation. If the pinion angle needs to change, wedge shaped blocks must be inserted between the axle pad and the spring to change the angle of the pinion. Always check the pinion-to-driveshaft angle with all of the weight on the springs.
The spring eye height determines the rear moment center height. Lowering the spring in relation to the chassis means lowering the rear MC height and causing more rollover at the rear. This serves to tighten the car all of the way around the corners.
Changing the front eye height will affect the rear steer characteristics and alter the handling of the car in the middle and off the turns. Many chassis builders will only place one hole in the front mount and position it where it will do the most good under most racing conditions.
Steel vs. Composite
The newest innovation in racing leaf spring technology is the composite design, or what some would call a fiberglass leaf, although there are other materials involved in the construction of these products. Composite leaf springs are not a new concept. The Chevrolet Corvette has been using this design of spring for quite some time now.
The composite leaf spring...
The composite leaf spring is made of a resin and fiberglass type of material while the connections that attach it to the axle and chassis are steel. Care must be taken to protect the leaf from heat or damage from contact with mounting bolts or foreign objects that might fly up off the racetrack.
The primary advantages of using a composite leaf spring are a 60-70 percent weight savings over steel springs and the tendency of the composite spring to maintain its shape (doesn’t sag—according to the advertisements and feedback we got from users). The composite leaves, just like the steel, come in various rates from 35 to 250 pounds. They can be used as a “single” leaf in low rates or as a “stack” leaf, still a mono-design but thicker like a stacked steel spring.
It’s the characteristic of maintaining a constant arc and shape that most appeals to current users of composite leaves, not necessarily the weight savings. The minus we discovered was that the composite must be protected from heat and contact with anything that would chip it, starting a process that might lead to failure. Failure with a composite leaf means breaking whereas a steel leaf will definitely bend, but seldom break.
Nonetheless, lovers of leaf-spring cars swear by their simplicity and consistency and that’s what makes them so attractive. For the hobbyist
racer who doesn’t have the time or patience to decipher all of the complexities of the three- or four-bar systems, much less afford all of that “mess,” the leaf car offers a good system for a reasonable price that can, and does, win races.