Leaf Springs

Leaf springs are the oldest form of suspension in racing. Although they are the oldest, they seem to be the least understood. Leaf springs possess many desirable suspension features, such as dampening, forward bite, roll over steer, high anti-squat percentage, and high lateral stiffness. In addition, the leaf spring suspension is more forgiving on chassis set-up errors. Due to the fact of the popularity of the leaf spring system, we felt racers may want to understand more about how the suspension actually works. We will cover the different aspects of leaf springs from free rate to installed rate, applications, hook up points and other performance enhancing factors. There are four basic types of leaf spring systems in the racing industry today.

• Multi-Leaf Spring This type of leaf spring has more than 1 leaf in its assembly. It consists of a center bolt that properly aligns the leaves and clips to resist its individual leaves from twisting and shifting.
• Mono Leaf Spring Consists of one main leaf where the materials width and thickness are constant. Example the leaf will be 2 1/2 wide throughout its length, and .323 in thickness throughout its entire length. The spring rate is lighter than other styles of leaf springs and usually requires a device to control positive and negative torque loads as well as requiring coil springs to hold the chassis at ride height.
• Parabolic Single Leaf Consists of one main leaf with a tapered thickness. This style is sufficient to control axle torque and dampening, while maintaining ride height. The advantage of this style is that the spring is lighter than the multi-leaf.
• Fiberglass Leaf Spring The fiberglass leaf spring is made of a mixture of plastic fibers and resin; it is lighter than all other springs. However, the cost is three times greater. The disadvantage is that they produce inconsistent spring rates. In addition, fiberglass springs are sensitive to heat. The resins break down when exposed to heat and heat cycles (produced from exhaust and/or brake systems) which will cause the resin in the spring to become brittle, resulting in the spring collapsing. Another problem occurs with inconsistent resin mix which will cause the leaf to splinter and break. Furthermore, the fiberglass spring is susceptible to damage from rocks and debris.
• WAYS TO ENHANCE PERFORMANCE AND LONGEVITY OF LEAF SPRINGS Running dampening shocks not only will tighten the car on entry, but will also prolong the life of the spring. The leaf spring will last longer because the shock will assist in absorbing the dampening forces. Driving style plays a major role in the life of a leaf spring. Slinging the car into the corner or spinning out puts an extreme amount of lateral force on the springs which in turn, causes premature failure. Impact from tire hopping or grazing the wall can bend or unroll the main leaf eyes. This can cause an undesirable change in spring rate and wheel base settings. Choosing too light of a spring rate will cause the spring to be in a higher stress situation, thus losing ride height. Furthermore, the spring will absorb all the weight transfer, and not plant the tire securely on the track. Too light of a spring rate will also cause the rear end to lose an excessive amount of pinion angle under acceleration, leading to a loss of forward bite. When not racing, keep the springs unloaded by simply placing a jack stand under the chassis frame rail. This simple task will increase the life of the leaf spring dramatically. If the leaf springs hook-up points are installed on the chassis incorrectly the misalignment will produce high stress loads on the spring. In turn, the life of the spring and the number of cycles is reduced. Bushing choice will also affect spring life, and more importantly, spring performance. Rubber bushings tend to absorb more energy and loads from the chassis and rear end, therefore the springs tend to last longer. However, under racing situations, this may cause the chassis and rear end to have excessive movement, thus producing erratic handling. Pivot bushings are bushings that were designed to remove the bind between the chassis and the spring. However, the negative effects of these bushings greatly outweigh their intended purpose. Our extensive testing has proven this component to produce very erratic handling characteristics. When the front bushing is allowing the front leaf spring to pivot, it transfers all the side loads and lateral forces to rear portion of the leaf spring and shackles or sliders, which were not designed to handle the additional stress. This leads to bent shackles, warped sliders and misaligned axis points. Furthermore, because the front eye is allowed to pivot, it does not have any solid displacement to drive the car forward. Problems with front pivot bushings are:
• Loose on corner entry, loose coming off
• Tight on corner entry, loose coming off
• No forward bite
• Bent or warped suspension components
• Reduction of spring life due to high stress loads on the shackle end of the spring.
• Urethane and aluminum bushings tend to transfer more energy and loads directly to the spring. This prevents undesirable chassis and rear end movement, thus creating favorable synergy between the chassis, springs and rear end. These bushings produce a solid rear housing displacement for added traction. Furthermore, these solid bushings control and enhance chassis performance by resisting chassis roll/torque. Solid bushings provide stable and predictable handling characteristics that lead to more consistent lap times.

It is not difficult to detect if you need a replacement leaf spring. Some of the usual symptoms include excess body bounce, lean, or roll. Your vehicle may droop more, especially when carrying lots of items or towing a trailer, adding strain to the leaf springs. Furthermore, degraded leaf spring bushings can bring out shocks and vibrations in the cabin.

 

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