Rear Suspension Guide - Leaf Springs And Spring Wrapup

Here is a classic illustration of spring wrapup where torque has created an S bend in the forward part of the spring. Note that this effectively shortens the spring's length, which also pulls the rear shackle forward.

Here is a classic illustration of spring wrapup where torque has created an S bend in the

While the leaf spring is still around because of its simplicity, there are drawbacks. The springs themselves are heavy, which contributes to the car's unsprung weight. This is defined as the weight not supported by the car's suspension. From a dynamic standpoint, less unsprung weight is an advantage. Because of their weight and size, leaf springs are also more expensive compared with coil springs. There are composite material leaf springs available that do a great job of reducing weight, but they're also more expensive.

Another important step to help control unwanted rear axle movement is to invest in high-quality suspension bushings for the front and rear. Polyurethane is a popular and inexpensive upgrade, but you should consider the virtually bulletproof aluminum insert Del-a-lum bushings first created by Global West almost 30 years ago. The aluminum bushings use a Delrin insert that prevents metal-to-metal contact, enhancing wear while also offering near-zero deflection. Global offers these bushings for all popular performance body styles. If you're going to go fast, these bushings are an excellent investment.

Factory Coil Spring
The most popular factory rear suspension design for solid rear axle cars is the coil spring system. Under the coil spring umbrella are a number of subtle design variations that make coil spring suspensions more attractive to suspension tuners, compared with leaf springs. Because the coil spring's only job is to support the weight of the vehicle, designers still needed a way to locate the rear axle under the car. This necessitated control arms (also called trailing arms). The simplest OE coil spring rear suspension is the four-link. This design uses two parallel lower control arms located near the outboard ends of the rear axle. The two upper control arms are angled outward instead of parallel to the chassis. This creates a triangle that locates the rear axle laterally (side to side) under the car, eliminating the need for a Panhard bar or Watt's link. Popular examples of this rear suspension can be found in cars such as the '64 to '72 GM A-bodies and the '79 to '93 Ford Fox Mustangs. While the coil spring four-link system is more complex than a leaf spring design, it enjoys numerous inherent advantages. First off, the system is usually lighter than leaf springs. More importantly, leaf spring wrapup is eliminated, although wheelhop can still occur if the rear ride height is raised excessively.

This is the result of ultimate spring wrapup where the tire literally bounces off the pavement. It's hard to accelerate when there's daylight under the back tires.

This is the result of ultimate spring wrapup where the tire literally bounces off the pave

With a true parallel four-link rear suspension, the links form a right-angle box that allows the rear axle to move from side to side underneath the car. This system is most often used in drag cars and requires the addition of a Panhard bar or Watt's link (these will be described later in this story), which limits rear axle lateral movement. The main advantage of factory four-link rear suspensions is that the rear axlehousing is securely located. If you view the car directly from the side, the housing pulls on the upper links under acceleration, placing them in tension while pushing the driving force through the lower links. This pushes on the links, applying a compressive force. The point where all this force comes together is something called the instant center (IC--see Suspension Simulation sidebar). This point can be moved by adjusting the position of either the upper or lower links. Factory four-links do not offer adjustability, but aftermarket add-on components such as the original Lakewood No-Hop bars change the IC by raising the rear upper link position, which shortens the IC. Generally, this creates a situation where the car's body actually rises on acceleration.

Adding a bolt-on traction bar helps to prevent spring wrapup, which in turn keeps the tire on the ground. Look closely at the traction bar on the right rear of Troy LeCrone's 9-second Camaro. This Lakewood bar is actually bent from the force of controlling spring wrapup. The ideal place for the snubber is directly underneath the front spring eye.
Adding a bolt-on traction bar helps to prevent spring wrapup, which in turn keeps the tire
In early '70s NHRA Pro Stock racing, the rules required stock-type rear suspensions. Chrysler engineers designed what they called a leaf link with tubular upper control arms that were under tension, using the leaf spring as the lower link that was subjected to compressive loads under acceleration. While this arrangement worked well, it tended to bend the main leaf spring upward just behind the front spring eye. When the rules changed, racers replaced the leaf springs with tubular lower bars and coilover shocks to create the classic four-link.
In early '70s NHRA Pro Stock racing, the rules required stock-type rear suspensions. Chrys
This is a CalTracs bar created by John Calvert and used here on his original '68 428 Cobra Jet NHRA Super Stocker. Positioning the tubular bar below the spring places the compressive loads into the bar rather than the spring. This not only eliminates spring wrapup but also changes the position of the rear suspension's instant center.
This is a CalTracs bar created by John Calvert and used here on his original '68 428 Cobra