The leaf-sprung car can be...
The leaf-sprung car can be made to perform much like the three- or four-bar cars if properly constructed and set up. Even so, there are benefits and liabilities in this system.
I ’ve had many requests for information about the leaf spring rear suspensions. These types of springs are popular in Dirt Modified cars and earlier stock divisions. We collected information from several sources in 2005 and published an article. The following is basically a reprint, with some refinements, of that original piece.
The leaf spring suspension is the oldest suspension used for mobile devices with wheels. The leaf spring system was used most recently in 1970s production cars, some of which are still in service today as race cars. Many racers even prefer to utilize the leaf spring rear suspension design in their fabricated Late Model stock and Modified race cars.
The multi-leaf spring has...
The multi-leaf spring has several leaves that make up the rate. How the leaves are held in place is important to the endurance of the spring. Landrum springs uses a unique method to locate the leaves and this reduces the friction and corrosion effects.
There is a science and methodology to the use of leaf springs in stock cars, for both dirt and asphalt. The construction of the car may well depend on the particular type of leaf spring to be used and where it will be raced. Making installation mistakes can reduce performance.
Four Basic Designs
There are four basic designs of leaf spring that are used in stock car racing. They are:
1. The Mono-Leaf Spring: The mono-leaf spring is usually characterized by being a low rate, thinner spring that serves to locate the rearend fore to aft and laterally. It basically replaces the trailing arms and the Panhard bar used in three- and four-link systems. It offers little spring rate to hold the car up nor much stiffness to bending to help control axle wrap-up. The design of the car must include additional springs to support the car plus a third link or lift bar system for controlling accelerating forces that will try to rotate the rearend.
The multi-leaf spring has...
The multi-leaf spring has several springs of different lengths and thicknesses. The multi-leaf spring is rated higher than the mono-leaf and is the most common leaf spring used for stock car racing. Special material is used in the eye of the springs for both performance and endurance.
Multi-leaf springs are just as described, made up of multiple leaves of varying length. These tend to be increasing rate springs in bump and decreasing rate in rebound and are useful for supporting the car as well as controlling axle wrap-up.
3. Parabolic Leaf Springs: Parabolic leaf springs can be a single-leaf or multi-leaf design whereas the leaves are thicker near the axle and have a tapered thickness design out to the eyes. These too support the weight of the car without the need for extra springs and do a fair job of controlling axle rotation under acceleration and braking. They can provide a much smoother ride due to the fact that the leaves don’t develop the friction associated with standard multi-leaf designs.
4. Composite Leaf Springs: Composite leaf springs are a fairly new product in racing that have been further refined recently. They’re made of fiberglass instead of steel. The mounting portions are composed of steel that is bolted to the fiberglass leaf. These leaves come in various rates and, with the lower rates, may need additional coil springs to support the weight of the car.
As the car rolls, the leaf...
As the car rolls, the leaf spring bends and the two ends get farther apart. Because the front eye is bolted to the frame and the rear eye is attached to a free moving shackle, the rear axle will move rearward under bump. This is attractive on tight dirt racetracks. As the moisture in the track dries up and the track slows down, there is less g-force rolling the car and with less movement of the spring, there is less rear steer.
From the information we have gathered from several excellent sources, leaf spring suspensions are very forgiving on tacky and rough dirt surfaces. The leaf cars seem to be a lot more consistent under those conditions.
The leaf serves several functions that other suspension systems might need additional hardware to serve. The leaf does the following:
1. Supports some or all of the chassis weight
2. Controls chassis roll more efficiently by utilizing a higher rear moment center and a wide spring base
3. Controls rearend wrap-up when not mounted with birdcage-type mounts
4. Controls axle dampening
5. Controls lateral forces much the same way a Panhard bar does, but with very little lateral movement
6. Controls braking forces when not mounted with birdcage-style mounts
7. Better at maintaining wheelbase lengths (reduced rear steer) under acceleration and braking