The leaf-sprung car can be made to perform much like the three- or four-bar cars if proper
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 several leaves that make up the rate. How the leaves are held in
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 several springs of different lengths and thicknesses. The multi-
2. Multi-Leaf Springs: 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 spring bends and the two ends get farther apart. Because the fr
Advantages to Using Leaf Springs
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
The proper way to determine the moment center height for a leaf spring system is to measur
The reason why the leaf spring design is so good for higher g-force conditions is because the design has a very wide spring base. The wider the springs are mounted apart, the less roll tendencies there are. Also, the moment center height can be fairly high with a leaf spring design. This shortens the moment arm in the rear and that also produces less roll.
As the g-forces increase with a tighter track, the rear roll increases at a higher rate than the front. It’s important to control this increase in roll rate in order to provide better balance in the setup and more overall grip.
There is much less rear steer when going over heavy bumps or holes in the track with the leaf design and so the rough conditions that upset a three- or four-bar car are welcomed by the leaf-spring car.
The overall cost of a race car is less when using leaf springs mostly because you don’t need many of the mounts, linkages, and other hardware that must be used with three- or four-link rear suspensions. And, maintenance is less not having to worry about rusting Heim joints or broken mounting bolts that are in constant, high stress, high shear conditions.
If the spring blocks that are attached to the axle tube are not parallel, then when you bo
Also, a leaf spring can be used in conjunction with a coil or coilover spring to enhance the spring rate adjustability function for chassis setup balance. Teams will often put a high rate leaf spring on the left rear and a low rate mono-leaf spring on the right rear along with a coilover spring and shock. That way they can change the spring fairly easily on the RR to adjust for changing track conditions. The LR spring also controls most of the axle wrap-up and is about half as stiff in that regard as would be two stiff leaf springs.
Disadvantages of the Leaf Spring
There are pluses and minuses for every suspension system. The leaf spring system has a few disadvantages when compared to a three- or four-link system. Although the typical bar-link-type of system doesn’t handle tight, tacky, and rough dirt conditions as well, the money race is usually run once the track has become slick. So, at the end of the night under dry conditions, more times than not, the suspension that favors a slick track will win out.
The steel leaf springs may tend to lose shape and that means the car may loses ride height. If the “sag” is uneven between the two springs, it can alter the amount of bite, wedge or cross weight in the car which changes the handling. Some teams are said to go through three or more sets of springs per season. Spring sag also changes the rear steer characteristics of the car because the height of the front eye changes as the spring loses height and that changes the axle-to-mount angle.
There is very little, if any, adjustment for rear steer characteristics with a leaf spring system. Some builders will put several mounting holes at the front bracket and some only put one hole keeping with the theory that less adjustment means less rope to hang yourself with, setup wise.
The installation of the leaf spring must be done with care to avoid binding of the springs
To change rear steer, you would have to make height changes to the front mount and the same amount of change to the spring spacing between the axle tube and the spring to maintain the original ride height.
It might also be possible to have multiple height adjusting holes in the front mount as well as in the shackle. If the distance from the axle tube to the front mount and to the rear mount are different, then the hole spacing in the front and shackle must be different in order to maintain the original ride height with rear steer changes.
With the bar cars, the teams have a choice of adding lift bars, pull bars, push rods, front or rear spring mounting options, a wide range of rear steer adjustability, and variable moment center height adjustment. Available for the leaf spring cars is a mono-leaf floater that can be clamped or left open. This allows for a lift arm or pull bar to be added to the leaf system to control acceleration forces.
The rear moment center height is not easily adjustable on leaf-spring systems and the RR bite effect of angling the Panhard bar toward the RR tire contact patch can’t be done with the leaf spring system like it’s done with a four-bar system.
The angle of the leaf spring to the shackle should be very nearly 90 degrees when the car
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 shackle changes from 90 degrees, a series of events that will l
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 is made of a resin and fiberglass type of material while the con
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.