It has been said that the leaf spring suspension is the oldest suspension used for automobiles. The leaf spring system was in use more 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.
As has evolved with all other racing components, 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.
Four basic designs of leaf spring are used in stock car racing:
Monoleaf Spring. The monoleaf is usually characterized by being a low rate, thinner spring that serves to locate the rear end laterally, like a panhard bar, but offer little spring rate to hold the car up nor much stiffness to bending to help control axle wrapup. The design of the car must include additional springs to support the car plus a system for controlling accelerating forces that will try to rotate the rear end.
Multileaf Springs. Multileaves are just as described, made up of multiple leaves of diminishing length. These tend to be increasing rate springs and are useful for supporting the car as well as controlling axle wrapup.
Parabolic Leaf Springs. These can be a single leaf or multileaf design where 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 do not develop the friction associated with standard multileaf designs.
Composite Leaves. These leaves are a fairly new product in racing that has been further refined recently. These leaves come in various rates and, with the lower rates, will need additional springs to support the weight of the car.
Advantages of 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:
- Supports the chassis weight.
- Controls chassis roll more efficiently with a high rear Moment Center and wide spring base.
- Controls rear end wrapup.
- Controls axle dampening.
- Controls lateral forces in much the same way a panhard bar does.
- Controls braking forces.
- Regulates wheel base lengths (rear steer) under acceleration and braking.
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 apart the springs are mounted, the less tendency to roll. 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.
There is much less rear steer when going over heavy bumps or holes in the track and so the rough conditions that upset a bar car is 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.
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 coil-over 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 wrapup and is about half as stiff in that regard as would be two stiff leaf springs.
Disadvantages of the Leaf Spring
The leaf spring system has some definite disadvantages when compared to a three- or four-link system. Although the typical bar link type of system does not 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, more times than not, the suspension that favors a slick track wins out.
The steel leaf springs tend to lose shape and that means the car will 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. Spring sag also changes the rear steer characteristics of the car.
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, setupwise.
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. There is available for the leaf spring cars a Monoleaf Floater that can be clamped or open. This allows for a lift arm or pull bar to be added to the leaf system.
The rear Moment Center height is not easily adjustable on leaf spring systems, and the RR bite effect of angling the panhard bar towards the right rear tire contact patch cannot be done with the leaf spring system.
Leaf springs are not easy to install. The pinion angle is not easily adjustable and the mounting pad on the rear axletube 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 wrapup 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-link 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 rear end axletubes are not parallel from a side view, then when we bolt them up, the springs on each side will be bound in opposite directions. It is 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.
The angle of the spring shackles to the leaf is important too. We need for the shackle to be at 90 degrees to the rear portion of the leaf when the car is at ride height. That way there will be less influence in the spring rate due to mechanical binding that would take place if the shackle were angled more or less than 90 degrees.
Lovers of leaf spring cars swear by their consistency and that is what makes them so attractive. For the hobbyist racer who does not have the time nor 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.