It is said that the leaf-spring suspension is the oldest suspension usedfor automobiles. The leaf-spring system was used more recently in '70sproduction cars, some of which are still in service today as race cars.Many racers even prefer to utilize the leaf-spring rear suspensiondesign in their fabricated Late Model Stock and Modified race cars.

Like all other racing components, there is a science and methodology tothe use of leaf springs in stock cars, for both dirt and asphalt. Theconstruction of the car may depend on the type of leaf spring to be usedand where it will be raced.

There are four basic designs of leaf springs that are used in stock carracing:

1. The monoleaf spring is a low-rate, thinner spring that serves tolocate the rear end laterally, like a Panhard bar, but offers littlespring rate to hold the car up nor much stiffness to bending to helpcontrol axle wrap-up. The design of the car must include additionalsprings to support the car and a system for controlling accelerating anddecelerating forces that will try to rotate the rear end.

2. Multileaf springs are just as described, made up of multiple leafs ofdiminishing length. These tend to be increasing rate springs and areuseful for supporting the car as well as controlling axle wrap-up.

3. Parabolic leaf springs can be a single-leaf or multileaf designwhereas the leaves are thicker near the axle and have a taperedthickness design out to the eyes. These, too, support the weight of thecar without the need for extra springs and do a fair job of controllingaxle rotation under acceleration and braking. They can provide a muchsmoother ride due to the fact that the leaves do not develop thefriction associated with standard multileaf designs.

4. Composites are fairly new in racing and have been further refinedrecently. These leaves come in various rates and, with the lower rates,will need additional springs to support the weight of the car.

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 dirtsurfaces. The leaf cars seem to be a lot more consistent under thoseconditions.

The leaf performs several functions that other suspension systems mightneed additional hardware to serve. The leaf does the following:

1. Supports the chassis weight

2. Controls chassis roll more efficiently--high rear moment center andwide spring base.

3. Controls rear end wrap-up

4. Controls axle damping

5. Controls lateral forces much the same way a Panhard bar does.

6. Controls braking forces

7. Regulates wheelbase lengths (rear steer) under acceleration andbraking.

The reason the leaf-spring design is so good for higher g-forceconditions is the design has a very wide spring base. The wider apartthe springs are mounted, the fewer roll tendencies there are. Also, themoment center height can be fairly high with a leaf-spring design. Thisshortens the moment arm in the rear, producing less roll.

There is much less rear steer when going over heavy bumps or holes inthe track, so the rough conditions that upset a bar car is welcomed bythe leaf-spring car.

The overall cost of a race car is less when using leaf springs mostlybecause you don't need many of the mounts, linkages, and other hardwarethat must be used with three- or four-link rear suspensions.

Also, a leaf spring can be used in conjunction with a coil or coiloverspring to enhance the spring rate adjustability function for chassissetup balance. Teams will often put a high-rate leaf spring on the leftrear and a low-rate monoleaf spring on the right rear along with acoilover spring and shock. They can change the spring fairly easily onthe right rear to adjust for changing track conditions. The left-rearspring also controls most of the axle wrap-up and is about half as stiffin that regard, as two stiff leaf springs would be.

Disadvantages of the Leaf Spring

The leaf-spring system has some definite disadvantages when compared toa three- or four-link system. Although the typical bar-link type ofsystem does not handle tight, tacky, and rough dirt conditions, themoney race is usually run once the track has become slick. So at the endof the night, more times than not, the suspension that favors a slicktrack will win out.

The steel leaf springs tend to lose shape, and that means the car losesride height. If the "sag" is uneven between the two springs, it canalter the amount of bite, wedge, or crossweight in the car, changing thehandling. Some teams go through three or more sets of springs perseason. Spring sag also changes the rear-steer characteristics of thecar.

There is very little, if any, adjustment for rear-steer characteristicswith a leaf-spring system. Some builders will put several mounting holesat the front bracket and some only put one hole, keeping with the theorythat less adjustment means less rope with which to hang yourself, setupwise.

With the bar cars, the teams have a choice of adding lift bars, pullbars, pushrods, front- or rear-spring mounting options, a wide range ofrear-steer adjustability, and variable moment center height adjustments.For the leaf-spring cars, there is a monoleaf floater that can beclamped or open. This allows for a lift arm or pull bar to be added tothe leaf system.

The rear moment center height is not easily adjustable on leaf-springsystems, and the right-rear bite effect of angling the Panhard bartoward the right-rear tire contact patch cannot be done with theleaf-spring system.

Leaf springs are not easy to properly install. The pinion angle is noteasily adjustable. The mounting pad on the rear axle tube must either berepositioned to adjust for pinion angle, or wedge-shaped blocks must beused between the spring and the housing pad. There is no adjustment forthe amount of axle wrap-up damping, either. You get what the springgives you.

The use of rubber or other compliant material as bushings in the springeyes will help cushion acceleration and braking forces, but overall,compared to the pull-bar and lift-bar systems used with the three-linkand four-link system, there isn't much in the way of damping oradjustment for what is there.

Proper Installation

We must be careful to make sure the leaf spring is not inadvertentlybent 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 aside view, then when we bolt them up, the springs on each side will bebound in opposite directions. It is best to bolt up one side and thencheck 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. We need theshackle to be at 90 degrees to the rear portion of the leaf when the caris at ride height. That way there will be less influence in the springrate due to mechanical binding that would take place if the shackle wereangled 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 beinserted between the axle pad and the spring. Always check thepinion-to-driveshaft angle with all of the weight on the springs.

The overall spring height, as measured from the ground, and therelationship of the front eye center to the point where the springattaches to the rear end are important considerations when designing aleaf-spring car.

The spring's eye height determines the rear moment center height.Lowering the spring in relation to the chassis means lowering the rearmoment center height and causing more rollover at the rear. This servesto tighten the car all the way around the corners.

Changing the front eye height will affect the rear-steer characteristicsand alter the handling of the car in the middle and off the turns. Manychassis builders will only place one hole in the front mount andposition 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 compositedesign, or what some would call a fiberglass leaf, although there areother materials involved in the construction of these products.Composite leaf springs are not a new concept. The Chevrolet Corvette hasbeen using this design of spring for quite some time now.

The primary advantages of using a composite leaf spring are a 60-70percent weight savings over steel springs and the tendency of thecomposite spring to maintain its shape (according to the advertisementsand feedback we got from users, it doesn't sag). The composite leafsprings, just like the steel, come in various rates from 35 to 250pounds. 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 is the characteristic of maintaining a constant arc and shape thatmost appeals to current users of composite leaf springs, not necessarilythe weight savings. The minus we discovered was that the composite mustbe protected from heat and contact with anything that would chip it,starting a process that might lead to failure. Failure with a compositeleaf means breakage whereas a steel leaf will definitely bend, butseldom break.

Is it the Right Choice? Lovers of leaf-spring cars swear by theirconsistency, and that is what makes them so attractive. For the hobbyistracer who does not have the time nor patience to decipher all of thecomplexities of the three- or four-bar systems, much less afford all ofthat "mess," the leaf car offers a good system for a reasonable pricethat can and does win races.

AFCO Racing Products
P.O. Box 548
IN  47601
Landrum Springs
Hyperco Springs