Killer Kong Project Update: Ironing Out Leaf Spring Woes
There are a lot of folks who resolutely stand on the ill-informed soapbox preaching, “Classic muscle cars were never meant to handle,” when in fact, none could be further from the truth. The “R” and “T” in Dodge’s R/T trim level stood for “road” and “track” respectively. In fact, the original BOSS 302 and Shelbys, and GM’s Z/28 and Trans Am F-Bodies were packages specifically designed for road course duty.
In fact, the point is that across all brands, the higher optioned muscle cars not only came with bigger powerplants, but larger brakes, stiffer suspension and sway bars, not to mention larger and often lighter 15-inch rims and bias-ply tires with a wider footprint.
Although they struggled – and usually failed – to out-maneuver the smaller European sports cars of the period, they did perform admirably for their time.
While technology and a century’s worth of ever-improving engineering has provided us with far more tactile and responsive street machines, it doesn’t mean that yesterday’s muscle is antiquated junk.
So rather than scrapping your factory muscle car’s suspension setup wholesale as some aftermarket companies would want you to do, might we suggest a few simple and affordable mods and bring your classic up to snuff?
Looking at this subject from a 10,000-foot view, there really is no way that we could properly sum up all of the knowledge accrued by chassis builders, racers and drivers in one short technical article.
The years of applied experience and know-how dedicated to race car suspension and troubleshooting have filled hundreds of books, so for the sake of saving our sanity, we’re going to cover the key basics that anybody running factory-style leaf springs ought to know, and how we went about perfecting our perennial project Charger Killer Kong‘s rear setup for the street and strip with the help of Competition Engineering.
Leaf Springs And You
Dating back to Conestoga wagons, leaf springs are easily one of the oldest and longest-lasting technologies found on automobiles and trucks today. As such, leaf springs are one of the simplest ways to both localize the rear axle and suspend the vehicle’s weight.
Despite the design’s uncomplicated nature, leaf springs are surprisingly effective for maintaining optimal traction on both asphalt and dirt surfaces.
Leaf springs typically come in two varieties: mono- and multi-leaves. As the name denotes, the mono-leaf is comprised of a single tempered steel or composite spring bar hung between a forward eyelet and a rear shackle or slide, while multi-leaf springs are a stack of several springs of different lengths and thicknesses hung likewise. Consequently, the multi-leaf tends to be more rigid, making it less prone to binding and more common in production vehicles.
Whether it a mono- or multi-leaf spring, the advantage of using rear leaf suspension becomes evident when considering that leaf springs perform several functions aptly that other suspension setups would need additional equipment to do, such as controlling chassis roll, axle wrap, lateral forces (similar to how a panhard bar does), and better regulates deceleration forces.
The key to the leaf spring’s success – particularly under higher lateral g-force conditions – is in its wide spring base design. The wider apart the springs are mounted, the fewer body roll tendencies appear. Moreover, car designers found conditions like heavy bumps or holes that would cause considerable rear steer with four-link or coil-over setup cars were no problem for those with leaf suspensions.
Taking The Good With The Bad
While leaf springs have many benefits, they aren’t without their faults. Their most noted quirk – as any drag racer can attest – is how leaf spring-equipped cars have a tenancy to wheel hop during hard acceleration.
This violent loss of traction is a byproduct of axle or spring “wrap,” wherein the front half of the spring bends enough to bind the rear half, contorting an almost wave-like S-shape in the spring.
This wave is caused by torque transferring from the tires forward through the spring, virtually convexing the forward half of the leaf and pinching the latter half.
Naturally, the spring will try to fight the opposing forces by returning to its prescribed shape, but can only do so by disengaging from the source of torque – i.e. traction – and break the tire free from the ground.
When torque is transferred to the tires and begins to contort the leaf springs, the pinion attempts to rotate upward into the undercarriage, exacerbating wheel hop.
Ideally, a quick way to ease this would be to adjust the pinion angle slightly downward.
Unfortunately, to do so with leaf springs would require either the mounting pad on the housing to be ground off, manually repositioned and welded back on, or a wedge-shaped spacer block must be inserted between the spring and the housing perches.
Going From Good To Great
Over the decades, many have engineered ways to stamp out axle wrap (and consequently wheel hop) in leaf springs. Many top-optioned muscle cars came with either thicker springs or springs with extra leaves. For Killer Kong, our long-term ’69 Dodge Charger R/T project car, we scored a nice set of Mopar Performance Super Stock springs a few years back that would prove advantageous and will work nicely with our QA1 double-adjustable shocks we recently installed.
Designed specifically for those racing Chrysler vehicles in Super Stock classes, the main leaf is of a heaver rating and comes with an extra sixth leaf.
In fact, adding additional leaves was a common practice until it was outlawed by certain racing bodies. To work around the ruling, racers clamped the leaf springs from the axle forward at the tip of each leaf.
Often referred to as the “Moonshiner’s Trick,” clamping the individual leaves forward of the axle housing increases the rear roll stiffness while retaining enough flexibility in the rear to corner, and dates back to the original Hudson Hornet from the early 1950’s.
Another trick came in the form of wedging the rear leaves, which dramatically improves traction and handling stability, a modification that disqualified Glenn Dunnaway’s win at the first ever NASCAR race in 1949.
Employing yet another old trick developed by early drag racers, auto manufacturers included factory-installed pinion snubbers, a now common practice. Since our retro-fitted Dana 60 was cast for a front-axle application, there was no place to mount a snubber to the housing. We considered fabricating a reverse-mounted pinion snubber from the undercarriage down, but went with another option…
C.E.’s Bill Sief explained how Slide-A-Links fight binding, “As the vehicle’s rear half moves downward the leaf spring will flatten out. When this occurs, the distance between the spring eye centerline and the axle centerline changes.
“Picture a traction device that rigidly attaches to the front spring eye and the axle perch. In this scenario, as the spring flattens out through downward body travel, the distance between these two mounting points will attempt to increase, however, if these two points were rigidly attached, the axle will then rotate causing the pinion angle to change.
“The Slide-A- Link incorporates a sliding tubular member that allows the axle centerline to move rearward as the chassis travels downward, thus preventing any binding in the spring and spring mounts or pinion angle change.”
Not all traction bars are created equal, nor do all traction bars do the exact same thing. The most popular leaf spring traction bars are either “slapper” bars or “fixed-length” bars like Competition Engineering’s Slide-A-Links. As one comes to understand where axle wrap control comes from, selecting the right traction bar for your setup becomes clear.
Competition Engineering’s Thor Schroeder explained that “a ‘slapper bar’ is one of the most basic traction devices available. Originally pioneered by Bill “Grumpy” Jenkins in the mid-sixties, it gets its name from the way it works.
“When the housing begins to rotate during launch, the bar also rotates until it contacts or ‘slaps’ the spring. When contact occurs, the slapper bar becomes a lever trying to push the axle housing down and planting the tires in the process.”
Under torque, all the force is transferred to the front of the spring or the “spring eye.” Slapper bars can only stop the rotation of the axle and spring wrap. Sounds good, right? Unfortunately, they fail to control lift particularly as front spring eye is bolted to the frame while the rear eye is mounted to a free-moving shackle, permitting the rear axle to move rearward under bump, contributing to rear suspension bind.
Conversely, a fixed-length bar, mounted below the axle and intersecting at the spring eye, which acts as a lower control arm by applying its force at the mounting point to prevent spring wrap up while eliminating bind. Fixed link bars not only disperse torque transfer, but can be set with enough preload to maintain pinion angle and reduce axle rotation as rotational force of the rear is now redirected to the front pivot point.
Schroeder continued, “The Slide-A-Link [was] designed to outperform conventional ‘slapper’ bars [by] providing a positive displacement for the torque that is transmitted from the rear axle through the telescoping bar and special durometer shock pad.
“These forces, along with improved instant center geometry, provide better weight transfer for increased traction. Free travel and pre-load adjustments are made on the vehicle by adjusting the jack screw at the rear of the bar.”
Getting To Work
We returned to Competition Engineering – where we gotten Kong’s 6-point cage, sub-frame connectors and weld-in torque boxes years earlier – for their Slide-A-Link traction bars. Unfortunately, calling Competition Engineering’s Slide-A-Link a “fixed length” is a bit of a misnomer, as the Slide-A-Link is a composition of two telescoping end links with a special durometer shock pad which helps to absorb the forward transferred torque.
Although the installation is considerably more involved that a bolt-on slapper bar system, the Slide-A-Links offer improved instant center geometry and weight transfer for increased traction.
Note that each of the Slide-A-Link’s front bracket are two pieces that are sandwiched between each spring eye and the factory bracket with a spring clamp tying the spring eye bracket with the forward-most leaf. Competition’s new spring pads feature a boss where the Slide-A-Link mounts, connecting the front shackle to directly beneath the axle housing, and replace your factory shock perches.
Since we’re talking about suspension components here, we made sure to go a little liberal with anti-seize, and we’re running lowering blocks, we already had a pair of U-bolts. The Slide-A-Link is a two-piece unit and installs rather easy. With the rear suspension already assembled and torqued down, we assembled the Slide-A-Link bars separately according to Competition Engineering’s instructions before attempting to install them the forward and rear brackets.
In fact, installation of the links require that the vehicle weight be supported by the springs, so we lifted the rear housing with a floor jack just enough to lift the frame off of the jackstands. Now, we could install our Slide-A-Links, careful not to over-tighten the large 5/8″ bolts as torquing these down will cause severe suspension binding. Once installed, we lowered the Charger back onto the ground.
Clearly we couldn’t properly adjust our links precisely as to do so requires the car to be in race-ready condition (and even suggests using corner scales), a stage we’re still quite a few months away from. When we get Killer Kong out on the track for our first shake down, we’ll be able to return with a far more detailed review of how to adjust our new Slide-A-Links.