We ain't like them fancy magazines what get all uppity with them coilovers and chain-link suspensions. If two pairs of leaf springs were good enough for those WWII GIs, they're good enough for us.
Well, actually, coilovers, multi-link suspensions, and all that fancy jazz have their place. But for all the technology, geometry, and intricacies of making them work, it's really hard to beat good old-fashioned leaf springs. When you think about it, leaf springs are amazing critters. They locate your axles longitudinally underneath your vehicle while still allowing a semi-elliptic arc of travel to smooth bumps. And they do it all with a steadily increasing, linearly compounding spring rate that can deliver smooth-yet-predictable performance off-road. As the top leaf compresses, it encounters the leaves below it. The spring rate ramps up at a rate determined by the thickness and number of leaves in the pack. A good spring builder can tailor a spring pack based on vehicle weight and usage that will rival the plushness of most coilover systems without the worry about all that anti-sway, body-rolling, tire-jacking nonsense. So follow along as we check out a few leaf spring factoids that may make you go vintage on your next suspension design.
We noticed that most leaf...
We noticed that most leaf spring naysayers gained their voice during the advent of spring-over suspensions using stock Wrangler spring packs-a common modification starting 10-15 years ago. The super-soft YJ spring packs weren't the best choice for a spring-over application and they helped give leaf springs a bad rap. Draw a line between the tire's contact patch to the center of the axle tube and that's the rotational fulcrum. Mounting the springs below the axle centerline reduces the leverage on the fulcrum and resists spring wrap, but mounting it above the fulcrum increases the leverage greatly. Even with traction bars, spring wrap and traction bar binding became such an issue that most turned to link-type suspensions as a cure-all.
Here is a pair of custom spring...
Here is a pair of custom spring packs for a Jeep Speed Cherokee. Note the triple-military wrap on the spring-eye end, which not only adds a margin of safety in case a main eye snaps, but ups the anti-wrap characteristics of the pack. As the rear pinion tries to rotate upwards, the military wraps on the spring eye end and top half-leaf contact each other and the forward half of the spring pack acts like a traction bar without limiting overall spring movement or stiffness. The seven-leaf pack (top) has a lighter spring rate than the nine-leaf pack (bottom), thanks to the many, thin leaves; both ride softly in the initial rates of travel.
In contrast to the more expensive,...
In contrast to the more expensive, custom race packs with many, thin leafs, this off-the-shelf lift spring uses fewer, thicker leafs. Although two spring packs may have the same overall spring rate, using more, thinner leafs in the pack allows the spring rate to ramp up more gradually upon soft hits. The downside is higher manufacturing costs, more weight, and the possibility for greater internal spring friction between leafs.
One area where modern leaf...
One area where modern leaf springs differ greatly from their predecessors is in the various ways of combating internal friction and delivering a smooth, seamless increase in spring rate as the leaves make contact with each other. Older or heavy-duty packs may feature square-cut spring ends with no taper, but most modern light- and medium-duty springs feature Teflon or plastic anti-friction inserts along with tapered leaf edges. Diamond- or round-cut leaf ends also help reduce harshness and improve flex. The spring clamps are usually riveted in place and feature either high-movement bolt-on ends that allow a lot of leaf separation, or crimped ends which more closely hold the leafs together within the pack. Most off-roaders prefer the leaf separation afforded by the bolted clamps, but some say that the additional separation when the spring droops can induce fatigue and lead to snapped leafs.
Savvy spring builders will...
Savvy spring builders will add features such as the military double- or triple-wrap spring eye ends mentioned previously, half-leafs, progressive overload leafs, or Berlin eye (shown) to allow shackle clearance at full-stuff. The Berlin eye is more prominent in tension-type shackles setups where the shackle-to-frame mount is below the spring shackle eye (shown). Tension shackles allow a wider range of movement on the rear of the spring and the potential of more overall travel since the tension shackle can pivot in an arc greater than 180-degrees. A compression-type shackle, which mounts the shackle-to-frame mount above the spring shackle eye, won't have such spring-to-shackle clearance issues, but won't offer as much potential for spring-growth in highly arched springs since the spring eye will hit the framerail before arcing 180 degrees.