Leaf springs are one of the oldest forms of suspension still on the market today. As opposed to coilovers or MacPherson struts, leaf springs are a considerably more straightforward way of absorbing the shocks and bumps of the open road. In fact, the concept of leaf spring goes all the way back to the 18th century in England where it was used to make carriages less bone-rattling. This suspension even appeared on the Ford Model T.
Even though the leaf springs are time-tested and simple, they have persisted for so long for a reason. They are easy to construct, and unlike other modern forms of suspension, they spread the weight of the vehicle out over a wider space. This is why leaf springs are the defacto suspension system for larger vehicles, trucks, vans, and some off-roading vehicles.
The leaf springs themselves can't do all of their work alone, however. Without a free-hanging connector called a shackle, the leaf springs would cause more trouble to the vehicle than they fix. This small but critical suspension component is important to understand if you drive a vehicle that uses leaf springs.
How Leaf Spring Shackles Work
The shackle on a leaf spring set up is there to compensate for the leaf spring's change in length as it flexes. In their default state, leaf springs are slightly curved. As pressure is exerted on the leaf springs, they flatten out, effectively becoming longer. Herein lies some of the geometry involved with leaf springs and leaf spring shackles. If the leaf spring shackle were not present, then the leaf spring would have nowhere to go when it flattens out. This means that it couldn't function as a suspension system and would be considerably more rigid. By being able to move, the leaf shackles can account for the changing length of the leaf springs themselves.
Risks and Problems with Leaf Spring Shackles
Shackles do come by default on vehicles with leaf spring suspension, but they don’t last forever, and can potentially break to some pretty nasty results. A common problem with leaf spring shackles is that they can be overtightened. Since their functionality is dependant on their freedom to move with the leaf springs, overtightened leaf springs will not be able to prevent this from occurring. It's recommended to tighten the shackles until they are snug and then loosen them by a quarter turn.
Leaf spring shackles run the risk of becoming inverted and moving past their designed axis of movement. Once this happens, the force of the leaf springs keeps the leaf shackle locked in place, consequently negating any of the benefits of the leaf springs themselves. This can cause major damage to your vehicle if it isn't quickly corrected. In the following video, you can clearly see the passenger side leaf spring shackle invert, making a loud snap and kicking up a small cloud of dust. While leaf springs are good for a variety of offroading applications, they can fail in this way if pushed past their limits.
If your leaf spring shackle becomes inverted, it’s certainly not the end of the world. After lifting the vehicle off of the ground, a crowbar or any similar source of leverage can be used to right the shackle and return it to its original position. This correction should not be done cavalierly. Ample caution is necessary since there is enough energy stored in the inverted leaf spring to fling a crowbar across the room.
If this is a recurring fear of yours when off-roading with a leaf sprung vehicle, then there are anti-inversion leaf spring shackles that can be installed. These specialty shackles physically don’t allow the shackle to move past its intended range of movement, thus preventing them from bending too far and ruining your day on the trails. There are also leaf spring shackles that have braces welded to them to prevent any excessive side to side movement and improve the vehicle's handling.
Specialty Leaf Spring Shackles
There are plenty more purpose-built leaf spring shackles. The YJ Wrangler uses a particular type of rear shackle that's called a boomerang or a dogleg shackle that allows for both a longer leaf spring as well as being able to avoid parts of the Wrangler's body. There are sliding shackles, super heavy-duty shackles, and shackles that are meant to be greased to name a few.
There are more types of specialty shackles available as well. One of the more popular reasons to ever tamper with the leaf spring shackles besides wear and tear is to install longer leaf spring shackles. This modification effectively increases the performance of the leaf springs by giving them more room to travel. Not only does this slightly increase the ride height of the vehicle, but it can increase the pinion angle. These benefits are not without a cost, however, as the approach and departure angles decrease with larger shackles.
Although they are simple, Leaf springs provide robust articulation. While fancy coilovers and air-ride suspension systems are top of the line, high-tech suspension solutions, there are plenty of merits to the simplicity of the leaf spring approach. Because of the role of the shackle, leaf springs are quite dynamic and can be modified or upgraded to fit all of your driving needs.