How Leaf Springs Work

Before you start your towing trip, it's a good idea to go over a brief checklist -- for safety's sake. You take a good look in your mirrors, adjusting them correctly in order to see passing traffic on the road. You've chosen the correct hitch and connected the towing vehicle to the trailer properly. The brake lights and braking systems are working synchronously, assuring you of the ride's legality. With everything loaded up, you're pretty confident the truck is ready for the job, so you head out on the road toward your destination. Once you reach a steady speed, however, the trailer behind your truck starts to bounce and sway a little more than it should. Pulling over to the side of the road, you rack your brains to figure out what you missed. You start to wonder if your cargo weight is maybe too high -- but what can you do about it?

In this situation, if there's too much cargo weighing down a towed vehicle, causing everything to rock and sway, the issue may be with the suspension. If a truck's suspension is too rigid, its wheels will often leave the pavement after hitting bumps; a good suspension, on the other hand, keeps the wheels on the ground as much as possible. Many towers use leaf springs to stabilize their towed load and to keep their cargo grounded.

Although you may not ever have heard about or even noticed leaf springs on larger tow vehicles, the technology has been around for centuries and is one of the earliest forms of suspension. Even Leonardo da Vinci used leaf springs in his diagram for a self-propelled car. But how do they work? Are there different types of leaf springs? And how do you install them onto a vehicle?

Choosing Leaf Springs

Compared to most automotive technology currently available, leaf springs don't look too fancy. They're simply long and narrow plates attached to the frame of a trailer that rest above or below the trailer's axle. Slightly curved, they look a little like a metal bow from an archery set, except without the string.

Leaf springs come in several different varieties. There are monoleaf springs, or single-leaf springs, that consist of simply one plate of spring steel. These are usually thick in the middle and taper out toward the end, and they don't typically offer too much strength and suspension for towed vehicles. Drivers looking to tow heavier loads typically use multileaf springs, which consist of several leaf springs of varying length stacked on top of each other. The shorter the leaf spring, the closer to the bottom it will be, giving it the same semielliptical shape a single leaf spring gets from being thicker in the middle.

Leaf springs also have different ends, depending on where they're connected to the frame. On double-eye leaf springs, the top plate -- and therefore the longest -- has both ends curved into a circle. The ends make two holes, which you can bolt to the bottom of a trailer's frame. Open-eye leaf springs have only one "eye," or open hole. The other end of an open eye leaf spring can be a hook end or a flat end.

Properly installing leaf springs will ensure the best suspension for your tow vehicle; read about leaf spring installation on the next page.

Installing Leaf Springs

Leaf springs are positioned behind the wheel -- you can't see them clearly when they're attached unless you crawl under the trailer -- so the wheel needs to be taken off completely before installation begins. Once the wheel is off, you should notice a pair of hangers on either side of the axle. The hangars are usually small, extended pieces of metal with holes that hang down from the frame of the trailer. Between the holes of these hangers is where you line up the eyes of a leaf spring, securing them with the proper hardware. If the leaf spring is a double-eye, one hanger will have a shackle link.

Once the leaf springs are bolted into the hangers, the next step is to install a pair of U-bolts, two horseshoe-shaped metal rods. The leaf springs can be overslung (placed over the axle) or underslung (placed under the axle). Either way, the U-bolts fit around the axle and secure to a metal plate that rests against the leaf springs. The purpose of the U-bolts is to keep the leaf springs flush against the axle so they take the weight of the cargo and don't move around too much during driving.

It's always important to check leaf springs and their accessories for wear -- a worn or cracked leaf spring can cause untold damage to a trailer and its contents. When taking leaf springs off, it's a good idea to check the shackle links for wear. A round ring of wear that matches the shape of the nut is OK, but once the shape becomes an oval, it's time to replace the shackle link. Proper care of leaf springs will ensure a safe ride while towing by keeping the trailer's wheels on the ground and its cargo in place.

For lots more information on towing and towing accessories, see the next page.

How Leaf Springs Work

Choosing Leaf Springs

Installing Leaf Springs

Lots More Information