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The Wonder Of Spring-Under Suspension - 4-Wheel & Off-Road Magazine

Fred Williams Brand Manager, Petersen’s 4Wheel & Off Road

About 20 some years ago the trend for off-road leaf springs moved above the axle. The consensus was that if you were going to get crazy in the dirt you needed to have a spring-over conversion. There were plenty of 4x4s still running a spring-under axle suspension, but the hardcore guys all went spring-over. Spring-over gave them great flexibility, articulation, and ground clearance. But it often also resulted in axlewrap, worn-out springs, and problems with driveshafts.

the Wonder Of Spring Under Suspension prerunner Toyota Tacoma Photo 43831355

High-speed off-road guys were the first to return to a spring-under axle suspension—and with good reasons. Running the leaf springs under the axle helps lower the center of gravity, improves axle control under hard acceleration, allows the axletube to come up farther since there are no leaves between it and the frame, and with proper springs can still allow great wheel travel. Plus, most of the prerunner-style builds were less concerned with ground clearance when compared to rockcrawlers.

the Wonder Of Spring Under Suspension spring Over Suspension Flex Photo 43831370

In a spring-over axle scenario the leaves have a hard time controlling axlewrap, especially with a long, flat, flexible leaf spring, due to increased leverage. Spring wrap occurs under hard acceleration with large tires and good traction, such that the pinion of the axle wants to move upward in the opposite direction of the tire rotation. This in turn can twist or “S” the soft leaf springs. Shock placement (one fore, one aft on the axletube) and antiwrap bars can help counter axlewrap but can hinder axle movement, ground clearance, and articulation.

We’re not here to tell you to ditch your spring-over suspensions, but we don’t want you to write off spring-under as a bad option either. We visited Deaver Spring, King Shocks, and Randy Ellis Design to discuss how and why spring-under isn’t as antiquated as you might think.

the Wonder Of Spring Under Suspension spring Under Suspension Flex Photo 43831373

A spring-under suspension usually has more arch to the spring, and this helps control the axlewrap because it is harder to “S” the springs. Mounting the springs under the axle does reduce ground clearance, but at the same time it lowers the height of the weight of the springs and in effect lowers the center of gravity of the suspension. PhotosView Slideshow Randy Ellis has a well-built Toyota Tacoma, which he calls his adventure truck. It goes fast, hauls gear, and can still crawl over moderate trails with the help of the ARB Air Lockers. His rear suspension came from the factory spring-over, but he opted for a set of Deaver leaf springs under the axle and King bypass shocks.

Randy Ellis has a well-built Toyota Tacoma, which he calls his adventure truck. It goes fast, hauls gear, and can still crawl over moderate trails with the help of the ARB Air Lockers. His rear suspension came from the factory spring-over, but he opted for a set of Deaver leaf springs under the axle and King bypass shocks.

When hoisted up by a fork lift the Deaver springs flex very well. This is because rather than a few thick springs the pack is made up of many thinner leaves. Each spring pack is made of American-made 5160 spring steel with special heat treat to help it keep its arch. Each leaf is also angle-cut at the ends and uses antifriction pads that help each leaf move with ease.

The Deaver leaf pack also has a military wrap at the hanger end. This helps strengthen the main leaf and reduce the chance of bending it. Plus, it offers a security measure in the unlikely case that you break the main leaf.

On Ellis’s truck the leaf springs were mounted below the framerail. This allows him to install his shocks outboard the frame, and as close to the wheel as possible. The closer the shocks mount to the wheels the better they can control the movement of the axle.

With his shocks mounted outboard the frame, Ellis built a shock mount on the frame that is tight to the bed side and not very tall. The shocks need to be angled forward to keep them from crashing into the tire under articulation, but this also means you can run a slightly shorter shock than one mounted straight vertical and still get the same travel.

Another benefit of mounting the shocks outboard is it allows you to keep the bed for hauling stuff. Shocks that are capable of controlling the large amounts of travel offered by Deaver springs often have to come through the bed to get the axle to full compression. This results in a cluster of tubes for a bed cage and zero space for hauling large objects. Ellis’s bed has tubes that tie into his rollcage, but they still allow ample storage as compared to some prerunner style trucks.

Here is a Tacoma with a factory spring pack above the axle. The addition of small lift blocks only increases the leverage of the axle on the springs and can result in spring wrap. However, spring-over is the clear advantage in ground clearance.

These Deaver leaf springs may look like they will ride rough as a hay wagon with the massive arch, but don’t let that deceive you. The thin leaves and high-quality material will allow these springs to go nearly flat at full compression and still return to their ride height. Then when the axles droop they can go to full arch, resulting in around 16 inches of wheel travel. And copious wheel travel (when properly controlled) is important for going fast off-road because it allows your wheel to follow the irregularities of the terrain and your shocks to control the movement.

Mounting Deavers under our ’03 Avalanche, Ellis and crew frenched the springs inside the framerail, helping to keep the entire truck low and reduce the chance of getting hung up on the spring perch should this soon-to-be go-fast truck find itself on a boulder field.

The large arch of the Deaver springs and supple construction mean that the springs will be very long from eye to eye at full compression and very short at full droop. Thus we’ll need a shackle that allows this much movement. To determine where the shackle will swing, we disassembled the spring pack down to just two leaves and cycled the suspension. We eventually modified the rear of the frame and mounted the long shackle above the frame to get enough swing.

To control the Deavers, we’ll be fitting a set of King 21⁄2-inch bypass shocks. We initially went with a 16-inch-travel shock but soon realized that although the axle will move 16 inches we cannot fit 16s without getting into the bed and ruining the utility of the truck. By going with a shorter shock and mounting it outboard the frame and at an angle we’ll be able to keep the wheel travel without filling the bed with a tubular bed cage. This reminded us of a very important step when building any custom suspension: Wait to order your shocks until you know exactly what wheel travel and space your truck will afford. Otherwise you’ll be swapping shocks, as we will be.

The King bypass shocks are built to spec for each application. The bypass shocks are unique in that they allow multiple valving settings throughout the stroke of the shock. In a standard shock the internal shock oil must all pass through the piston of the shock shaft as it moves up and down in the shock body. But often you want a softer shock valving in the first few inches of travel up and down and then a more aggressive valving as it goes to full compression or full droop. (The monster shocks in the background are Kings built for desert racing Trophy Trucks.)

With a bypass shock the shock oil is pushed through the external bypass tubes as the shock shaft compresses or extends, and then fed back into the body of the shock on the other side of the piston. The rate the oil passes through these tubes can be adjusted easily without removing the shock from the vehicle via an external adjustable check valve.

We had a set of shocks made with two compression tubes and one rebound tube. This gives the shock three different compression settings so it can be flexible at ride height, get stiffer as the suspension compresses, and then ramp up to extremely stiff at full compression, allowing the shock to act as a bumpstop. The single rebound tube offers two separate rebound or extension settings for the shock. This can allow the axle to drop away quickly and then slow before full compression so it doesn’t kick up the back of the vehicle.

The reservoir on the Kings is piggyback-style, meaning it is attached directly to the shock body. This makes it easier to mount on the vehicle as long as there is room for the larger shock and reservoir combo. The reservoir is plumbed into the bottom of the shock so that the top of the shock stroke can act as a bumpstop as the suspension goes to full compression. In a future issue we will delve into the how-to of shock valving and tuning.

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The Wonder Of Spring-Under Suspension - 4-Wheel & Off-Road Magazine

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