March 28, 2023 7 min read 2 Comments
Modifying your truck’s suspension is the most effective way to increase its performance off-road. But, suspension is a complex system in which each part must work together, and what we mean by “performance” is often misunderstood. With a lack of effective information, it’s common to see drivers spend a bunch of money on suspension components only to reduce performance. Let’s fix that.
It’s the job of a vehicle’s suspension to support weight. But because that weight must be supported in a dynamic environment on a moving vehicle, that simple job rapidly becomes incredibly complex.
Suspension connects your vehicle’s frame—its sprung components—to its unsprung parts—wheels, tires, brakes, axles, and some of the stuff it takes to steer. Ideally, those sprung components will move as as little as possible, while all that unsprung stuff moves as fast as possible. Making that even harder, the orientation of unsprung to sprung parts changes as each moves, so in addition to the obvious parts like springs and dampers, there’s also a long list of equally important, but less celebrated stuff like control arms, sway bars, and a whole pile of bushings and bearings that must all work together with every other component to create a harmonious whole, even as thousands of pounds cycles up and down, in and out, and back and forth multiple times each second.
This is the live rear axle on a Ford Bronco. Here, you can see the large axle supported by coil springs, and located by a diagonal panhard rod, and the lower trailing arms on each side. (Photo: Ford)
Most new 4x4s include both types of suspension. Let’s briefly explain how each works.
Independent suspension, as is found at the front of most pickup trucks and SUVs, uses control arms bolted to the frame to control the up and down path of the wheel. This is great for ride quality and handling, as each wheel on an axle is free to move separately. But adding weight to an independent suspension alters its geometry, which limits outright capacity. Articulation is also typically limited by the short arc through which the control arms are able to travel, and because the wheels move independently, ground clearance at the front axle is constantly changing.
Live axles are much simpler. On one, the wheels remain parallel no matter the load, and when one wheel moves up, its counterpart moves down an equal amount, so ground clearance remains unchanged. This movement is typically able to occur independently of the body, which is free to remain relatively level even as the axle moves underneath it. But while articulation is typically superior, the unsprung weight is higher and both wheels are impacted by a given bump or obstacle, so ride and handling are typically inferior.
This is the independent front suspension on that same Bronco. Two control arms pivot on the frame, while a coil spring encloses a shock bolted to the lower control arm and frame.
It’s the job of a spring to support weight while facilitating movement. As such, the length and weight (in this case referring to how stiff the spring is, not its measurement on a scale) are a spring’s variables. Run too heavy a spring, and too much of the up and down movement of a wheel will be transferred to the body and frame. Run too light a spring, and the weight of the sprung components will sag.
Without a damper, that spring will continue to move uncontrollably with any force applied to it. The job of a shock is to dampen and control spring movement. Shocks do that by moving a piston through oil. The rate at which that piston moves determines how much dampening it’s able to apply. In an ideal world, a shock will enough dampening so that movement of the spring is limited to the duration and size of a given bump—the spring will cycle the movement of the wheel, then return to its static state. This is rarely achieved on a stock vehicle, and is one of five primary goals with modification.
Wheel Travel: By increasing the distance a wheel is able to travel you increase articulation and improve ride quality. The tire will be able to remain in contact with the ground across more extreme obstacles, and bumps of a given size will use less of the suspension’s travel, improving ride quality.
Spring Rate: The amount of weight a spring is designed to support is always a compromise. From the showroom floor, a vehicle must prioritize ride comfort even with only the driver onboard, while also attempting to achieve the ability to carry a full compliment of passengers, and heavy cargo safely. On a passenger car, that weight range is relatively small. On a big pickup, there may be thousands of pounds between and empty and fully laden vehicle. That’s why pickups typically ride so poorly in stock form. If you’re modifying a vehicle with permanently installed, heavy parts like bumpers, and campers, or routinely carrying a heavy load, then replacing the springs with ones tailored to your unique weight will eliminate sag, restore (or improve!) full articulation, make handling safe, and improve ride quality. If you're carrying a range of weights, then progressive springs may help deliver both unladen ride quality along with the ability to support heavy loads.
Heat Dissipation: Shocks translate movement energy to heat, so the ability to dissipate that heat is key to their ability to control the springs. This is the single largest limiting factor stock equipment brings in off-road environments. And adding weight worsens the problem, since more weight equals higher forces equals more heat. If a shock overheats, it will stop controlling spring movement, and your vehicle will begin to bounce uncontrollably.
Dampening Rates: Spring and dampening rates are related. A stiffer spring will require more dampening, as will more weight on the vehicle. This can then cause issues because too high a dampening rate—which may be necessary for safety on a heavily loaded vehicle—will then limit the small movements necessary to provide good ride quality. This is where adjustable dampening and bypass suspension enter the picture.
Ride Height: Lift. By increasing the height of the suspension you can improve approach, breakover and departure angles while making room for larger tires.
In this Old Man Emu BP-51 suspension system, we can see rear leaf packs (for solid axle rear suspension) with added leafs designed to support more weight, front coilovers (for an independent front suspension) with remote reservoirs to facilitate additional travel and improved cooling, and rear shocks complete with adjustable compression and rebound damping rates. (Photo: ARB)
Wheel Travel: On an independent suspension, you can improve travel through both the shocks and control arms. Oil inside a shock is incompressible. So, a remote reservoir attached to the shock body can provide a place for that oil to escape as the shock piston compresses, increasing the stroke length of that piston. This needs to be paired with an upper control arm designed to operate at more extreme angles, so that arm doesn’t bind against the steering knuckle as it moves downward. Such a setup is typically referred to as “mid-travel,” and it normally increases wheel travel by 33 percent or so.
You can also improve articulation by fitting longer upper and lower control arms, to create a “long-travel” system. A wider arc is a longer arc, so travel can be improved dramatically. But this must be paired with extended CV axles and tie rod ends, may not be compatible with a sway bar, and often requires new fenders be added to the body to give the tires room to move. Such systems are expensive, complicated, require frequent maintenance, and may not be easily repaired away from your home shop.
On a live axle, travel can be improved by replacing the springs with longer items, and modifying or replacing the components connecting the axle to the frame if those springs are coils.
Another way to improve wheel travel is by removing or disconnecting the sway bar, if one is present. Sway bars are another item that must exist in constant compromise, and which are typically victims of cost cutting. Removing a sway bar entirely, especially on a lifted truck, can cause unsafe amounts of body roll on the road. Leaving a sway bar fitted can artificially impair wheel movement off-road. Many of the most capable off-roaders feature some sort of sway bar disconnect, allowing you retain safety on the road, and articulation off. Such disconnects are also available in the aftermarket.
Spring Rate: Much simpler to talk about. Just figure out your all-up weight, and order a spring designed to support it. Fixed rate springs are best if you’re typically carrying a set weight. Progressive rate springs allow for varying loads.
Heat Dissipation:That same remote reservoir you need to add wheel travel also helps cool shock oil. Adding an oil line and a reservoir canister drastically increase the surface area, evacuating heat into the surrounding air. In extreme cases, cooling fins can take cooling capacity even further.
Dampening Rates: Channels within a shock piston allow oil to flow through it at a set rate. On aftermarket shocks, these channels are controlled by shim stacks. Varying their number and the size of the apertures within them allows shock makers and suspension shops fine control of damping rates.
High end shocks may also include bypass channels, which alter damping rates during different phases of a shock’s travel by allowing oil to flow around the piston at certain points in its movement. Damping may also be externally adjustable, which allows you to tailor performance to changing conditions or weights.
Ride Height: The amount of total additional height you add to a vehicle is determined by both suspension lift and tire size. It’s common to hear people talking about lift heights as a user-determined value, but often they’re simply the product of best practices applied by a suspension maker. Because height is added to an independent suspension partially by pushing its control arms downwards, you don’t want to lift it too far, or you risk pushing the CV axles into too extreme an angle, and also reduce available down travel. Two inches is usually the right amount. Things are a little more variable with live axles, but unless you’re designing your own custom setup, you’ll be limited to the height ranges of production suspension kits, which will start at about two inches and maybe go up to five in extreme cases.
On trucks with IFS front and live axle rear suspensions, always try to lift both ends of the truck by comparable amounts. Simply pushing the front end of a truck skywards is a cheap solution that too often leaves rear axle performance falling well behind the front.
All this sound complicated? It is, and replacing components that must cycle dozens of times a second for hundreds of thousands of miles also requires much care and expertise. GFC works with each of its partner shops to help customers find the best possible solutions for their own unique needs, and those shops will be capable of performing safe, honest work. And you can get advice from other GFC owners on our forum.