The suspension system is one of the most important components of any vehicle. The system must be engineered to handle expected and unexpected conditions when driving. When considering suspension systems, be prepared to make compromises regarding ride quality, load capacity, price, space and durability.
There are a variety of suspension types on the market today and they all function differently, each with their own strengths and weaknesses. Here is our guide to understanding them.
Independent Versus Solid Suspensions
It’s important to understand the differences between independent suspensions and solid (also called live) axle suspensions.
On independent suspensions, the wheels on an axle act independently from one another. Nearly all vehicles today, apart from some heavy-duty full-size pickups, have independent front suspensions. If one side hits a bump or a dip, it acts independently of the other side to compensate for the disturbance. This helps to smooth out the ride and keep it from rocking from side to side as it does with a solid axle suspension.
Independent suspensions can free up space for other components in certain applications, such as the rear cargo area of SUVs. Independent suspensions have higher ground clearance, which makes them good for off-roading, and a smooth ride during uneven desert running. For example, the military M1152 HMMWV has 18 inches of ground clearance underneath the center differential. The downside is that in most cases, independent suspensions offer less wheel travel than solid axles do.
When pickup enthusiasts hear the word axle, often the first thing that comes to mind is solid axle. A solid axle connects the wheels with a bar or a tube. If it is a driven axle (meaning it transfers power from the engine through the shafts), the differential will be placed somewhere in the middle of the axle. When a wheel on a solid axle hits a bump or a dip, it directly affects the wheel on the other side of the axle. This can make the vehicle rock from side to side and create a rougher ride. Solid axle suspensions are usually less complex than independent axles; they often are thought of as more durable and easier to work on. That said, many military vehicles — which see some of the worst driving conditions in the world — have independent suspensions.
When driving off-road, solid axles create more articulation. The increased articulation comes from the pivot point that is created when one wheel is on higher ground that the other. The pivot point is where the spring suspension connects to the axle; this motion can apply more load to the lower wheel, increasing its traction. Solid axles have lower ground clearance, but they are still preferred for rock crawlers for their extra articulation and strength.
Coil Springs Versus Leaf Springs
Coil springs can be used in both independent and solid axle suspension systems. They are the most common spring used for suspension on vehicles today, and they are useful in a wide variety of applications. They can have a variable spring rate, meaning that the more the spring compresses, the stiffer it becomes. This helps create a smooth ride while allowing a pickup to carry a heavy load.
Coil springs weigh less than the longer leaf springs and can provide lots of articulation. When used in solid axle suspensions, coil spring suspensions don’t have nearly as much wheel hop as leaf springs do. This is because control arms are used to keep the axle in place. Coil springs typically aren’t as durable as leaf springs and can sag over time. Coil springs also are used in coil-over suspension systems, where the shock is placed inside the coil spring to create a compact suspension system. Coil-overs can be found in SUVs such as the Ford Expedition and Toyota Sequoia, and pickups such as the Titan and Titan XD.
Leaf springs are the oldest type of automotive springs. They are used on solid axle suspensions and are currently only being made for the rear axles of pickups, vans and commercial vehicles. Leaf springs hold the axle in place as well as suspend the weight of the vehicle, which means control arms are not necessary. Like coil springs, leaf springs have a variable spring rate and can even have extra springs, called overload springs, that only engage once the vehicle has been loaded to a certain weight. More leaves can be added for extra stiffness and load-carrying capacity. If one leaf breaks, there likely will be enough support left in the other leaves to get the vehicle to a repair shop without damaging it. The disadvantages of leaf spring suspensions are that they can be noisy, they ride roughly and are prone to wheel hop.
Torsion bars, also known as torsion springs, aren’t as common as other suspension systems; however, GM still uses them on its pickups. Torsion bars are only used in independent suspension systems. Torsion bars are long, round bars that act as a spring because of their twisting strength. One side of the bar is mounted solidly onto the frame, while the other end is mounted into the lower control arm of the front suspension. Torsion bars resist wheel movement by twisting when a wheel moves up or down. They are the simplest automotive spring. Like leaf springs, they are durable and will likely outlast the vehicle. The drawbacks are that they don’t have a lot of wheel articulation when compared with coil and leaf springs, and they don’t have a variable spring rate.
Air Springs/Air Bags
Air suspensions are the most complex suspension systems offered. They include valves, an air compressor, air lines, height sensors, control modules, air reservoirs and the air springs themselves. Air springs are used on both independent and solid axle systems, and air bags are most often used with solid axles in the rear suspension to level a loaded vehicle.
Air suspensions are the most versatile suspension systems. Vehicle ride height can be adjusted on the fly. This means when the vehicle is in a rough off-road spot, the suspension can lift it to give it more ground clearance. When cruising down the highway, the suspension can lower the vehicle to reduce drag and increase fuel economy.
Many air suspension systems are included with optional packages that have shocks with adjustable damping as well. Between the adjustable air suspension and adjustable shocks, the user can set the vehicle to ride like a 1980s Lincoln Town Car or like a one-ton diesel truck, depending on his or her preference. Other benefits include leveling the vehicle when it is loaded, and lowering the vehicle for exit/entry and cargo loading. However, these systems make it easy to overload a vehicle, they are expensive to repair and there is a higher likelihood of a suspension component malfunctioning.
Summing Things Up
Automakers design vehicles to meet the needs of the most people possible so that they can sell more vehicles. This means that most SUVs will have independent front and rear suspensions for a smoother ride on the road. Heavy-duty trucks will at least have a rear solid axle; Ford and Ram HDs have a solid front axle as well. Most HD trucks have leaf springs in the rear. Luxury vehicles and some factory off-roaders such as the Jeep Grand Cherokee, Land Rover Discovery and the Ram 1500 Rebel have air suspension with adjustable-ride height.
When selecting a new vehicle, seriously consider how it will be used and then determine which suspension system will work best for those applications. That could narrow your choices considerably. The suspension system most likely won’t be the deciding factor on a new- or used-vehicle purchase, but it should play an important part in that decision.
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