Off-Road Suspension 101

by AJ Deysel

Generally there are two main types of suspension, or rather drive-line setups. Solid axles or independent axle setups. Then suspension wise, you get four different methods of providing springing to a vehicle: coil springs, leaf springs, air bags and coil overs. For Independent suspension you basically only get coil springs and coil overs, however another older option fitted to some vehicles was tension bars.

What is suspension and what does it provide?

Suspension consists of the parts and component that suspends the wheels down from a vehicle. Suspension provides the important function of ride dampening, thus isolating the main body of the vehicle from road conditions, allowing for comfort, more grip and control of the vehicle. Thus also preventing more damage and wear and tear on the vehicle too. Suspension consists of multiple factors like steering geometry, drive-line angles, spring rates, suspension travel, flexibility and articulation, all working in unison to provide the correct ride quality, comfort and performance.

Steering Geometry

Steering geometry so that the vehicle can turn, and go in the direction that the driver intends, and do so in a stable manner. Steering geometry is set in various ways, first being the steering linkage which consist out of the tie rod (connecting the two steering knuckles to each other), the drag link (which connects the pitman arm to the steering knuckle), the pitman arm (which connects the steering box to the drag link), and the second being the steering box, which takes the input of the steering wheel, and turns the steering linkage.

There are other factors that affect steering geometry as well, like the front axle to, which can impact the steering sensitivity, bump-steer (the feedback into the steering wheel when a wheel hits a bump and could try to steer the vehicle in another direction), and the feeling of drifting over the road. An important consideration on a solid front axle (SFA) vehicle with a 3-link, or control arm based suspension setup, is to ensure that the trackbar / panhard bar, is running at the same angle as the drag link, and follows a similar range of motion during axle articulation, as this is what prevents bump steer and other dangerous steering feedback.

Drive-line Angles

Drive-line angles for instance cv angles and propshaft and universal joint angles ensure that the rotational power gets transferred down to the wheels and tires from the engine and gearbox through a transfer case. The angles have to be within their intended design angles in order for the driveline joints to work effectively.

Spring Rates

Spring rates, is that rate at which the wheel and the vehicle move closer and further away from each other. This rate need to be controlled in order to ensure driveability and traction. These spring rates are controlled by the shock absorbers and they control the rates at which the spring returns to its neutral position.

Suspension Travel

Suspension travel is the distance the wheels can go from closest to the vehicle to the furthest point from the vehicle. It needs to be limited at both ends; on the vehicles side usually with bumpstops to prevent the tires from hitting or scratching the body or the axles hitting the frame or chassis of the vehicle; and on the furthest end away from the vehicle to prevent the shock absorbers and springs from over stretching and breaking or falling out, but also to prevent too extreme angles on the driveline joints.

Flex

Flexibility or articulation, to try and ensure that the tires stay down on the ground as much as possible in order to maintain traction.

Ride Quality

Ride quality is the comfort, feel, driveability and overall quality of the suspension setups to ensure the comfort and performance required.

Comfort

The comfort in a vehicle, not only makes the ride more enjoyable, but also prevents driver fatigue and possible injuries.

Performance

Performance is determined by how effective the suspension allows for the vehicle to achieve the task required from it. How high a suspension system on a off-road vehicle is, is not the most important, and lifting too high can be detrimental to performance, as is evident by our Lift Paradox.

Drive Line Components

Solid Axles

Solid axles, being the original stalwart of off-roading, have come a long way in the past 40 years, but still maintain the same original idea, concept and design. They offer good strength, and when set up with the correct suspension, offer great traction with offering good articulation and travel, which translates to more wheels on the terrain.

Independent Suspension

Although independent suspension is a newer technology, developed to improve ride quality and comfort, it has always been regarded as being less capable than Solid Axles for most off-road disciplines except for high speed off-road racing, because of the limited suspension travel and axle strength it offers. However, with newer technology from companies like Axle-Tech, the independent suspension options are growing sometimes even offer better performance for disciplines such as rock-crawling, which used to be considered a solid-axle only type terrain.

Spring Types

Leaf Springs

A leaf spring suspension consists of a series of relatively long strips of steel attached at both ends to a frame and suspending the axle in the middle. A leaf spring is a slender arc-shaped length of spring steel of rectangular cross-section in form. 

Leaf spring suspensions are simple in terms of functionality as the axle is suspended by the spring, and does not require the additional suspension geometry and anchoring of a coil-spring set-up. Leaf springs are also much sturdier as compared to the coil springs, capable of handling heavier loads with less deflection than coils. Trucks with leaf spring suspension setups are also simpler to lift or lower.

Whilst being a simple solution, leaf springs have their pros and cons. Since the spring is attached at fixed points on the chassis, it leaves a small room for adjustment and customization of the suspension geometry. These springs also flex a great deal less than coil springs, resulting in a loss of wheel-to-ground contact under extreme conditions. They can flex a lot if some of the smaller arc load leafs are removed, but this then sacrifices the load carrying abilities of the spring pack.

Coil Springs

Coil springs, as it is called, is a rod of steel, coiled into a spring. It sits between the top of the axle and the chassis or in independent suspension applications, between the lower control arm and the upper control arm. A coil spring, also known as a helical spring, by definition, is a mechanical device which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces.

Coil spring suspensions can offer increased range of suspension movement, and allow the user a wider tuning envelope through the suspension, and increased steering angles range than leaf springs, due to the fact that it is not as long as leaf springs, so it does not protrude in front and behind the tire or wheel, so in turn it gives more clearance for the wheel or tire to turn inwards towards it. Most high-speed or high-performance applications use coil springs where possible.

Drawbacks to a coil spring suspensions setup are cost and load-bearing ability. Cost isn’t so much an issue if the vehicle is originally equipped with coil springs, significantly lifting, or changing the suspension geometry becomes quite expensive, and retro-fitting a coil spring suspension setup in a vehicle that was originally leaf spring also can be quite expensive and time consuming. Coils are not ordinarily preferred for very heavy vehicles, as the coil on axle setup isn’t remotely as stable or strong as a proper leaf spring, at the same cost. This is however starting to change with the recent advent of technology.

Air Bags

Often taking the place of leaf springs and coil springs, but also as often simply used to supplement a leaf of coil spring suspension setup, air bags also provide another option of suspending a vehicle on it’s axles. Often very cost effective where heavy load bearing abilities are required, air bags have become common place in the trucking and heavy-duty machinery industry.

Aftermarket air bags are also added to modern vehicles with coil spring rear suspensions, just used at times of carrying or towing heavy loads to supplement the coil springs preventing sag or damage to them.

Although having a shorter life span than most coil and leaf springs, air bags are relatively cheap to replace by comparison.

Shock Absorbers

Shock absorbers, simply put, absorb the shock. To explain it more clearly, they control the spring and rebound rates. Simply put, they control how fast the spring compresses and extends and eases the shock associated with such compression and extension.

Essentially, all shock absorbers work in much the same way as a coffee plunger. A piston is attached to a circular disc with holes in it, that disc moves through a cylinder containing oil which provides the damping effect through friction of the oil through the holes. The energy from the bouncing spring is converted to heat which is then dissipated through the shock to the atmosphere.

Shock absorbers fail if heat is generated more quickly than can be dissipated, then the shock heats up to the point where the damping no longer works effectively, either from the oil becoming more viscous from the heat, or the oil foaming up, which then causes the friction to lower and the piston to move quicker through the shock tube. This affects both ride and handling, and can ultimately lead to complete shock failure. This is one of the main reasons why, when 4wd vehicles are used for more than just your average road and highway commute, one of the first things to upgrade, is the suspension system, starting with the shock absorbers.

Gas Shocks

Shocks are designed to typically use a compressible, but heat-resistant gas such as nitrogen so when the piston goes down, the gas is compressed.  Both monotube and twintube shocks work on that same basic principle.

Foam Cell Shocks

It is a twin-tube design, but instead of pressurized gas it uses a foam layer made of cells filled with nitrogen which compress when the piston enters the shock cylinder, and decompresses when it exits. One advantage of this design is that the oil is spread more evenly around the shock body, as opposed to gas shocks which compresses the gas into a small area of the shock. This means the surface area of the oil against the tube is increased, allowing for greater heat dissipation. Another advantage is that there’s no way gas and oil can mix, so aeration cannot happen. The foam cells also take up less room inside the shock tube that the gas in a conventional monotube, which means there’s more space for oil and the more oil you have, the cooler the shock runs.

Oil Shocks

Remote Reservoir Shocks

On high-end of the spectrum is the remote or external-reservoir design. These shocks, primarily designed for ultra-high performance and racing applications, use a small, lightweight shock body that is connected through a hose to a reservoir of oil mounted in a different part of the vehicle. This reduces the weight at the wheel without compromising oil volume, provides more oil which provides for better cooling and heat dissipation, provides more air space within the reservoir for heated oil to expand without blowing seals and lastly allows adjustability of the flow to and from the shock body.

These added benefits come at a significant additional cost to produce, and as such, are usually overkill for most users. The benefits of them only become worth while in high-speed or high performance areas.

Coilover Shocks

Coil-over shocks, are shock where the coil spring is integrated with the shock absorber, with the coil spring running either outside around the shock, or inside the shock tube, thus requiring only one mount for both on either end of the suspension mounting points. The not only offer more clearance, but often because of the nature of the design, also allow design for more suspension travel as the coil spring cannot easily slip out at full extension. Again coming at a significant added cost of design and production, these shocks are usually only used in high performance or specific applications.

Adaptive / Semi-Active Shocks

Adaptive or semi-active systems work by changing the viscous damping coefficient of the shock absorber, by changing the viscosity or consistency of the oil, or medium used inside the shock absorber to control the rate of movement.

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