Understanding suspension for 4×4 performance
For buyers of AWD vehicles, choosing a drivetrain appropriate to their needs is only one consideration. Less understood is the suspension’s influence on driving dynamics, off-road capability and comfort.
Image: Ford
For those requiring proper 4×4 and off-road capabilities, the choice comes down to vehicles offering part-time or full-time AWD, with low-range gearing, diff locks, and brake traction control all contributing to the vehicle’s capability. While the choice of drivetrain will determine how, and under which conditions, drive will be delivered to each wheel, it is the suspension that has a major influence on how that drive is effectively transferred to the tyre contact patch. Its performance largely defines not only a vehicle’s capability in more extreme off-road conditions, but also its on- and off-road driving dynamics and comfort.
Types of suspension
At the most fundamental level, 4×4 suspension designs can be categorised as being either independent or solid (live) axle, with different types often being fitted to the front and rear. Each offers distinct advantages that influence such things as off-road performance, comfort, handling, durability, and maintenance.
Independent suspension allows each wheel to move vertically without affecting the opposite wheel. This allows them to maintain optimal contact with uneven road surfaces, reducing vibration and improving traction during cornering or high-speed travel. On rough gravel or corrugated dirt roads, it enhances driver comfort and directional stability, while its lower unsprung mass – the mass not supported by the suspension – translates to a quicker response to bumps and better absorption of surface irregularities, which also benefits braking. On the downside, it is mechanically more complex, involving multiple arms, joints, bushes and driveshafts, which can be more prone to wear and damage in harsh off-road conditions. The design also generally offers less wheel articulation, which can be countered to a limited extent by increased ride height or the fitment of height-adjustable air suspension.
A solid axle (live axle) connects both wheels on an axle with a single rigid beam, meaning that the movement of one wheel affects the other. This setup is recognised for its strength, simplicity, and durability. It excels in low-speed, technical terrain, such as rock crawling or deep ruts, because it allows greater articulation and traction. The design also provides superior ground clearance under the differential since the axle housing moves as one unit. Because of their robustness, solid axles are preferred in vehicles designed for heavy loads and expedition use, where reliability outweighs their harsher ride quality and less responsive on-road handling. Maintenance is also easier, as solid axles contain fewer moving parts exposed to dirt or debris.
Only a few off-road-focused vehicles like the Jeep Wrangler or Toyota Land Cruiser 70 Series still retain solid front axles, where their articulation, ground clearance and traction are prioritised over the all-round performance of independent – typically double wishbone – designs. In contrast, live
rear axles remain a common fitment, located by leaf springs in the case of most double-cabs or double-cab-derived SUVs, while others utilise coil springs and locate the axle with control arms and a Panhard rod or Watts Linkage.
Image: Peet Mocke
Stabiliser bars
Also known as sway bars or anti-roll bars, stabiliser bars provide stability and control body roll but unfortunately have a negative effect on off-road performance. Linking wheels on the same axle, they operate in torsion, controlling body roll by limiting the relative movement between the wheels. In an off-road situation, this means limiting the travel of a lightly loaded wheel, compromising its ability to extend and retain traction.
On some more sophisticated 4x4s this is overcome by a de-coupling mechanism incorporated into the stabiliser bars or one of the links connecting it to the suspension. This allows each wheel to articulate freely when disengaged. While disengagement is often controlled by a switch, for safety reasons, re-engagement is often automatic and linked to vehicle speed.
Shock absorbers
In simple terms, the primary role of springs in a vehicle suspension system is to support the vehicle’s weight and absorb impacts from road irregularities by compressing and rebounding – storing and releasing energy – which helps to smooth out bumps while maintaining ride height and stability. They work in conjunction with shock absorbers, also known as dampers, which control and dampen the motion of the springs by converting the kinetic energy generated by spring compression and rebound into heat, thereby reducing excessive bouncing, and ensuring better tyre contact with the road, improving handling and ride comfort.
They perform a vital role in both technical off-roading where long-stroke wheel movement needs to be properly controlled, and in overlanding where higher loads, combined with higher speeds over longer distances, have the potential to cause heat buildup and fading, leading to compromised handling and vehicle control. Several types of shock absorbers are available, each with unique designs and advantages, the main types including twin-tube, foam cell (foam-filled), monotube, remote reservoir, and adaptive (adjustable) shock absorbers.
The most common type fitted as original equipment to 4×4 vehicles is the twin-tube design which consists of an inner working tube with a rod-mounted piston and a foot valve, while an outer reserve tube holds additional hydraulic fluid and a reservoir of low-pressure nitrogen gas to help prevent aeration of the oil. With suspension movement, oil is forced through valves in the piston and the foot valve, providing the damping effect. This can be tuned for different rates of suspension movement (piston speeds) by varying the valve characteristics. On high-performance units, these valve parameters can also be varied at different stages of suspension travel using internal bypass valves.
Twin-tube shock absorber (left) and monotube shock absorber. Image: CAR magazine
Regular twin-tube units provide good ride quality for mixed road- and light off-road use, their design making them less prone to impact damage, but relatively poor heat dissipation can lead to fading on long, rough trails or heavy loads. This is improved with larger-diameter, increased-oil -capacity units, or the use of foam cell designs, where the nitrogen gas in the outer tube is replaced by a foam sleeve, further reducing the likelihood of oil aeration and improving performance in severe conditions.
Monotube shocks feature a single cylinder, also with a rod-mounted piston, but include an additional floating piston separating the oil and a high-pressure nitrogen gas chamber. The monotube improves heat dissipation, which improves damping consistency and is less prone to fade, but is also more susceptible to impact damage. Some monotube designs include an external reservoir which increases oil and gas capacity, further reducing temperature buildup and helping to maintain damping performance under prolonged use in harsh driving conditions.
Both twin and monotube designs can feature manually adjustable, driver-selected, or dynamic electronic damping systems. These allow the driver or suspension control unit to change valve characteristics and optimise damping force for different operating conditions.
Manufacturers design and calibrate the suspensions of their 4×4 vehicles to accommodate the requirements of their intended customers. Fortunately, for those owners wishing to optimise their vehicles for specific operating conditions, a wide range of specialist aftermarket shock absorbers, springs and bushes are available to meet their specific needs.
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