Suspension on a heavy goods vehicle has two jobs that pull in different directions: it must keep the wheels in firm contact with the road for safe handling and braking, while insulating the load and the driver from the shocks of an uneven surface. On a modern long-haul tractor this is achieved with separate suspension systems for the cab and the chassis, supplemented by devices that control the way the body leans in corners.
The cab is not bolted rigidly to the frame. Instead it is carried on its own dedicated cab suspension, so that the road and chassis inputs reaching the occupants are filtered a second time, independently of whatever the chassis is doing beneath it. A common arrangement uses a four-air-bag mounting — an air spring at each corner of the cab — with automatic levelling that keeps the cab sitting square regardless of load or road attitude. The result is that vibration and harshness transmitted into the chassis are substantially attenuated before they reach the seat, reducing driver fatigue over a long shift.
The chassis suspension connects the axles to the frame and determines how the vehicle as a whole rides and sits. Long-haul tractors typically use different systems at the two axles.
The driven rear axle of a long-distance tractor typically carries full air suspension: load-bearing air springs, or air bags, in place of steel springs. Air suspension gives a soft, well-damped ride that protects the load, and because the pressure in the bags can be regulated it also allows control of ride height and load height — useful for matching the coupling height to a trailer and for keeping the vehicle level as the load changes.
The front steer axle, which carries a smaller and more constant load, may instead use a single parabolic leaf spring on each side. A parabolic spring is a leaf tapered in thickness along its length so that one or few leaves do the work of a traditional multi-leaf pack. Increasingly these are made in composite material rather than steel, which is lighter still while providing the same spring rate, saving weight at the front of the vehicle.
When a vehicle corners, its body tends to lean outward, an effect known as body roll. To limit this, tractors are fitted with anti-roll bars, also called stabiliser bars, at the front and rear: a transverse torsion bar linking the two sides of an axle so that the suspension resists the body leaning while still allowing the wheels to move together over bumps. Reducing unsprung mass — for instance by fitting light-alloy wheels in place of steel, as described on the Tyres page — further improves comfort, because lighter wheels allow the suspension to react more quickly to the road.
The choice of suspension involves a compromise. Where the optional full air suspension is not fitted at every axle, and a leaf-sprung front is combined with a different rear arrangement, the front and rear suspension responses can fall slightly out of step with one another over undulating ground, and the amount of body roll the occupants feel increases. Specifying full air suspension throughout addresses this, at the cost of additional weight and complexity, so the configuration is matched to the operator's priorities of ride quality against payload and simplicity.
The air-suspension and anti-roll-bar arrangements described above shape the ride, but the dampers that govern how quickly the suspension settles are, in a conventional installation, fixed in character. A semi-active or adaptive damper instead has its rate varied continuously and electronically, moment by moment, in response to the movements of the road and the body. Where a passive damper offers a single fixed compromise, an adaptive damper can range between two extremes — soft, for the most comfortable ride, and firm, for the tightest control of body movement — and select the appropriate degree of damping for the conditions of the instant. It is termed semi-active because it can only dissipate energy, modulating its resistance, rather than feeding energy in to drive the suspension as a fully active system would.
The logic that decides how firm each damper should be is commonly built on the Skyhook principle. The name describes the idea notionally: the control behaves as though the body of the vehicle were tethered by a damper to an imaginary fixed point in the sky, so that the body's own motion is suppressed while the wheels remain free to follow the contours of the road beneath. In this way the heaving and pitching felt by the occupants is calmed without the dampers having to fight every small movement of the wheels. Systems working on these principles are marketed under various names, among them Continuous Damping Control (CDC).