Combine a deteriorating North American road network with a general trend towards increased weights and you have a situation that is pushing suspension design to new levels.

Not only do heavy duty suspensions play a considerable role in driver and client retention by cushioning both the load and the driver from road shock, they must “locate” the axles, keeping them in line and negating the effects of road shocks and vibration as well as the twisting effect of engine torque, while at the same time allowing them to move independently so that each of the wheels remains on the ground. The available options are varied with some truck manufacturers offering their own suspension models. Understanding the peculiarities between the main market choices is what makes for a smart spec. To help you out, following is a description of the basic spring and suspension terms you will need to know to get started.

SPRING TYPES

Made of rubber and inflated by the truck’s compressed air system, air springs or air bags as they are often called, can provide a smoother ride than is possible with metal leaf springs, and the bags maintain their springiness even when the temperature heads into the subzero section. The air bags are part of an elaborate network of arms, bars, beams and rods, the configuration of which differs with each manufacturer. But the focus in recent years has been on producing lighter systems.

Steel leaf springs come in three main designs: flatleaf, taperleaf and parabolic.

Flatleaf springs are the simplest. A flat metal leaf with a uniform thickness and width is bowed at rest and straightens when it has to support a load. This design provides for a single rate, the load-bearing capacity remaining constant throughout the flex range. For rear axles, metal leaves of different lengths are stacked. When supporting a load, the longer leaves start to straighten and become stiffer. The shorter leaves then start carrying some of the load burden.

Taperleaf sping design is a bit more complicated. The metal leaf is thick at the centre and thinner at the ends. This design requires fewer leaves to achieve equal spring capacity as compared with the flatleaf design. And, of course, the fewer leaves used, the greater the savings in weight. Also, taperleaf is claimed to provide better ride, in empty or loaded hauls, because there is less leaf friction.

Parabolic spring is the more complicated design. The metal leaf has a thickness that is different at several points. When these leaves are stacked they are often separated by spacers; friction does not hamper their flexing.

SUSPENSION TYPES

Rear suspensions include walking beam, multi-leaf, four-spring, and air-ride, as well as several design variations of those terms.

Walking beam suspensions consist of two beams, one at each side of the chassis, which pivot in the centre and connect at the front to one axle of a tandem and the rear to the other axle. The axle ends are allowed to move up and down without affecting the critical distance between them. The weight is shared by the four wheels. An offshoot of this design involves splitting the beams into two pieces and joining them with hinges to reduce the shock that’s carried from the forward axle to the rear axle.

Multi-leaf suspensions rely on a pair of long packs of springs. One spring pack is placed on each side of the vehicle, tying the two axle ends together. This equalizes and cushions the load. The advantage is weight savings since no other springs are used.

Four-spring suspensions, as the name implies, employ four sets of spring packs. One is positioned at each wheel. The main advantage with this design is also light weight and its primary use is with on-highway applications.

Air-ride suspensions, although initially designed for the on-highway market, are starting to make their way into the on/off road market as well. They are usually designed with two air bags per axle. Each bag’s role is to support one end of a leaf spring or follower arm so the truck’s weight is not completely borne by the air bags. Compressed air is supplied by the same engine-driven compressor and reservoir tanks which provide air to the air brake system. To prevent trucks from “leaning” into turns, air suspensions use a height control valve with a sensor to monitor and then adjust the pressure between the bags on either side of the vehicle.