Often the "forgotten component," the suspension receives less spec'ing attention than most other vehicle systems. But, when one considers the responsibilities imposed on a suspension, it deserves a second look.
There's more to a suspension than leaf springs and U-bolts. A suspension is a complex system with several jobs to perform. And there are several types of suspensions, each giving special priority to specific functions. Understanding the suspension's tasks and knowing which types of suspensions are best suited to specific jobs can make your job easier at spec'ing time and down the road.
A truck suspension handles no fewer than five distinct functions. It must:
Support the vertical load imposed by the truck, body and payload
Transfer longitudinal, acceleration and deceleration forces between axles and frame
Provide chassis isolation or cushioning from axle motions (ride quality)
Maintain wheel location and attitude (alignment)
Provide stability or rollover resistance.
Also, a sliding trailer suspension can be locked in any of several fore and aft positions, thus adjusting load distribution between tractor and trailer. To further complicate matters, some suspension functions are at cross purposes with others. For example, a suspension that provides high resistance to rollover is usually stiffer and has less travel, which degrades ride quality. The spec'ing decision also must consider type and weight of payload, as well as terrain and road conditions. Heavy, liquid loads, for example, are prone to surging, so special consideration must be given to rollover resistance. On the other hand, some produce can be ruined by road shock, so ride quality can be especially important. We will address various suspension types and components, discuss what applications they are best suited to and offer some savvy spec'ing tips.
Leaf Spring
The leaf spring is a common suspension for on-highway trucks, tractors and trailers, owing to its low cost, lightweight and ease of maintenance. These features make leaf springs popular for trailers and converter dollies, as well as for tractors and straight trucks. At each axle end, a single leaf or pack of spring leaves is attached to the frame at two points, one being through a shackle or slide to accommodate the change in length as the spring compresses and rebounds. The multi-leaf spring has a rudimentary ability to increase damping with load, due to an increase in inter-leaf friction. So, under load, ride quality is acceptable to good. Lightly loaded, however, the leaves act more like one thick spring, and ride quality suffers.
The air suspension is another popular choice for over-the-road applications. It consists of two air bags at each axle, fastened between the frame and a rigid or flexible trailing arm. Air pressure inside the bags is adjusted for load and height. Maintaining constant ride height requires a height-control valve. Increased load activates the valve and channels air into the bags. The heavier load is repositioned at the original height. In a tandem-axle application, axle-load distribution is achieved by passing air between the front and rear sets of airbags. This also provides a degree of damping, although the bags themselves dissipate little energy. Improved ride quality is the major benefit claimed for air suspensions. Compared to a typical, mechanical suspension, air protects fragile loads better, and lowers vibrational stress. Also, where mechanical suspensions tend to ride harsher when unloaded, air maintains a relatively consistent ride throughout its load range.
The walking beam system is usually used where roll resistance, off-road stability and traction equalization are important - dump and construction applications, for example. With this arrangement, rigid, longitudinal beams or, in some examples, spring packs are attached between tandem axles, one on each side of the vehicle. Each beam pivots at the chassis, or at a point suspended by a spring from the chassis. This provides a high degree of inter-axle articulation, while minimizing overall suspension travel and enhancing stability. The spring usually is a high-stiffness leaf spring pack, fastened to the beam at its center and to the vehicle frame at its end points. Rubber or air springs can be used in place of leaf springs. Axle deflection due to road irregularities is accomplished mainly by inter-axle rotation of un-sprung mass about the walking pivot. The two axle masses bounce, out of phase, against each other and against the spring of the loaded tires. Ride quality, therefore, is generally poor.
A rubber-spring suspension uses a deformable rubber block or cylinder, in place of a leaf spring, between the frame and un-sprung mass. A metal half-can is often fitted over the rubber member to prevent excessive deformation under load. This containment of the rubber provides stability and adds capacity to the system. Because suspension travel is relatively short, ride quality suffers, while cornering stability is enhanced. Again, bulk tank operators have reported good results with this suspension.
Although successfully used in some passenger-car applications, composite leaf springs enjoy only limited use on medium and heavy-duty trucks. The composite spring, and expensive, fiber-reinforced polymeric material, is stronger and lighter than steel leaves.
Moreover, when the composite spring fails, it does so in a "soft" mode. It sags and carries the load lower, but the member does not completely break apart. The composite's two worst enemies are heat, which causes delamination, and metal mounting hardware, which initiates cracks and stress risers.
Shock absorbers and stabilizer bars are two suspension-enhancing devices that can be applied to virtually any suspension type. Shock absorbers are cylinder/piston assemblies that cushion road impacts by forcing oil through a small opening, or orifice. This slows down impact events, spreading them over a longer time period. The resulting cushioning improves ride and control, and extends suspension component life. While they require periodic replacement, many fleet operators view them as cheap protection for more expensive components. Stabilizer bars (also called anti-roll bars) do their thing by transmitting suspension travel on one side of an axle to the other side. In other words, when a vehicle leans to the left -as when making a right turn - chassis-to-axle distance decreases on the left side and increases on the right. A stabilizer bar, which is a lever attached to each axle end, and usually to the frame, works by squaring left and right-side chassis-to-frame distance, thereby helping to keep the vehicle more level while cornering.
Equipment managers and product engineers offer the following points to consider when spec'ing a suspension:
Look, in great detail, at the intended use of the vehicle. This includes GVW, type of cargo, off-highway considerations, and any special performance parameters, such as deflection rate (the distance the suspension travels per 1,000 pounds of load)
Keep in mind that some types of cargo can be damaged by specific vibration frequencies transmitted by certain suspension configurations. Engineering expertise is needed in this phase of selection
Compare suspension weight by type and manufacturer, but don't let a concern for weight and payload override safety, durability and maintainability considerations
Ask other fleet operators, in applications similar to yours, about their experiences with various suspension types
Be sure repair parts are readily available in your area
The ultimate concern should always be safety. A suspension must not contribute to vehicle instability in a specific application with its rated load. |
