Considerations for Worktruck Suspension Spec'ing

By Tom Kelley | September 28, 2010

Different applications, different operations, different regulations, and different priorities all contribute to the complexity of choosing the right suspension. Far from a "one size fits all" proposition, spec'ing a suspension system on a construction truck is one of the most important and most complex aspects of choosing your new truck.

Part of the problem arises from the fact that most construction hauling is an "all or nothing" scenario, running either at tare weight or at maximum gross. It's fairly simple to suspend a truck that spends most of its life operating inside a very narrow weight range.

With construction trucks, the spring rate that provides the best ride when empty won't be sufficient for running fully loaded, and conversely, the spring rate that rides best when loaded will beat you to death when running empty. Because of this suspensions generally designed to ride best when they are loaded.

"In the past there typically was a trade-off between ride quality and performance (traction, stability, durability)," says Gerry Remus, Vocational Market manager at Hendrickson Suspension Systems. "Today, due to highly engineered systems, there are products available that strike the perfect combination of ride and performance."

Another problem that adds to the complexity of choosing a suspension is the wide variation of local, regional, state, and federal weight regulations that can dictate the number and spacing of axles required to maximize payload in a given operating area.

What To Consider?

At the most basic level, there are six key considerations when selecting a suspension.

  1. Capacity — How much weight can the suspension support?
  2. Payload — How much does the weight of the suspension reduce or increase the truck's maximum payload? Today, it's more important than ever to consider the suspension weight. With engine emission changes driving the weight of the truck up several hundred pounds, the suspension can be a way to mitigate some of the weight increase.
  3. Ride Quality — How well does the suspension absorb input from the road, whether the truck is loaded or empty?
  4. Articulation — How much of an input can the suspension absorb before transferring that input to the chassis, and how well does the suspension keep the wheels engaged with the ground while absorbing input?
  5. Durability — How well does the suspension stand up to the rigors of jobsite operation, and how much regular maintenance is required?
  6. Stability — How well does the suspension mitigate lateral forces imparted by dynamic or high-center loads?
  • What Does A Suspension Do?

    Suspension systems typically involve components that can be divided into two major tasks. In the simplest suspension systems, a single component may perform both of these tasks. In the most complex systems, each component is dedicated to one task.

    One task is to insulate the truck from road input while keeping the wheels in contact with the ground, and the other task is to maintain the attitude of the suspended axle(s) relative to the truck, while allowing the axle to articulate through the suspension's range of travel.

    Spring Fever

    The first task is performed by a spring, most frequently a half-elliptical steel leaf spring, sometimes by a rubber pad/cushion, or with increasing regularity, an air spring. Each of these spring types has both positive and negative attributes.

    The simplest spring type, the rubber pad/cushion, provides excellent stability, which is important for trucks hauling a dynamic load such as ready-mix concrete, and is also the most durable, important in severe service applications such as mine or quarry operations. On the negative side, the rubber pad/cushion springs provide the harshest ride quality. Newer designs employing variable-rate rubber springs can offer improvements in both articulation and unladen ride.

    The most common spring type, the half-elliptical steel leaf spring, provides a better ride quality than the rubber pad/cushion spring, and is repairable virtually anywhere due to the wide availability of locally fabricated replacement spring leaves. On the negative side, the leaf spring weighs more and is less durable than the rubber spring.

    The air spring is quickly gaining popularity due to its lighter weight and its ability to provide an optimum spring rate whether the truck is loaded or empty. The negatives associated with air springs are lower stability and lower durability than the other spring types.

    Most air suspension systems that saw early use in the construction market were the same systems sold for strictly on-highway use in the over-the-road truck market, occasionally up-rated one size to gain some durability. More recently, one of the pioneers in the vocational suspension business, Hendrickson, has developed an air suspension system purpose-built for vocational applications, in an effort to mitigate some of the durability, stability and off-road maneuverability concerns.

    Attitude Adjustment

    The next area where different suspension systems are differentiated is how they accomplish the task of maintaining the axle's attitude, while allowing a range of suspension articulation.

    Some older leaf spring suspension designs accomplish the articulation task by forming an eye in one end of one of the topmost spring leaves that is then connected to the truck frame through a hinge pin to allow articulation. A second suspension member, typically a torque rod, but sometimes a second "eyed" spring leaf in older designs, is installed parallel to the main leaf to help maintain the axle's attitude, and resist much of the torque reaction made possible by the flexibility of the main leaf.

    Most modern spring suspension systems separate the positioning/travel tasks from the spring's tasks, replacing the eyed spring leaves with rigid torque rods. In addition to these torque rods mounted longitudinally to the truck, additional suspension members may be mounted laterally to prevent side loading of the longitudinal members.

    The linking members on most air suspensions operate in a geometrically similar fashion to the leaf spring systems, although the two longitudinal members may be replaced by a single larger "trailing arm" that performs both the task of permitting articulation and the task of mitigating torque reaction.

    Still more suspension members may be employed to maintain the axle's attitude when longer ranges of articulation are needed.

    Tandem Teamwork

    So far we've only addressed the suspension of a single (rear) axle. Most trucks and trailers employ axles in tandem pairs to maximize their legal payload capacity. While many tandem suspensions simply use two identical single-axle systems, this can create some issues with balancing the load between the two axles in the tandem set, especially when operating in uneven terrain on a job site.

    The answer to balancing the weight between a tandem pair is another innovation from Hendrickson, this one having been around almost a century. A "walking beam" suspension uses a large, extremely rigid beam to evenly distribute the weight between the two axles in the tandem pair.

    With the axles riding on each end of the beam, and the connection to the truck chassis at the center of the beam, the lever action of the beam reduces the amount of deflection transferred from the axle to the truck chassis. As one axle in the pair travels over a bump, this lever action also serves to keep the other axle in contact with the ground, important for weight distribution, and for traction when employed on a drive axle pair.

    A walking beam suspension can use any of the three spring types — leaf, rubber cushion or air — or in some rare cases where the need for durability totally eclipses the need for ride quality, no spring at all, using only the beam's articulation and tire deflection to absorb road input.

    Out In Front

    Although rear suspensions get most of the attention when spec'ing a truck, the front suspension is nonetheless important. However, due to a narrower range of operating weights on this end of the truck, and the requirements of positioning the steering axle, there hasn't been nearly as much differentiation in suspension offerings for this end of the truck.

    While leaf spring configurations account for the vast majority of front suspensions, air springs are gaining in popularity at this end of the truck as well. Hendrickson is currently developing air ride steer axle for vocational trucks. Key benefits include: excellent ride quality, weight reduction and the stability to match current steel leaf designs.

    The goal is to create an "all-air" platform for vocational vehicles, resulting not only in ride improvement, but also enabling load sensing for on-board scales.

    One front-end variation that is permitted in a handful of states is to allow the use of a second steer axle working in tandem with the first. Not only does this increase the allowable weight carried by the front of the truck, but it can also improve flotation and close-quarter maneuverability on job sites.

    While most "twin-steer" configurations simply employ two single steer axle suspensions, a Canadian company, Raydan Manufacturing, had developed an air-ride walking beam front suspension for military twin-steer applications, that is now available for civilian use.

    Between And Beyond

    Once the primary suspensions at the front and back of the truck have been considered, the spec'ing job isn't necessarily done. Many states allow the use of "lift axles" that can be retracted during unladen operation, and then be deployed when running loaded to maximize payload. The lift axles can be installed between the main front and rear axles, or in some cases, behind the rearmost drive axle.

    These lift axles use an air spring to both suspend the load and to allow for remote deployment/retraction through pneumatic controls. Depending on the axle spacing and the position on the truck, the lift axles may also include a self steering feature to minimize tire scrubbing when making tight turns.

    Out Back

    Although we've mainly focused on suspensions for the truck, virtually all of the same issues and attributes apply to trailer suspensions. Granted, a trailer axle doesn't share the kingpin angle concerns of a steer axle, or the pinion angle concerns of a drive axle, so maintaining the attitude of the axle isn't as critical, but the torque loading, weight balance, articulation, and ground engagement issues are the same.

    One might question the justification for using an air suspension on a dump trailer, as the load is neither fragile nor likely to present ride quality concerns, yet air suspensions are gaining in popularity in this application as well. The concern here is not for the load, but rather, to protect the trailer itself from harsh input when running unloaded.

    Trailers do, however, add another layer of complexity to the suspension spec'ing process, not so much in the type or capacity of the suspension, but in its location. In addition to laws regulating the maximum weight on any single axle and on any tandem pair of axles, there are also weight regulations aimed at the spacing between the axles on the trailer and those on the truck. Designed to limit the amount of load imposed on any single span of a bridge, the Federal Bridge Law (see sidebar) specifies a minimum length between the first and last axles in a truck-trailer combination to allow the maximum permitted weight.

    Putting It All Together

    Whether it's a single axle, or tandem pair; whether it uses rubber, leaf or air springs; depending on the number and location of the axles; as well as the configuration of the hardware involved, there is no single answer for every construction truck.

    For lightweight material hauling trucks dedicated to inter-plant runs, the low tare weight and smooth ride of an air suspension may be the best option. For dynamic, high center loads such as ready-mix, the stability of a rubber pad suspension may be the best choice. For operations that need to strike a balance between ride quality, cost and stability, a leaf spring suspension may be preferred.

    Advances in materials and technology have resulted in improvements to virtually every type of suspension over the past few years, so even if one type of suspension might not have been a practical choice in the past, it may be worth fresh consideration.

    One doesn't necessarily gain a competitive edge by choosing the same configuration that everybody else in town is using. Your local dealer can work with the factory's chassis and suspension engineers to find the best configuration for your specific needs.


    Federal Bridge Law

    The formula for calculating the allowable weight for any group of two or more axles is as follows:

    W = 500 [ ( LN / N-1 ) + 12N + 36 ]


    W = The maximum weight in pounds allowable for the axle group.

    L = The distance in feet between the outermost axles in the group.

    N = The number of axles in the group.

    The law is intended to limit the amount of load that would be imposed within any single span of a bridge. Even though a truck may be well within the weight limits for any single axle or any group of axles, if those axles are too closely spaced, the potential exists to concentrate too much of that load on a single span.

    Here are some definitions that help to explain the weight limits involved:

    Gross Weight — The weight of a vehicle or vehicle combination and any load thereon. The federal gross weight limit on the Interstate Highway System is 80,000 pounds.

    Single-Axle Weight — The total weight on one axle, or multiple axles spaced less than 40 inches apart. The federal single-axle weight limit on the Interstate Highway System is 20,000 pounds.

    Tandem-Axle Weight — The total weight on two or more consecutive axles spaced more than 40 inches but not more than 96 inches apart. The federal tandem-axle weight limit on the Interstate Highway System is 34,000 pounds.

    In order to legally carry the full 80,000 pounds allowable on the Interstate Highway System, the length (designated as "L" in the formula above) between the first and last axles on a five-axle tractor-trailer combination must be a minimum of 51 feet.

    However, there is another limit that comes into play on this same tractor-trailer configuration. In order for the combination of the tractor tandem and trailer tandem axles to carry the full 68,000 pounds allowed (34,000 pounds times two tandems), the length between the forward axle on the tractor tandem, and the rear axle on the trailer tandem, must be a minimum of 39 feet.

    As either of these dimensions are reduced, the allowable bridge weights are reduced from the maximum gross weight. Several states permit higher weights based on configurations that were "grandfathered" when the federal bridge laws were enacted. Check with the authorities in the state(s) in which the truck will be operated.

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