Spec’ing the Right Suspension for Your Truck
Choosing the right suspension can affect not only the performance of your vehicle, but it can also have a resounding impact on your company’s bottom line.
Trucks are expensive. As with any piece of expensive equipment, priority should be taken when spec’ing components that have an effect on the performance of the vehicle, the life of the vehicle, the frequency of maintenance, component lifecycle costs, and the comfort and safety of the operators. Without a doubt, spec’ing the right suspension should be a high priority for decision makers in the trucking industry.
One of the first suspensions widely used in the trucking industry was the walking beam or equalizing beam tandem suspension. Walking beam suspensions are able to equalize the truckload between two drive axles by allowing the wheels to adjust to uneven terrain by way of a central pivot point. Some walking beam suspensions contain steel leaf springs designed to offer support and stability. Walking beam and steel leaf spring suspensions are very robust and demonstrate good stability characteristics, with adequate ride quality for most operations.
The next generation of suspensions incorporated rubber into the springing medium. As applications became heavier and more severe toward the middle of the 20th century, rubber block suspensions were designed to increase stability over mechanical spring systems. Later developments in rubber technology provided designs that offered an improvement in ride over traditional walking beam and spring systems by dissipating road shock via rubber bolster springs. Some rear suspensions were designed as a two-stage system to offer excellent ride quality in the unloaded condition and additional stability when the truck is loaded and the center of gravity increases.
Compelled by the drive to improve ride while maintaining the same level of stability and handling, suspension manufacturers in more recent years began developing systems that incorporate air spring into the suspension design to provide even greater ride quality. In such suspensions, the air springs adjust to maintain proper ride height as the load of the vehicle changes, thereby providing a relatively constant ride whether loaded or unloaded. Air springs were first used on trailers and on-highway tractors to protect cargo from excessive road shock and to provide enhanced comfort for the driver. However, recent developments over the past decade have produced vocational air suspension designs that offer greater protection of the cargo, chassis and body equipment in severe environments.
Over the years, research and development, intense testing and experience have resulted in current designs developed to address past shortcomings. As a result, the components used in air suspensions have become more robust and lighter weight. More recent air suspension products have also been developed to increase stability and handling, and to package within the tire envelope for component protection. In addition, more advanced designs use system geometry that controls torque wind-up and corresponding frame rise, and significantly reduces driveline vibration that can cause undue wear and tear on the chassis and body equipment.
What to Consider
Upfront costs should not be the sole factor used to determine which type of suspension to spec for your particular operation. There are four major factors to consider when spec’ing a suspension:
Evaluate the application and duty cycle requirements for the vehicle to make sure the suspension meets your needs.
Consider the performance of the suspension (stability, handling, ride, etc.) as it relates to your specific industry.
Lifecycle costs, including purchase cost, maintenance and downtime, can have a major “unseen” financial impact if not considered while purchasing the truck.
Keeping the weight of the truck as light as possible by spec’ing lightweight suspensions can translate to numerous benefits, especially as the weight of other components continues to increase.
Application and Duty Cycles
First and foremost, individuals purchasing new trucks need to take a look at the intended application to make sure that the suspension design is right for the duty cycle of the truck. For instance, if the application is extremely severe (heavy loads, rough terrain, frequent turns, etc.), more durable products are required. The continual stop-and-go nature of refuse vehicles can cause pre-mature suspension wear if the wrong suspension is spec’ed. Furthermore, trucks that have higher centers of gravity require suspensions that provide superior roll stiffness characteristics.
In addition, the suspension should have a capacity rating that matches the intended use for the vehicle. Your choice in capacity should take into consideration the gross vehicle weight, the weight of the body and the weight of the intended load, as well as any regional weight laws that might restrict the amount of load that can be legally carried. The suspension should also be rated the same as or higher than the axle capacity rating, so that it is not the limiting factor for how much load can be carried while in operation. Ensuring the proper suspension spec is essential to making sure the vehicle operates in the most efficient manner possible.
The term “performance” encompasses stability, handling, ride quality, articulation and mobility. Stability is a measure of how well the suspension and chassis resist rolling or swaying, especially when in a turn. Typically, a vehicle’s stability is based on its rollover threshold, which is determined by measuring the maximum lateral acceleration a vehicle can sustain in a steady turn without tipping over—less roll = more stability. Higher rollover thresholds allow a vehicle to negotiate a steady turn at higher speeds. For vocational vehicles, higher rollover thresholds allow improved vehicle performance on the job site, and during lane changes and turns. The amount of roll stiffness found within a suspension is the best way to directly affect the stability of the vehicle (more roll stiffness = more roll stability).
Handling is a measure of how well the suspension and chassis optimize the lateral cornering performance of the vehicle in reaction to steering inputs. Handling determines how fast you can drive and what kinds of turns that you can make while maintaining directional control (an engineering term known as “yaw” stability), and it has strong effect on the amount of movement or sway in the tail end of the truck, which can dramatically affect vehicle performance. A suspension with good handling characteristics can give the driver greater control in both on- and off-road conditions.
Ride quality is a measure of how road shock affects the vehicle and driver as the vehicle encounters bumps, potholes and other road irregularities. Driver and passenger health and comfort are directly affected by ride. Rubber suspensions can provide excellent ride quality, oftentimes bridging the gap between the stability of a mechanical suspension and the ride quality of an air suspension. Because of their inherent design, air suspensions provide superior ride quality. Air springs provide a constant spring rate, meaning that the ride is consistent no matter what condition the body is in—loaded or unloaded.
Articulation refers to the amount of vertical wheel travel experienced when traveling over uneven terrain. Articulation has a direct effect on the traction of the vehicle’s wheels. Suspensions with greater articulation and designs that allow the wheels to move independent of each other enable the wheels to maintain continual contact with the ground, thus providing better traction and mobility.
Lowering costs is a priority for almost every business, community or industry across the continent. The trucking industry is no different, and suspension lifecycle costs play a big part in keeping costs down over the life of the truck. Not only is the upfront purchase price to be considered, but the regular maintenance required on the suspension and propensity for downtime should also be taken into account. Let’s face it—trucks can be driven hard with significant load constraints over undulating and rugged terrain. Choosing a suspension that will live in the environment where it is used will help to keep service intervals as far apart as possible, and thus minimize downtime and reduce the costs that result from recurrent vehicle repairs. In regards to maintenance, consider whether there are any lubrication requirements or special maintenance requirements, as well as how quickly wear items will need to be replaced. In some applications, an extended warranty plan, if available through the dealer or suspension supplier, might be appropriate in order to ease the burden of repair costs in the event of unexpected failure.
Not only should the suspension itself last, but the design of the suspension should also protect the chassis and body equipment from excessive vibration, road shock and premature wear. Air suspensions provide greater equipment protection over traditional mechanical suspensions due to the air bags’ ability to cushion the equipment and passenger load. Some air suspensions have a unique geometry designed to optimize torque reactivity, significantly reducing driveline vibration that causes wear on components.
When choosing a company from whom to buy your next suspension, take into consideration the company’s customer service track record. Examine the design of the suspension and determine whether or not it will be easy to maintain. For times when maintenance is necessary, check to see if parts are readily available. The goal is to reduce the amount of downtime in which this expensive piece of equipment is unavailable for use.
More than ever, fleets are faced with the challenge to reduce weight while EPA engine regulations, as well as anti-idling laws, continue to add weight to the vehicle. While the obvious benefits of lightweight suspension systems include the opportunity to consolidate routes, as well as offset engine, emission and idle reduction system weight increases, numerous articles have been written regarding additional weight savings benefits. According to industry studies, reducing truck weight can improve fuel economy, decrease wear and tear on vehicles, reduce horsepower needed to overcome mechanical resistance of the drivetrain and tires while driving, lower lifecycle costs and improve carrying efficiencies, especially during driver shortages. In addition, lightweight suspensions can provide greater freedom for the end-user to spec more sophisticated equipment by offsetting the weight of the additional components. Spec’ing products that can save weight, whether suspension products or other truck components, can help the end user to realize these numerous benefits.
Impacting the Bottom Line
As with any component, your company’s needs should be evaluated before spec’ing suspension products. Choosing the right suspension can affect not only the performance of your vehicle, but it can also have a resounding impact on your company’s bottom line.
Jacob Madden is the Segment Manager for vocational truck suspensions at Hendrickson, located in Woodridge, IL. Hendrickson is a premier, global manufacturer and supplier of truck and tractor suspensions and heavy-duty springs; trailer suspensions, controls and nonintegrated axles; truck and trailer lift axles; and bumpers and trim components to the commercial transportation industry. For more information, call Hendrickson at (630) 910-2800.