Every vocational truck suspension option comes with its own unique performance trade-offs. The best choice requires an understanding of how the trade-offs match up to your application requirements.
“Mechanical suspensions are still the most popular for the vocational market, thanks to their great stability, articulation and maintainability,” says Tim Wrinkle, Mack Trucks construction product manager. “Air-ride suspensions are becoming more common, as they offer a better ride when unloaded. However, these suspensions can be limited in articulation and stability. Driver knowledge and training are also very important with air suspensions because drivers need to evacuate air bags before dumping the load. Depending upon the setup and a driver’s experience, this can also slow the dumping process for dump trucks.”
Daimler Trucks North America“Although there is a trend spec’ing air suspensions for vocational applications, there are trade-offs between off-road and on-road performance,” says Peter Schimunek, marketing segment manager, Western Star.
The job of the suspension is to support and cushion the load. The suspension absorbs shock loads that would otherwise be transferred into the frame. If the truck frame is doing the job of the suspension by twisting and absorbing shock, then it stresses every component of the vehicle, eventually causing failures and maintenance issues.
Understanding the application and performance demands is the first step. The capacity of the suspension; whether it is for a single, tandem or tridem configuration; if the system is for a tractor or straight truck; and the nature of the terrain all play a role in narrowing the choice between air, rubber or steel springs.
There are multiple suspension options available to meet the many niches in the vocational truck market, each with unique features and benefits. Compare the suspension choices available to meet application needs — reducing weight, improving off-road mobility, increasing stability, minimizing torque reactivity or all of the above — while balancing upfront cost.
The type of loading and unloading performed must be considered. A dump truck hauling aggregate requires a different suspension than a mixer truck, for instance. There is more of a shock load factor when aggregate is loaded vs. ready-mix. Additionally, a stationary load has different needs than a moving load, such as liquid bulk haul or a concrete mixer.
Be sure to account for worst-case scenarios. Sometimes the truck may be required to take on heavier loads than it usually does. These temporary loads may require a higher creep rating on the jobsite, thus requiring a heavier-duty suspension. Where the truck will operate — the type of terrain — can create different needs for articulation and ground clearance.
Serviceability gains importance in remote locations. For example, if it is going to be in the middle of an oil field with no immediate access to a dealership, it’s important to spec the suspension to be serviceable by the driver.
Narrowing the Gaps
Avoid relying on past history to spec today’s suspensions. Suspension designs have become more streamlined and versatile over time, reducing trade-offs.
In the past bigger, bulkier and heavier suspensions suited for tougher applications also provided a rougher ride. With the introduction of air ride suspensions as well as other innovations, drivers now have a more comfortable ride, and the loads are better protected from rough terrain.
There used to be a large gap between the comfort of on-road suspensions and the durability and stability of their off-road counterparts. The gap has closed significantly in recent years, with off-road suspensions getting very close to matching the comfort and performance of on-road suspensions.
Mack recently made improvements to its mRIDE mechanical suspension. “The Mack mRIDE spring leaf over rubber block suspension is ideal for customers needing improved stability and greater traction on jobsites,” notes Wrinkle. “The mRIDE’s parabolic springs are thickest at the center to ensure the clamp load area offers the greatest strength and are tapered to improve driver comfort.
Hendickson“The mRIDE provides extreme articulation and constant tire contact, delivering superior traction on the rough terrain,” adds Wrinkle. “The spring leaf design and V-shaped torque rods help ensure stability to efficiently transmit forces into the truck’s frame. The Mack mRIDE has been our lightest vocational proprietary rear suspension and earlier this year we lightened it event further, now offering additional weight savings for increased payload when paired with proprietary Mack axles. “Available at 40,000-lb. and 46,000-lb. suspension ratings and with standard track axles, customers can save 140 lbs. with drum brakes and 146 lbs. with air disc brakes as a result of casting suspension components into the Mack axle housing.”
With advancements in technology, the gap between lighter suspension weights and life-cycle durability has also narrowed. For example, Hendrickson’s next generation HAULMAAX EX rear tandem rubber suspension is lightweight yet doesn’t sacrifice on durability. It is designed for applications such as dump, concrete mixer and crane/boom with 40,000-, 46,000- and a new 52,000-lb. capacity. The suspension utilizes a unique rubber spring design that balances empty-ride quality and loaded stability. Angled bolster springs and a unique progressive load spring provide extra stability for demanding applications.
Kenworth has recently added the HAULMAAX EX suspension as an option for its Class 8 vocational models. “The Hendrickson HAULMAAX EX is an excellent choice for Kenworth vocational customers seeking a lightweight, durable suspension equipped to handle rugged operating conditions and provide a smooth on-highway driving experience,” says Genevieve Bekkerus, Kenworth marketing director. “Now that this suspension line is also available with a 52,000-lb. suspension rating, Kenworth customers operating in heavy applications have a viable new suspension option to consider.”
To further reduce weight and increase ride quality, lightweight air ride suspensions have gained off-road capability. Newer air suspensions use casting technology and innovative designs for increased durability and performance.
The suspension choice helps drive ownership cost. An improperly specified suspension can result in costly repairs down the road, wearing out components faster and increasing overall maintenance costs and downtime vs. a properly specified product.
Balance maintenance requirements and expected performance. Different suspensions have different requirements for maintenance, such as frequency and type of lubrication and replacing bushings. The application and operating environments may also affect maintenance requirements.
The complexity of the suspension helps determine maintenance costs. Generally speaking, steel suspensions are the least expensive to maintain, while air ride suspensions are more complex and therefore more likely to require maintenance. This is because air ride suspensions have air in the system that needs to be regulated with control valves. Also, depending upon the type of environment or application, air springs can get cut or cracked if stones get embedded.
High Articulation Solutions
Mechanical suspensions typically offer greater articulation than air suspensions. They can also be designed to maximize stability. A trade-off between articulation and stability is not necessary. Mack offers its Camelback and mRide as examples that both offer high articulation and stability.
It is important that any mechanical suspension used in a high articulation environment be able to evenly distribute the load. Freightliner recommends a trunnion suspension such as TufTrac, which is Daimler Truck North America’s proprietary suspension for Freightliner and Western Star trucks. Trunnion suspensions are best suited to handle bumps and ridges without bottoming out. TufTrac specifically keeps a uniform load over each wheel during articulation.
Walking beam suspensions have traditionally been favored for high articulation applications. Floating walking beam suspensions allow maximum articulation and can offer maximum roll stability. The floating beam suspension with soft ride restrictor can provide a superior ride quality that compares favorably with air ride when on the road. This feature is available for specific truck applications and tractors, as well.
Walking beam suspension systems are also extremely durable. Depending upon the spring medium design, walking beam suspensions can have a little bit harsher ride and may require more maintenance, because they have a more complex design than spring steel suspension systems.
ChalmersThe primary benefit of the walking beam design is its ability to equalize the load. Modern walking beam designs are integrated into advanced rubber spring systems, which bring additional benefits to the suspension system. Walking beam suspensions have advanced to provide the highest amount of articulation while achieving the stability required for off-road work.
In high center of gravity applications, you will want to consider the benefits of a mechanical suspension. Mechanical suspensions with high roll stiffness and high roll centers are best suited for high center of gravity applications like concrete mixers.
Concrete mixer trucks are typically associated with a high center of gravity. “Suspensions are important to the performance and safety of concrete mixers, which typically need to perform both on- and off-road,” says Wrinkle. “Mixers have a high center of gravity and extra offset center of gravity with the spinning barrel carrying approximately 40,000 lbs. of concrete, so they are more prone to rollover than other vocational applications. With this in mind, high-stability mechanical suspensions are typically used to help reduce the risk of rollover. Mechanical suspensions with high roll stiffness and high roll centers are best suited for high center of gravity mixers.”
The stability of a walking beam suspension can be a definite advantage in these applications. It is important to choose one that can distribute the load equally to all four wheels even during severe articulation. That can be done by choosing a floating beam suspension, which is suited specifically for high center of gravity applications.
Walking beam suspensions are best suited for high center of gravity applications because they are the most roll stable due to limited movement and compression within the suspension spring.
The walking beam can be paired with rubber spring technology. Rubber spring components are less weight, easier to maintain and provide greater ride quality and stability over steel spring suspension systems.
Western Star offers a walking beam suspension in the form of the six-rod TufTrac Gen2. “The inherent stability of the design provides an excellent platform for high center of gravity applications of this high-performance and durable vocations suspension,” says Schimunek. “The high ground clearance feature of this suspension performs extremely well in vocational applications, including construction, asphalt spreader, heavy haul, mining and logging.” It is available in 40,000-lb., 46,000-lb. and 52,000-lb. tandem axle capacities.”
“The TufTrac walking beam design allows the suspension to maintain a uniform load over each wheel in laden and unladen conditions as the suspension articulates,” says Schimunek. “By maintaining a uniform wheel load the TufTrac suspension maintains maximum traction, providing increased confidence to cross even the toughest jobsite, maximizing productivity. By isolating the moving joints in rubber elastomer bushings, this suspension provides one of the quietest rides among vocational suspensions, keeping the operator comfortable.”
The Comfort Ride version of the TufTrac Gen2 suspension is available in 40,000-lb. and 46,000-lb. tandem axle capacities providing a softer ride for increased ride quality and driver comfort. “The reduced spring rate combination with four shock absorbers and a stabilizer bar provides a smoother ride, whether loaded or unloaded, minimizing wear and tear on both truck and driver,” says Schimunek. “Comfort ride is ideal for applications that require both on- and off-road usage, such as dump trucks.”
For high articulation applications, Western Star also offers the Chalmers 800 Series low maintenance and high articulation tandem rubber spring rear suspension. “This rear suspension provides rubber bushings without greaseable joints, reducing maintenance costs and increasing uptime. High articulation greatly increases stability and cornering, improving driver comfort for increased productivity and safety,” says Schimunek. “Chalmers 800 Series rear suspensions are available ranging from 46,000 to 70,000 lbs. The suspension is ideal for on/off-road applications, such a dump trucks, logging, heavy haul, oil field and bulk haulers and can be used in straight truck or tractor applications.”
Air Suspensions Provide Best Ride Quality
It’s hard to beat air ride for driver comfort. Creature comfort and the reduction of shock to other components of the chassis are real reasons to look at air ride. That said, it does have limitations. No manufacturer has been able to offer both stability and articulation with air ride.
Air ride suspensions continue to expand in certain vocational applications, such as dump trucks and heavy-haul tractors. Air ride suspensions provide consistent ride quality in any load condition, which will help lessen driver fatigue while protecting equipment and chassis components from damaging vibration and road inputs.
Western Star’s proprietary Airliner rear suspension design reduces weight, which allows for a greater payload. “The maintenance-free double-bonded rubber bushings on moving joints ensure a long life expectancy,” says Schimunek. “The stability and cushioning of this suspension system significantly reduces freight damage and driver fatigue, improving the overall driving experience. A full selection of single (21,000 to 23,000 lbs.), tandem (40,000 to 46,000 lbs.) and tridem (69,000 lbs.) drive suspensions are available and tuned for almost any application. Some air suspension models are best suited for highway applications however, so we recommend adhering to manufacturer recommendations for your specific applications and weight carrying capacity.”
Weight savings is also an advantage with air ride systems. Some mandated federal requirements, such as SCR and DPF, have added weight to trucks, which is one reason customers are moving to the lighter air ride suspensions. While they are not the best for every job, they provide ample capability and capacity for dump applications.
Purpose-built air suspensions can overcome many of the previous limitations — primarily stability and off-road mobility. Stability does not need to be sacrificed if choosing the proper vocational air suspension. For example Hendrickson’s PRIMAAX EX promises up to two times the roll stability of competitive air suspensions, equaling the stability of a steel walking beam.
“This suspension features a robust structural design with optimized suspension geometry for exceptional stability, durability, reliability and handling characteristics,” says Schimunek. “The PRIMAAX EX minimizes suspension induced driveline vibration and significantly controls suspension windup and corresponding frame rise compared to competitive trailing-arm suspensions, resulting in higher driver comfort and less premature wear on truck and body equipment. This suspension is ideal for a variety of vocational and severe-duty applications including construction, mixer, logging and heavy haul applications. It’s available in 46,000- and 52,000-lb. tandem axle capacities.”
The Neway ADZ Series air suspensions feature a streamlined design that has reduced weight by more than 220 lbs. per axle (compared to previous models). “The durable suspension is ideal for high center of gravity, severe-duty and heavy-duty vocational applications optimizing operating efficiencies,” says Schimunek. “High cross-articulation provides superior traction in wet, muddy and uneven terrain such as logging tracks, hillsides, oil sands and fracking sites. The ADZ Series suspension is available in single, tandem and tridem axle configurations ranging from 26,000 to 78,000 lbs.”
Steel Springs Offer Simplicity
Simplicity rules in certain applications. Spring steel suspensions are simple, stable and durable. Walking beam suspensions are usually heavier, depending upon the design of the steel springs.
Steel spring suspensions are known to be simple suspension designs resulting in an economical upfront purchase. Common use of the simple design also leads to familiarity of maintenance.
Drawbacks for steel spring suspensions will often be heavier weight and the unladen ride quality. Maintenance requirements actually depend on the design of the steel spring suspension. The more complex designs may increase maintenance costs. For example, a multi-rod or six-rod type of suspension will require substantially more maintenance due to the fact that they have more bushings and end rod connections, as well as leaf springs.
Three major drawbacks with steel springs are breakage, deflection change and weight.