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How to Spec a Truck Chassis and Suspension

January 2018, Work Truck - Feature

by Lauren Fletcher - Also by this author

Photo: Getty Images
Photo: Getty Images

From driver comfort to cost savings, a truck’s chassis and suspension systems are integral components to ensuring you have a vehicle that can accomplish the job at hand.

Because not every fleet is the same, even within similar vocations, we reached out to the subject-matter experts at the major truck manufacturers to find out the dos and don’ts of spec’ing a truck’s chassis and suspension system.

“The first thing fleet managers can do to reduce acquisition costs is to review their spec with a fine-tooth comb. Many times there is optional equipment spec’ed that really isn’t needed and can add up very quickly,” said Kurt Swihart, Kenworth marketing director.

This includes a truck’s chassis and suspension.

What to Spec

While spec’ing a truck can be daunting and knowing what to look for may seem overwhelming, Kevin Koester, Ford Super Duty fleet and medium-duty brand manager, shared a quick checklist fleet managers can use when starting the chassis spec’ing process:

  • Required GVWR.
  • Required GCWR.
  • Front/rear GAWR needs.
  • PTO provision?
  • Cab size (standard, super, or crew).
  • Seating. 
  • Cab to Axle dimension (CA).
  • Wheelbase dimension (WB).
  • Resistance to bending movement (RBM) frame strength.

What to Consider When Spec’ing a Truck’s Chassis

When you are spec’ing a truck’s chassis, the absolute most important factor to consider is what the truck will be required to do.

“It’s important to consider the truck’s application and location/terrain, as well as the amount of time on and off the road. Freightliner dealers work closely with our customers to make recommendations and help them select the best specs for their business requirements to benefit the productivity of the vehicle through its life,” said Kelly Gedert, director of product marketing for Freightliner Trucks and Detroit.

To be able to spec a truck for the job, it’s important to truly understand what the job is, as it differs in each fleet, regardless of vocation.

“Know your drivers and how they use their trucks on a daily basis. Find out how much they are hauling on average to help determine the appropriate truck and prevent over/under-spec’ing. Understand the importance of fuel economy to the customer and how many miles they are driving annually. Also, consider the customer’s future business growth — can they get by with a larger body or will they need additional vehicles?” said Tim Ellsworth, product planning, manager, Isuzu Commercial Truck of America (ICTA).

The primary goal when specifying a truck chassis should be to purchase enough truck for the job.

“Because lower-GVW trucks are less expensive, it is tempting to order the lowest GVW chassis possible. Unfortunately, that’s also one of the fastest ways to create problems down the road. Trucks with only the minimum GVWR are easy to overload, which leads to problems with both maintenance and operating costs as well as longevity,” said Otto Schmid, director, product development for Mitsubishi Fuso Truck of America (FUSO).

Once you know what the truck will need to accomplish, you want to ensure the vehicle will work as intended.

“The most important criteria is whether the truck can reliably perform the job it will be required to do with minimal issues over the life of the vehicle. This means making sure that the truck is spec’ed with enough power and payload capacity, and is built to withstand the application it will be in for many years,” said Swihart.

Cost is also a factor. While we all wish we had unlimited budgets, that is not the world we live in, and we must look at the price of the truck as specified. Increased customization will add to the final price of a truck.

“Fleet managers and owner-operators truly have a challenge on their hands with the task of spec’ing truck chassis. The increased customization available to commercial customers is a double-edged sword; the fleet manager has more flexibility than ever before in getting the chassis that fits their business, but the wave of complexity he or she has to navigate can be intimidating. It boils down to achieving the total cost of ownership (TCO). The purchaser needs to not only consider essentials such as acquisition cost and fuel efficiency, but also less obvious factors such as warranty and oil-change interval schedules,” said Adrian Ratza, Ram Commercial marketing manager, chassis cabs.

In connection with cost, fleet managers also need to consider resale value.

“When looking at lifecycle costs, the potential resale value should be a large consideration. Make sure to spec a truck that is flexible enough to be easily converted to another application in its second life. Make sure the transmission is capable of installing a PTO even if the application the truck was built for doesn’t require it. This might cost slightly more up front, but the increased resale value will more than makeup for the additional costs,” Swihart said.

Another important consideration is the safety of the vehicle. Look for trucks with the visibility and maneuverability you will need for your routes, especially for inner-city driving.

“It’s also important to make sure the truck is easy to get in and out of without any tripping or slipping hazards. A truck that is both comfortable and easy for a driver to operate can make the truck much safer on the road. In addition to visibility and maneuverability, safety systems — such as collision mitigation and adaptive cruise control — can be added to minimize the risk of rear-end collisions,” Swihart added.

What NOT to Overlook When Spec’ing a Truck Chassis

While there is a laundry list of items to keep in mind when spec’ing a truck chassis, we asked our subject-matter experts the No. 1 items not to overlook:

Truckload. “It is important to look not just at the total load the truck will be asked to carry, but also the nature of that load and its distribution. Individual axle loads must be accounted for, as well as total load. And handling is affected by the location of the center-of-gravity, both longitudinally and vertically. Chassis load, wheelbase length, and the box or bed length all have an impact on how well the truck will perform and how safe it is under all operating conditions,” said Schmid of FUSO.

Weight distribution. “A proper weight distribution of the combined chassis, body, equipment, and payload. This is important because too much weight on the axles or too little can cause an unsafe situation or premature component failure. Also, this is the only way to ensure you have the correct chassis and wheelbase needed for body mounting. Additionally, vehicle center of gravity is often overlooked and can be equally as important as weight distribution for stability and braking,” said Ellsworth of ICTA.

Gear ratio. “This selectable variable (gear ratio) will determine the cruise speed for the fully laden chassis along with the start-ability and grade-ability thresholds. Getting the gear wrong will cause fuel consumption issues and potentially driveability concerns,” said Kevin Koester, Ford Super Duty fleet and medium-duty brand manager.

Truck frame. “With the right frame and suspension selection, a customer can avoid an over- or under-spec’ed truck for budget, weight optimization, and truck longevity,” said Peter Schimunek, marketing segment manager for Western Star.

Fleet application. “It depends on the application. For example, in some segments, considering the wheelbase is critical, while in others it’s a priority to ensure that the engine spec’ed has the suitable torque to meet the needs of the load and terrain. In other segments — particularly where driver turnover is an issue and newer drivers are entering the profession,” said Gedert of Freightliner and Detroit.

Fleet size. “From a fleet manager’s perspective, an overlooked factor could easily be that one size doesn’t fit all. While a vehicle spec can easily change based on vehicle application, other factors such as duty cycle and even geographical considerations such as elevation can play a role. Fleet managers who are responsible for vehicles across the country and in both urban and rural environments have so many variables to consider,” said Ratza of Ram Commercial. “Small business owners may have a solid handle on those considerations with the solid understanding of their local market, but a factor that can be easy to underestimate for many are the leaps in technological advances that are being made with commercial vehicles.”

Advice for Spec’ing a Truck Chassis

Overall, the top recommendation from chassis subject-matter experts is to ensure that you are spec’ing the truck for the job it needs to do. But, don’t just continue with the same specs year-over-year. Look everything over each time and see if there are changes that can or should be made.

“Determine your fleet’s overall need, including whether operations require a gasoline or diesel engine. Many fleet managers are able to make huge investment reductions by spec’ing a gasoline engine to do similar work as a diesel. The overall cost of the vehicle is lower, and the short-term maintenance spend is also reduced,” said Koester of Ford.

Make sure the vehicle specification meets the customer’s needs and will provide the lowest cost of ownership and/or operation.

“Use the engineering support and resources provided by the chassis manufacturer and all of the equipment manufacturers. Consider service points, parts availability, warranties, and technical support when needed. Also, consider the resale value when the vehicle needs to be replaced,” said Ellsworth of ICTA.

But, be careful of what you may sacrifice for cost savings.

“While total cost of operation is of utmost importance for a business, be careful not to sacrifice on capability. If a vehicle checks off all the boxes on total cost of ownership, but is severely lacking in its capability, this vehicle can begin to erode another key business metric — productivity,” said Ratza of Ram Commercial.

Work with a salesperson who knows your segment and understands the environment your vehicle will be operated in.

“Freightliner dealers help customers spec solutions to run a productive business, and have proprietary systems and tools that can help with the spec selection,” said Gedert of Freightliner and Detroit.

And, don’t forget about upfitting and needs body builders may have.

“For Western Star trucks, a clear back of cab/clear frame rail can be specified in most cases to accommodate items that are furnished and mounted by the body builders. Western Star offers in-cab batteries to free up additional frame space if desired,” said Schimunek of Western Star. “Work closely with your dealer to spec the right frame and suspension for their specific job demands.”

Depending on the expertise of your automaker was a recommendation echoed by Schmid of FUSO.
“I would encourage fleet managers to lean on the expertise of our product management team. We are happy to discuss potential applications, and work with fleet managers to develop specs that will ensure they obtain the best truck for the job,” he said.

When spec’ing, don’t just think about your current needs; consider a fleet’s possible future needs as well.

“Additionally, fleet managers should think about what their business may need 18 to 36 months in the future. A fleet manager who is aligned to the overall long-term business needs can make better decisions on correctly spec’ed trucks, reducing the likelihood of having wasted capability, or worse yet, insufficient capability,” Koester said.

Top Items to Consider When Spec’ing a Truck’s Suspension

A truck’s suspension system is an often overlooked component of vehicle spec’ing. A common misperception is that the only reason suspension matters is when dealing with on- or off-road vehicle performance, but there is far more involved and it’s a large component in driver comfort.

“Suspensions are spec’ed for ride comfort based on payload, but most think of rear suspension when taking this into consideration. The front suspension must also be addressed when auxiliary upfit equipment, such as a snow plow blade, will be used to add further weight to the front of the vehicle. Type of consideration (spring, air, dual leveling, etc.) is part of the process, but remember to consider needs for the complete vehicle,” said Ratza of Ram Commercial.

According to Gedert of Freightliner and Detroit, the top items to consider when spec’ing a truck’s suspension include “what’s being hauled, the application, and the environment. Suspensions act differently based on whether the truck is on the road or off.”

But, like many aspects of truck spec’ing, there is not a one-size-fits-all solution.

“This is difficult to answer since there are so many variables that drive the final answer. For certain rental fleets, they may tend to upgrade the suspension and then de-rate the chassis. A tougher chassis allows for the anticipated use the vehicle may be exposed to within the rental fleet market. Another consideration is ride quality. Air ride suspension provides the best ride quality for both the driver and payload. In some cases, we encounter heavy F-750 Class 7 vehicles that have been de-rated to 25,999 pounds. The same truck could be spec’ed as an F-650. The biggest difference is one of payload. Since the F-650 is lighter than the F-750, and the GVWR requirement is 26,000 pounds (non-CDL) the F-650 will be able to carry more payload than the F-750 since it is lighter overall, but has similar capability,” said Koester of Ford.

In addition to driver comfort, consider what the trucks will be hauling.

“Is the payload sensitive to vibration and/or movement? Ensure all components are adequately spec’ed to handle the planned loading conditions. Does the suspension support the intended GVWR?” said Ellsworth of ICTA.

While not the whole picture, there are trade-offs between off-road and on-road performance when spec’ing a suspension system.

“Performance depends on the type of suspension. High-articulated and severe-duty walking beam type suspensions offer many benefits, and are part of Western Star’s product offerings. Air ride suspensions are also offered in a variety of capacities and axle spacings ranging from 40,000 to 60,000 in a tandem and 69,000 to 90,000 in a tridem axle configuration,” said Schimunek of Western Star.

What Not to Overlook When Spec’ing a Truck’s Suspension

Remember, a truck suspension consists of the axle, spring, and tire.

“All three items affect the GAWR and performance. The weakest link will dictate the axle GAWR. In my opinion, the tire is most often overlooked. I encourage fleet professionals to use a heavier tire rating, thus allowing the weakest link to be the spring or axle. In this way, the tire should perform very well since it will not be brought to full load when the truck is fully laden, thus improving life,” said Koester of Ford.

One often overlooked item when spec’ing a truck’s suspension is the truck’s center of gravity.

“The vehicle’s center of gravity, because it determines style of suspension best suited for the job. Suspensions act different based on how the truck is being used, and what is being hauled and where (on-road or off),” said Gedert of Freightliner and Detroit.

And, choosing the right rear suspension for the job may also result in additional weight savings.

“Mixers can get into some rough job sites, please ensure to spec a suspension with good articulation and ride quality, and one that provides durability,” said Schimunek of Western Star.

Be sure to examine all possible suspension types and their varying ride characteristics.

“The ride characteristics of various types of suspension differ, such as taper leaf, multi leaf, solid, and air ride suspension, to protect the valuable cargo being carried and ensure the suspension can carry the load. Also, consider the cost of maintaining various types of suspension,” said Ellsworth of ICTA.

Advice for Spec’ing a Truck’s Suspension

Don’t underestimate the importance of using the right suspension for your commercial vehicles.

“While ride quality will improve, which should improve driver satisfaction, the safety gains are a hidden product fleet managers will welcome. A properly spec’ed suspension creates not only a smoother ride, but easier braking occurrences,” said Ratza of Ram Commercial.

Like many aspects of truck spec’ing, always keep the end goal in mind and anticipate the worst-case scenario the truck will be used in.

“If this is a dump truck, going off road fully laden and traversing over uneven terrain, then the suspension will work harder than a dump truck that works on hard level pavement. With the more severe duty requirement, I would suggest increased spring rate, increased axle rating, and even increased frame RBM. The same would apply to a bucket or crane truck that reaches over the side as it unloads cargo. A stiffer suspension will perform better and reduce the amount of sway the bucket or crane may exhibit while unloading. Suspensions do more than just carry a load, they also determine how well the load will be managed over time during severe duty cycle and also how the truck may behave while being loaded, or unloaded,” said Koester of Ford.

Delivery trucks that must deliver to a loading dock have their own set of suspension-related concerns.

“If the truck is to be used for dock loading/unloading, consider the height of the cargo floor under varying load conditions. Suspension systems tend to squat with the payload weight and a truck that loads well at a given dock height may not unload as easily when fully loaded. Typically, chassis OEMs provide suspension deflection charts/values to assist with loaded chassis height calculations. Consider the types of payloads to be hauled and select the correct suspension type to protect the load and provide the lowest maintenance cost,” said Ellsworth of ICTA.

If a truck will have a lift axle installed, be sure to take this into consideration during the spec’ing process.
“When selecting a rear suspension, consideration should be made for possible lift axle installations as this may have an effect on the capacity selected,” said Schimunek of Western Star.

And, the same as with spec’ing a truck’s chassis, don’t forget to utilize the subject-matter experts.

“When spec’ing a truck’s suspension, work with a salesperson who understands your business and the environment the vehicle will be operated in,” said Gedert of Freightliner and Detroit.

The Bottom Line

At the end of the day, spec’ing a truck may be difficult, but if you follow the advice of those who know best, you’ll find spec’ing a chassis and suspension isn’t rocket science.

“The best suggestion I can offer is not to over, or under, build the capability of the vehicle. With over-building, you will sacrifice payload capability. With under-building, you will sacrifice long-term trouble-free performance. Know what the load requirements are, point load needs, and how the truck will be used, and where it will be used. Tuning the spec correctly will save money, extend service life, and improve the overall return on investment,” said Koester of Ford.

And if you still have questions, do not be afraid to ask manufacturers questions regarding technical items.

“Trucks can be expensive pieces of equipment. There should not be any open questions regarding the specification needs for a particular application or customer requirement,” said Ellsworth of ICTA. 

Comments

  1. 1. Mark W. Brochtrup, CAFM [ January 11, 2018 @ 01:12PM ]

    Great article, you see this data all over – and segmented. Thanks for putting it together in a single, brief and concise article. Know your needs, predict your future and build the right vehicle on the right platform.

 

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