Spec’ing Vocational Trucks For Minimal Maintenance

Vocational vehicles (work trucks) continue to grow in complexity and sophistication. At the same time, all companies are under increasing pressure to maximize their vehicle investments.Consequently, all fleets need to re-think all aspects of how they design, maintain and manage their vehicles. This process needs to begin start well before a new work truck is ordered.No longer is it prudent for a fleet to just buy what it always bought. It has to figure out how to keep its vehicle maintenance and operation costs down prior to starting the truck acquisition process.

The best practice is to take a systematic approach to specifying a new vocational truck to ensure that the completed vehicle will be optimized for its intended application and will be able to handle the different types of wear and stresses vocational trucks are subjected to, advises Bob Johnson, director of fleet relations for NTEA, the Association for the Work Truck Industry. NTEA represents companies that manufacture, distribute, install, sell and repair commercial trucks, truck bodies, truck equipment, trailers and accessories (www.ntea.com)


The first step is to accurately define the truck’s functions and identify its requirements, Johnson says. These may include:

  • Type of truck body and mounted equipment, such as crane, generator, welder, liftgate, etc., required.
  • Capacity and performance of the mounted equipment.
  • Truck performance requirements (starting, gradeability, road speed, etc.).
  • Trailer towing requirements, including trailer loading (gross combination weight rating).
  • What payload weight and volume will the vehicle have to carry?

He adds that it also important to identify any conditions or limitations that may apply to the completed unit, such as:

Operating environment

Think about such things as:

  • Does the vehicle need to haul equipment?
  • What kind of material will be hauled and how much?
  • Will the truck have different uses at various times of the year? For example, will it be used to plow snow in the winter?
  • How can available features improve or hinder operator productivity?
  • Maintenance environment.
  • Projected life cycle of the application.
  • How many workers will the truck carry regularly?
  • Regulatory requirements.


Once the truck’s functions and requirements have been determined, the next step is to select the appropriate components and equipment to achieve the design goals.

When choosing individual components, Johnson says keep track of their weight and center of gravity. Having this information up-front will make it easier to perform the second unit weight distribution analysis.

Further, he suggests making sure the various components are compatible with each other and that they match the previously identified criteria. Incorrect components and options can result in upfitter delays, unplanned modifications costs and high long-term maintenance costs.

Additional considerations:

  • The size of special equipment to be upfitted to the chassis, the weight of these components, cargo storage needs, component installation requirements and operational requirements, such as power sources for equipment, equipment access, etc.
  • What are the dimensional requirements, based on the size and shape of materials to be transported?

For many vocational vehicles, accessory items like generators, hose reels and compressors must also be taken into account, he notes, and says local vehicle equipment distributors can be a useful resource in this process.


After choosing the necessary components and equipment, Johnson recommends drawing a basic layout to be sure that all of the components fit together and that no safety, productivity or maintenance issues have been created. Don’t depend on a visualization of the components, he warns. You may think that everything will fit together, but a simple sketch may reveal that there are problems.

After creating the layout, indicate on the drawing the weights and centers of gravity for each component, plus the payload, using the front end of the body and the frame mounting pads as reference points, he adds. Then, determine the total weight of the body and payload combination and locate the center of gravity by performing a weight distribution analysis.


Properly matching the chassis to the application to obtain the necessary gross payload and weight distribution needed to properly carry body and equipment and to provide the required towing capacity helps increase productivity and reliability, reduce maintenance and operating costs, extend vehicle life and improve safety.

A chassis that is too “small” for the intended application will be overloaded, resulting in poor reliability, says Johnson. If too “large,” there is unnecessary initial cost and a weight penalty. If the wheelbase is wrong, the vehicle will handle poorly and will be unable to carry the full gross vehicle weight rating (GVWR).

When making the chassis selection, he says there are a number of key factors to consider. Among them:

  • GVWR.
  • Gross combination weight rating (GCWR).
  • Gross axle weight rating (GAWR).
  • Wheelbase needed to accommodate the selected body with a chassis OEM-specified back-of-cab-to-body clearance allowance.
  • Frame rating. Be aware that frame selections may de-rate the GVWR on a long wheelbase chassis.
  • Chassis style (conventional, set-back conventional, low cab forward, etc.)

The frame, axles and associated brakes and suspension carry the weight, provide the structural strength needed for durability and steer and stop the truck, says Johnson. In order to maintain a safe, certifiable truck, none of these components can be overloaded.

The manufacturers of some types of equipment, such as cranes, aerial devices and dump bodies, may specify minimum frame requirements over and above what the chassis manufacturer requires for a given GVWR, he points out.

While the individual axles and suspensions cannot be overloaded, it is wise to provide some margin of reserve, he says. Recommendations vary, but in general, the front axle of a truck should not be loaded to more than 90 to 94 percent of its rated capacity. Rear axles should be limited to around 95 percent of their capacity.


The next step in the design process is to decide upon the powertrain necessary to provide a truck that meets the established performance criteria while also offering optimum fuel economy. An improperly specified engine, drive axle ratio and transmission will not operate efficiently, resulting in poor performance, higher fuel costs and increased maintenance and operating costs.

Most chassis dealers have access to computer programs that will calculate the optimum powertrain for an application, Johnson says, or it can be done using available charts and tables.

When making these selections, be sure to establish realistic vehicle performance guidelines, he warns. If the guidelines are set too low, vehicle performance will suffer and you may end up with a high-maintenance unit. If the standards are too high, fuel consumption will be excessive.

Other elements that will help determine the correct selection of engine, transmission and other vehicle components are:

  • How often the vehicle will be driven in the city, on the highway or off-road.
  • Will it be used mostly in level or hilly terrain?
  • Operational (drive and duty) cycles, including desired cycle time and daily hours of operation.
  • Loading cycle, climate and maintenance requirements.

For example, Johnson says that if the vehicle will be used in temperatures above 90 degrees Fahrenheit for an extended period of time, it would be wise to upgrade the engine and transmission cooling systems, select high-temperature-rated tires and specify deeply tinted glass in the cab. In a very cold climate, it would be advisable to relocate air system tanks or use remote drain systems to facilitate manual draining, install upgraded air dryers and specify heated mirrors and windows.


Once the foundation and powertrain components have been selected, it is time to complete the chassis by choosing the rest of the truck’s components, says Johnson. These include the cooling, electrical, fuel braking and exhaust systems, along with power take-off provisions, the cab and working environment and steps, bumpers, fairings, etc..

While the individual selections in these categories seem rather simple, they can have a significant impact on the final performance of the truck, he points out.

Great care needs to be taken when developing specs to ensure they meet the needs of the intended application of each vocational truck, he stresses. Under-spec’ing can lead to premature component failures, resulting in unscheduled repairs and additional downtime, plus increased life cycle costs. Over-spec’ing adds unnecessary weight to the vehicle, increasing the cost of daily operation through lowered fuel economy and added stresses.


Not to be overlooked is the importance of reviewing the maintenance histories of existing vehicles in the fleet, Johnson counsels.

Vocational trucks used in off-road applications are subject to different types of wear and stresses compared to their on-road counterparts. Failure points depend on type of equipment, operating environment, duty and drive cycles and overall quality of maintenance.

Typically, the most neglected items on vocational trucks are the work equipment installed to adapt the vehicle for a specific job, he says. All too often, maintenance on this equipment, if performed at all, is limited to servicing easily visible or accessible grease fittings.

Look for common failure patterns to see if there are areas where vehicle specifications may need to be upgraded, he advises.

Going through this process can also alert you to other potential issues within your operation. If a particular truck has higher maintenance costs than similar vehicles in the fleet, that truck’s driver may be responsible. A high incidence of repeat repairs within a short period of time may indicate poor maintenance and repair.


Johnson suggests performing a final weight distribution and capacity analysis, incorporating all of the options and accessories added to the vehicle. This will ensure that the completed truck still has the necessary payload capacity and that the individual axles are not overloaded.

If the original chassis selection was marginal, you may have to go back to the initial chassis selection process and make changes in the foundation components or even select a larger truck, he says.