Since well-designed equipment enhances productivity, your objective when designing a work truck is to optimize the vehicle to achieve the best overall return for dollar spent. But three common mistakes can jeopardize your results and lead to a work truck that just doesn’t work.

Here’s some advice on how to defeat these spec killers:

Spec Mistake #1: Working Out of Order

Many people start the specification-writing process with the truck chassis, but it is actually most logical to address the body and equipment first. An effective, productive work truck is designed to perform a specific job or series of jobs. Therefore, it makes sense to start the process by defining the job or jobs the truck will perform.

From there, you can address the equipment and associated truck body needed to perform those tasks. Finalizing the body and equipment will define the vehicle’s required payload, necessary clear cab to axle chassis length and the loaded body’s center of gravity.

This information enables you to select an appropriately sized chassis and perform an accurate weight distribution and payload analysis to determine the appropriate chassis. Finish by choosing the powertrain that meets your performance criteria.

Spec Mistake #2: Doing What You’ve Always Done

It’s an easy trap to fall into. You’re replacing an existing vehicle, so you figure all you have to do is duplicate the old unit. Not so fast. For starters, the current vehicle may not have been optimized for the job it is doing. In such a case, the end users probably figured out how to make it work as well as they could, but that does not mean you shouldn’t improve the new unit.

Even if the vehicle was properly spec’d at the time it was initially acquired, requirements change and new/ improved products are constantly being introduced. It is possible that there is a new body, chassis or piece of equipment that would offer improvements over what was previously available.

In particular, chassis selection should be carefully thought through every time. Chassis specifications change from year to year, so a current Class 3 chassis, for example, may not have the same net payload as an older Class 3 chassis from the same OEM.

One of the challenges facing work truck designers is keeping up with these changes. To some degree, you can accomplish this by reviewing new product literature and reading industry trade journals. However, nothing really beats seeing the equipment firsthand and talking directly to representatives of the companies developing this new equipment.

Spec Mistake #3: Taking Your Best Guess

After the preliminary selection of a suitable chassis has been made, perform a detailed weight distribution analysis to determine individual axle loadings. When necessary, optional axles, suspension components and frames may be required to ensure that the chassis is not overloaded and all equipment-mounting requirements have been met.

Once the correct foundation components have been selected, it is time to move on to the powertrain (engine, transmission, axle ratio, etc.) and other components, such as the electrical system, fuel tanks and cab content. All of these components have an impact on the completed vehicle’s overall performance and productivity, so it’s important to make selections based on valid requirements.

In too many cases, vocational truck designers make powertrain selections based on guesses instead of a careful analysis of horsepower and engine torque demands. Start by defining your performance criteria (road speed, starting gradeability, reserve gradeability, etc.) and then calculate your actual requirements. This will allow you to select the proper engine, transmission and axle ratio to ensure that the truck performs as required. The payoff in this process is a truck that has better fuel economy and lower maintenance costs.

The powertrain design process is not difficult, but it’s critical that you know how much the completed vehicle will weigh (including trailers for combination vehicles) and that you will make realistic selections when establishing your performance criteria. Chassis dealers have access to sophisticated computer programs that will make these calculations for you, but the output is only as good as the information you put in.

Writing a Killer Spec

Designing an integrated work truck is not difficult. It does require that you understand the design principles involved and that you research your requirements before starting the design process. If you take the time to write a killer spec, you will be rewarded with a more productive, cost-effective vehicle.

Learn more about specifying vocational trucks at The Work Truck Show 2014 on March 5-7 in Indianapolis. The Work Truck Show educational conference (March 4-6) offers 60 educational sessions, including a series of three sequential truck specification seminars:

  • “Developing Optimal Specifications for New Work Truck Bodies and Equipment,” presented by Robert “Bob” Johnson, NTEA director of fleet relations.
  • “Designing Your Next Truck Chassis to Meet the Job Requirements,” presented by Robert J. Aquaro, vice president, TARA Commercial Vehicle Consulting Services (Lake Placid, Fla).
  • “Optimizing Efficiency and Performance: Properly Spec’ing Your Next Powertrain,” presented by Joe Johansson, senior applications engineer, Allison Transmission, Inc. (Indianapolis).

For more information about The Work Truck Show, visit

By Robert “Bob” Johnson, director of fleet relations for NTEA (the Association for the Work Truck Industry)