Specing a truck for a vocational application can be a daunting task. Get it wrong and it will definitely impact the bottom line.
Customers typically start by getting a truck and trying to make it work often with disappointing results, notes Bob Johnson, director of Fleet Relations at the National Truck Equipment Association. Johnson understands the topic well. He holds a mechanical engineering degree, has 40 years of experience in utility fleet and heavy equipment maintenance and design and currently conducts seminars for customers on how to spec vocational trucks.
Both Johnson and David McKenna, director of powertrain sales and marketing, Mack Trucks Inc., agree that the first step is to define the exact vocational application.
For instance, a dump truck that is used primarily on a jobsite or largely off road would be specd very differently than a dump truck that operates mostly on maintained roads, says McKenna, yet both vehicles may be tasked with hauling similar loads.
After you have defined the application, you need to identify the limiters, says Johnson. These include restrictions on the overall length and weight of the truck, as well as the grades and terrain it will encounter. Are there any unusual items that will constrain the vehicle? he asks. Does it have an unusually steep grade that it has to climb? Will there be a certain size garage door someplace?
Topography can play a major role in the specs required. Midwest dump trucks may operate mostly on relatively flat ground, whereas a dump truck in West Virginia is going to experience significant grades and tight twisting roads, says McKenna. You need to examine the Gross Vehicle Weight Rating (GVWR) in each of these applications and determine the best solution.
When working in an on/off-road environment, you need to take into account both the off-road and on-road criteria. It is extremely difficult to design an on/off-road truck without some compromises, says Johnson. It is just deciding which compromises you can live with the best.
This involves looking at more than simply the duty cycle. A lot of people make [the mistake of] saying my duty cycle is 70% off road, [so] I will tailor the truck for that, Johnson states. But that 30% on road may involve a 100-mile route. Therefore, the ability to get over the road quickly is a critical factor.
Always start with the application and identify your functional requirements, he advises. Once you have defined the limitations, you know what the dimensions and weight are going to be. Then you can spec a truck for the application.
Also factor in running the correct components at rated capacity, says Bryan Howard, vocational sales manager, Daimler Trucks North America. For instance, if a customer took a standard delivery truck chassis and threw a dump bed on it to be used off-road 75% of the time, he is probably going to have some suspension issues, he points out. There are major design differences between on-road and off-road suspension packages, even if they are both rated for the same GAWR.
If the customer properly specs a truck with the correct components for the application, appropriately loads the chassis and operates it responsibly with proper servicing, he adds, then we would expect to see good component life and a good service life even when operated at near capacity much of the time.
The drivetrain solution is also determined by the defined application. An off-road, low-speed, high GVW truck will require much lower transmission and rear axle ratio gearing, and a high torque rise engine with plenty of peak torque, than a chassis that will see highway speeds most of the duty cycle, says McKenna. In that case, higher rear axle ratios and higher horsepower are required.