November 06, 2008
How to Spec Medium Duty Trucks
Judy McTigue, medium duty marketing manager for Kenworth Truck Company, offers spec’ing tips for medium duty trucks that fall into Class 5, 6 and 7 (16,001 to 33,000 lb. gross vehicle weight rating).
McTigue stresses the importance of considering life cycle cost instead of initial cost when buying a truck. Common customer expectations when buying trucks are fuel efficiency and dependability.
Body Wisdom
Some of the first things to consider are the type of load, annual mileage and type of operating environment. Buyers can choose from a wide range of bodies that can be mounted differently depending on the truck’s wheelbase and local weight regulations.
Each medium duty application is different. “Bodies are mounted flush with the cab or with a space in between,” McTigue explains. “They can also be on top of the frame or extend below it. Kenworth’s body builder options help the body builder with this process.
“Frame strength and length, can be determined when we know the type of body selected. Typically, the big issue with a frame isn’t just the kind of load, but also the body type and vehicle application. For example, there’s no problem with a truck-mounted crane when it’s traveling down the road mounted. But when it’s parked and lifting loads, we need to have a better understanding of the weights and stresses that will be placed on the vehicle,” McTigue says.
“If it looks like the frame will be subject to a lot of stress, a larger frame size for a higher RBM (resisting bending moment) will be spec’d by Kenworth and may include an additional frame insert or heavy duty cross members. Applications that require higher strength frames include fire trucks, tankers and dump trucks.”
Driveability
Medium duty drivers often work a normal shift close to home, or are trained in another field and trucks are simply tools of the trade, so a truck’s driveability has become a top priority for many buyers. A truck that is easy to drive and comfortable to operate will help control driver turnover.
Other important driver factors are startability, maneuverability, visibility, and ease of entrance and exit into and from the truck. “If your truck is going to be operated on busy roads, or there’s a lot of loading and unloading in tight quarters, these factors become crucial,” says McTigue.
Numerous items factor into the vehicle’s actual performance. Take, for example, turning radius. “A standard 50-degree wheelcut provides a tight turning circle. But if you need to get more weight up front, say for a crane application, you may need a heavier axle and larger tires and that could affect your turning radius,” McTigue notes.
“Longer wheelbases will also affect the turning radius. When spec’ing a unit, take into consideration the body and load carried in order to calculate optimal weight distribution and wheelbase,” McTigue says. “If it’s going to be an inner-city truck, you may want to give up a foot or two in body length for better maneuverability. But if the truck will be doing a lot of highway miles and making few deliveries, you may want to extend the body length to carry additional weight.”
Cab access is more a function of truck design, but there are still choices. “The wider the steps and the less climbing drivers have to do to get in and out, the better,” says McTigue. “Going to low-profile 19.5-inch tires can make life easier for a driver in an application with a lot of stop-and-go by helping to reduce fatigue.”
A final driveability issue is ride quality. McTigue says rear air ride suspensions are currently being spec’d on around 45% of all Kenworth medium duty vehicles.
“We see them in a variety of applications ranging from expeditors to dumps. A lot of it is how drivers prefer them for the smoother ride,” he says. “But some maintenance managers don’t want them because they feel they cost more to maintain than a leaf spring type. In general, if you don’t need air suspension to protect the load, you can still get a good ride with a leaf spring. Longer leaf springs on the front axle also help smooth and stabilize the ride,” McTigue says.
The Drivetrain
A carefully spec’d drivetrain can lower your total operating costs over the truck’s life. Be careful not to over-spec the engine, says McTigue.
“In a fire and rescue application, you’re going to need high horsepower and torque to get good acceleration. But in most pickup and delivery applications, you aren’t going to need nearly the same amount.”
Your transmission choice is primarily based on performance requirements and driver ability. “A manual transmission may be best if you either have an experienced driver or don’t make frequent deliveries,” McTigue says. “An automatic or automated transmission may be a better fit if you either have a new driver or routes with frequent stops.”
Whether you select manual or automatic, you should choose the rear axle ratio carefully to get the best fuel economy. “Pick something that will provide the startability needed based on the load that will be carried, but also keeps the engine in the most fuel-efficient operating range as long as possible.