Selecting proper medium-duty specifications

Feb. 1, 2003
For years, a key to specifying medium-duty trucks has been the gross vehicle weight rating. Users select trucks primarily on the basis of weight class.

For years, a key to specifying medium-duty trucks has been the gross vehicle weight rating. Users select trucks primarily on the basis of weight class. So a Class 6 vehicle may be suitable for one application, and a Class 7 may work better for another. For instance, a Class 6 vehicle may be fine for foodservice distribution, while a Class 7 truck may be needed for the heavier payload of dairy delivery.

While the vehicle's GVWR is still important, buyers are increasingly focusing on other factors when making their selection, says Mike Parrish, medium-duty product marketing manager for Kenworth Truck Company.

“The expectations that people have of medium-duty trucks has continued to increase in recent years,” Parrish noted. “Many of these truck users are looking for a business solution rather than just a vehicle. Dependability is high on their list because many are hauling perishable goods, and they can't afford breakdowns.

“Buying the least expensive truck available off a dealer lot may seem like a good deal at the time, but in the long run it may be more expensive than one carefully built to detailed specifications. The key concern should be life cycle cost, and sometimes users lose sight of that. Working with a dealer, a buyer should do a thorough analysis of how the truck will be used and then put together specifications that will match the buyer's business goals and minimize life cycle cost,” Parrish says.

Consider the job

Some of the first things to consider are the type of load, annual mileage, and 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.

“Bodies can be mounted flush with the cab or with a space in between,” Parrish explains. “They can also be on top of the frame or extend below it.

“In order to calculate the correct frame strength, we first need to 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.

“If it looks like the frame will be subject to a lot of stress, we can beef it up with inserts and/or go to a larger frame size for a higher resisting bending moment. Applications that require higher-strength frames include fire trucks, tankers, and dump trucks.”

Parrish says Kenworth dealers use a special vehicle specification software called Prospector that makes it easier for a salesperson to put together the best vehicle. “We always review every order that comes in to make sure the specifications are adequate,” he adds. “Some clients like to overbuild, which adds cost unnecessarily and which we discourage. But we don't allow under-spec'ing.”

Consider driver experience

With fleets finding it harder and harder to hire qualified drivers, a truck's driveability has become a top priority for many buyers. A truck that is easy to drive and comfortable to operate is desirable.

This holds particularly true for vocations where the main job is not driving, says Parrish. “Take a look at the distribution industry. Often the driver is a salesperson first and usually isn't hired only for driving skills.

“That's why automatic transmissions have become very popular in distribution. Between 80% and 90% of all the distribution trucks we build have them. That compares to a 40% rate of automatic transmission in the total number of medium-duty trucks we build,” Parrish says.

Other important factors to drivers are startability, maneuverability, visibility, and ease of entrance and exit of the truck. “If a 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 Parrish.

Numerous items factor into a vehicle's actual performance. Take, for example, turning radius. “Our T300 comes standard with a 50-degree wheelcut for a tight turning circle. But if an application needs more weight up front, the truck may need a heavier axle and larger tires, and that could affect the turning radius,” Parrish notes.

“Longer wheelbases will also affect turning radius. When writing specifications, take into consideration body length and load in order to calculate optimal weight distribution and wheelbase,” Parrish says. “If it's going to be an inner-city truck, think about giving up a foot or two in body length for better maneuverability. But if the truck will be doing a lot of highway miles with just a few delivery stops, you may want to extend the body length to carry additional product.”

Visibility matters

Visibility and cab access are more a function of truck design than specifications, 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 Parrish. “Going to low-profile 19.5-inch tires can make life easier for a driver in a multi-stop application. Well-placed steps are one key.

“How well the driver can see to the front and sides is influenced by window area and the slope of the hood. With a 20-degree hood slope, an average-size driver can see the ground 13 feet in front of the truck. We enhance side visibility with a wrap-around windshield, large door windows, and a peeper window,” says Parrish.

“Be sure to note how the mirrors are mounted. We mount mirrors on the cab rather than the door so the view out the window isn't blocked by mirror brackets,” he says.

“This reduces vibration and keeps the mirror from going out of adjustment every time the door is slammed,” he says. “For urban delivery, we have numerous mirror options to fit specific needs.”

A final driveability issue is ride quality. Parrish says rear air suspensions are currently being purchased for about 30% of all Kenworth medium-duty vehicles. “We see them in a variety of applications. A lot of it is that drivers prefer them for a smoother ride,” he says. “But some maintenance managers don't want air suspension, because they believe air ride costs more to maintain than a leaf spring suspension. In general, if air suspension isn't needed to protect the load, spring suspension still provides a good ride. Longer leaf springs up to 64 inches — the longest available — on the front axle also help smooth and stabilize the ride.”

Choose engines carefully

A carefully designed drivetrain can lower total operating costs over the truck's life. Be careful not to overrate the engine, says Parrish. “In fire and rescue applications, high horsepower and torque are needed for acceleration, but most distribution applications don't need that sort of performance,” he says.

In anticipation of the October 2002 EPA emissions regulations, Kenworth engineered a new cooling system for the T300. “It was designed to handle not only the new Cummins ISB EGR engine but also the current Cummins ISC and Caterpillar 3126,” says Parrish. “We'll see other emission-related engine changes in 2003, but we don't expect any significant performance or maintenance issues because of them.”

Transmission choice is primarily based on performance requirements and driver ability. “A manual transmission may be best with experienced drivers or routes with few stops,” he says. “An automatic transmission may be a better fit with newer drivers or routes with frequent stops.”

With manual or automatic transmissions, choose the rear axle ratio carefully to get the best fuel economy. Pick something that provides the startability needed based on the anticipated load, but also keeps the engine in its most fuel-efficient operating range as much as possible. As it is with most specification choices, it all comes down to finding the right balance, Parrish says.

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