Spec’ing Medium-Duty Chassis for Utility Applications

Fleet managers dread receiving a call from an upfitter who says that the chassis delivered to their shop won’t work with the original body design and will require expensive changes to make it right.

At stake could be several thousand dollars to correct the issue and costly delays in vehicle delivery. And when you multiply that across five, 10 or 20 trucks ordered with the same spec, you get a clear idea of the potential financial fallout.

How do you reduce the risk of error when spec’ing a medium-duty (Class 4-7) chassis? Follow these four principles to write a chassis spec that’s suitable for the upfit – and the job.

1. Begin with the upfit in mind.
Whether it’s a digger derrick, an aerial platform or a service body with a crane, the upfit dictates what the chassis specs should be, said Mark Steffens, medium- and heavy-duty truck consultant at Donlen (www.donlen.com), a full-service fleet management company based in Northbrook, Ill., and wholly owned subsidiary of The Hertz Corp. (NYSE: HTZ). “So work closely with upfitters from the beginning because they can help you navigate potential pitfalls with the chassis, which are expensive to correct after it has been built.”

This is especially important because new diesel emissions technologies – such as diesel particulate filters, selective catalytic reduction systems and diesel exhaust fluid (DEF) tanks – added to medium-duty trucks in 2007 and 2010 have significantly reduced available frame rail space, requiring utility fleet managers to rethink fuel tank configurations, PTO locations and openings, exhaust systems and chassis lengths.

“A dual vertical exhaust was common in the past for many utility fleets, but it’s not really a good option these days with the addition of the DEF tank,” said William J. Byron, senior truck specialist, medium- and heavy-duty trucks, at Donlen. “You’ll find that many fleets are going to the horizontal exhaust because of the [frame rail] real estate issue.”

The use of alternative fuels also impacts upfit design, said Jim Palin, truck application engineer at GE Capital Fleet Services (www.gefleet.com), a leading full-service fleet management company based in Eden Prairie, Minn. “Extra battery packs [for hybrid-electric] or fuel tanks for propane or CNG could take up space on the frame rail, and the upfit will need to account for it. So, you may need to select a longer [cab-to-axle] chassis to allow for extra space,” he said.

2. Include a buffer with gross payload requirements.
What is the total weight of the body, the equipment mounted on the body and the expected cargo at maximum load?

Calculate that weight, also known as gross payload, and add a little extra to serve as a cushion. Byron suggests adding 2,000 pounds. Then select a chassis that offers the capacity to handle the maximum estimated payload, including a buffer. This way, you reduce the risk of overloading the truck, which could lead to safety concerns, a shorter life cycle, and premature maintenance and repairs.

3. Remember the trailer.
“Gas and electric utilities tend to use trailers a lot, particularly with their medium- and heavy-duty trucks, to pull backhoes, trenchers and other equipment,” Palin said. “But a common mistake fleet managers make is that they underestimate the weight of their trailer and the machine on it. We have to be very vigilant to get the make and model of the machines and an idea of any extra items that will be placed on the trailer so we can ensure the chassis is spec’d to the proper towing capacity.”

Will the truck pull a trailer? If so, how much does the trailer weigh? What will go on the trailer and how much will that cargo weigh at its heaviest load?

Once you’ve calculated these weights, make sure the chassis offers a sufficient gross combination weight rating – the maximum allowable combined weight of the truck and trailer at full loads – to support your towing requirements.

4. Account for terrain.
“The bulk of our utility clients want as many wheels turning as possible just because they have to deal with all types of environments, whether the trucks are on dry pavement, off-road, mud or snow,” said Mark Stumne, truck application engineer at GE Capital Fleet Services. “But if the truck isn’t going to be used off-road or on harsh terrain significantly, they’ll shy away from spec’ing four-wheel drive because it usually adds about $20,000 to those trucks.”

Palin said that terrain also impacts fuel tank selection. “If you’re going off-road, a key issue is ground clearance, especially with Class 6 to 7 trucks. In these cases, we recommend selecting shallow fuel tanks versus deep tanks, which can hang too low.”

Another consideration is gradeability, or steepness, of the terrain, Steffens advised. “Will the truck operate in the Rocky Mountains or in areas where it’s flat? Spec the optimal powertrain combination of engine horsepower and torque, transmission, and drive-axle ratio to ensure the truck has the power to get off the starting line with a heavy load and handle steep grades.”

What is the horsepower sweet spot for diesel engines in utility fleet applications? Byron offered this rule of thumb: “If it’s a severe-duty vehicle, anywhere from 275 to 330 horsepower is widely acceptable.”

What about transmission selection? “If the truck operates primarily off-road, a manual transmission might be a good idea,” Byron said. “But you also have to take into consideration how many drivers you have for that vehicle. If you have multiple drivers, and they’re not skilled with a manual, the automatic transmission would be the better fit.”

The Bottom Line
When in doubt, seek input from your upfitter or fleet management company to ensure the medium-duty chassis you’re about to order fits the application. Taking a little extra time upfront to get the spec right will pay dividends in peace of mind and money saved down the road.

About the Author: Sean M. Lyden is a nationally recognized journalist and feature writer for a wide range of automotive and trucking trade publications, covering fleet management strategies, light- and medium-duty trucks, truck bodies and equipment, and green fuel technologies. He blogs at Strategy + Writing (www.seanmlyden.com).