Where do you obtain frame strength numbers to review? Contact the chassis OEM or dealer rep for more information. In addition, consult your body and equipment upfitter to confirm the frame strength level to select.  -  Photo: Isuzu

Where do you obtain frame strength numbers to review? Contact the chassis OEM or dealer rep for more information. In addition, consult your body and equipment upfitter to confirm the frame strength level to select.

Photo: Isuzu 

If you feel overwhelmed by medium-duty truck specifications, you’re not alone. While light-duty work trucks and cargo vans typically have a handful of total options to consider, medium-duties aren’t as straightforward, with myriad options to filter through. When spec’ing a truck that can cost $60,000-$100,000 or more, stakes are high, leaving fleet managers very little room for error.

How can you make this process less intimidating? Begin by understanding the fundamentals of medium-duty chassis and suspensions. Then the other options, which may seem complex at first, (such as engine, transmission, and rear-axle ratio choices), will fall into place more easily.

How do you determine the right chassis and suspension specs for your application? Here are seven principles to follow.

1. Calculate Accurate Payload Needs

What exactly will your truck haul? How much do those items weigh?

Avoid thinking in general terms, such as, "I’m thinking maybe 4,000 lbs. or so," when in reality, you’re looking at 6,500 lbs. Imprecise payload projections risk under-spec’ing the truck, causing potential safety and maintenance issues.

Here are key factors to include when projecting payload requirements:

  • Bed weight. The body manufacturer can provide this number.
  • Equipment weights. If hauling a Bobcat or other pieces of equipment, what’s the curb weight? This is the weight of the equipment including a full tank of fuel and fluids. Consult the equipment rep for these amounts.
  • Fluid weights. Suppose you’re hauling a 600-gallon water tank. How much does that water weigh when the tank is full? Use 8.4 lbs. per gallon as the multiplier (See sidebar, Fluid Weight Multipliers.)\
  • Weight of driver and occupants at all seating positions. Truck manufacturers across the board attribute a nominal 150 lbs. per seating position in payload calculations. For five-passenger seating, multiply five by 150 lbs., which equals 750 lbs., the minimum amount to factor in payload projections.
  • As-spec’d chassis curb weight. This factor is defined as the shipping weight of the cab and chassis (without body and aftermarket upfits), including all standard equipment and options, fluids, and a full tank of fuel. Consult the truck OEM or dealer rep for curb weight estimates on chassis closest to your spec requirements.

Totaling these components helps accurately assess what gross vehicle weight rating (GVWR) truck best suits your need. GVWR is the maximum allowable weight (chassis, occupants, and payload), as determined by the manufacturer, for the vehicle to safely start and stop.

Your objective is to select a truck that offers a slight buffer in payload capacity but is not overkill, which would unnecessarily drive up cost.

2. Verify Sufficient GVWR

If the truck will pull a trailer, verify the GCWR is sufficient to meet both payload and trailer needs. Gross combination weight rating (GCWR) is the maximum allowable weight (as determined by the manufacturer) of chassis, fluids, occupants, body (including equipment and payload) combined with the weight of the trailer at maximum load. This is the key number from which you calculate trailer capacities.

To illustrate, take two chassis with the same GVWR. Both the Isuzu NRR W-5500 cab-over (also known as low cab forward) and Chevy C5500 Kodiak conventional cab offer the same 19,500 lb. GVWR. However, if you’re pulling a heavy trailer on a regular basis, which is a better fit for your application?

At this point, GCWR becomes important. The combined weight rating for the W-5500 is 21,000 lbs. and the C5500 is 26,000 lbs., a 5,000-lb. difference. Since the W-5500 cab weighs approximately 1,500 lbs. less than a comparably equipped C-series, there is a 3,500 lbs. net trailering advantage for the C-Series.

Do you need that additional trailer capacity? Factor GCWR in spec requirements.

 -  Source: Work Truck

Source: Work Truck

3. Select Rear Suspension

Choose a rear suspension that best matches your application. Too much suspension rigidity for the wrong application may cause driver complaints and discomfort. Yet, if the truck will handle constant, heavy loads, you may need to spec a stiffer, heavier-duty suspension or risk premature performance and maintenance issues.

The following terms detail three primary rear suspension choices for most medium-duty truck manufacturers and when to spec each.

  • Taper leaf – best fits a shuttle bus, any application that transports people, or an application for which you don’t want the back end bouncing hard. A taper leaf is spec’d for a smoother ride, not maximum loads.
  • Multi-leaf – provides extra stability and rigidity for dump bodies or other applications requiring heavier loads on the rear axle. To mitigate harshness in the ride, some truck manufacturers offer rear shock absorbers as a compatible option.
  • Air suspension – typically available on Class 6 trucks and larger. In addition to contributing to greater driver comfort without sacrificing stability under maximum loads, air suspension offers the ability to lower truck height for easier loading and unloading.

Which type of suspension would work best for your truck’s purpose?

4. Confirm Wheel & Tire Size

You’ve ordered a Class 6 (26,000 lb.- GVWR) truck from dealer stock, originally spec’d for a rollback application, but you intend to put an 18-foot box on it for a warehouse delivery truck. The rollback spec seems a good fit for your driver because it offers better comfort and convenience options with power windows, locks, and air-ride seats.

When you take delivery, however, you notice a problem: the truck is not dock high. Instead of the 22.5-inch wheels your application requires to achieve proper loading and unloading height, the truck is sitting on the smaller 19.5-inch wheels. You now must deal with a crew productivity and efficiency issue.

The opposite also holds true. If you intended to upfit the truck as an industrial rollback to load and transport Bobcats and other heavy equipment, the tire size directly impacts the body load angle as it tilts. In this case, if you spec the 22.5-inch tires, the tilt angle will be too steep to roll the equipment up.

Avoid this mistake by double-confirming wheel and tire size fit your application.

5. Select the Frame Strength

For some applications, the standard frame strength may be sufficient. Why pay several hundred to more than a thousand dollars additionally for a stronger frame without a corresponding benefit? Yet, if you’re using the truck for a heavier-duty application, you may compromise the frame’s integrity, contributing to increased maintenance issues and shortened truck life.

In reviewing frame strength options, the following are terms and measures to know.

  • Section modulus. This measurement, in cubic inches, is based strictly on frame side-rail height, width, and thickness. As a frame of reference, the section modulus for the standard frame on a Chevrolet Kodiak C7500 is 9.58 cubic inches, while the strongest frame option for that chassis offers a section modulus of 17.93 cubic inches.
  • Yield strength. This factor refers to the maximum weight in pounds per square inch (psi) that can be placed on the frame allowing it to return to its original position without permanently bending or creasing. Typical yield strengths for Class 4-7 trucks range from 50,000-120,000 psi.
  • Resistance bending moment (RBM). This calculation combines section modulus and yield strength and seems a fair and accurate measure when comparing frame strengths between truck manufacturers. The formula: Section Modulus x Yield Strength = RBM. Revisit the previous example with the standard frame strength of the C7500. Here’s how the numbers compute: 9.58 cubic inch (Section Modulus) x 50,000 psi (Yield Strength) = 479,000 RBM. In comparison, the strongest frame option on that truck offers 2,151,600 RBM, based on a section modulus of 17.93 cubic inches and 120,000 psi yield strength.

Where do you obtain frame strength numbers to review? Contact the chassis OEM or dealer rep for more information. In addition, consult your body and equipment upfitter to confirm the frame strength level to select.

6. Review Fuel Tank Capacity & Placement 

A single 25-gallon fuel tank in a Class 5 truck equipped with service body and crane for on-site equipment repair may be sufficient. The truck primarily stays on the job site, not required to travel many miles during the day. Fueling intervals aren’t a critical issue.

However, this 25-gallon fuel capacity in another Class 5 truck designed for on-highway transport of pipe and other electrical supply materials presents a productivity and efficiency problem. The driver must stop more frequently to refuel during routes, causing slower delivery times and greater crew frustration and complaints.

Another fuel tank issue is placement on the chassis frame. If the tank configuration gets in the way of the body company’s requirements, you pay to modify it. It costs significantly more in labor and downtime to alter the configuration after the fact than to get it directly from the factory. Ask your upfitter up-front what tank configuration your equipment requires.

Here are the common terms to know when discussing fuel tank positioning:

  • Side saddle. This tank is mounted outside the frame rails and placed on the driver’s side, passenger’s side, or both.
  • Mid-ship. This refers to tank placement inside frame rails toward the middle of the chassis, before the rear axle.
  • Aft axle. This tank is mounted behind the rear axle.

7. Select Compatible Exhaust Systems

Just as with fuel tank configurations, the wrong exhaust placement drives up truck costs.

Two options are available with exhaust systems: horizontal and vertical. The horizontal exhaust is often standard and works fine for most applications.

It makes sense to select a vertical exhaust when exhaust heat can become a safety issue to a crew operating truck-mounted equipment, such as a crane, from outside the cab. The vertical exhaust diverts the heat away from the driver and equipment.

However, depending on the chassis manufacturer, the vertical exhaust option may shorten effective cab-to-axle (CA), requiring body modification to fit properly. Therefore, work closely with your upfitter to ensure sufficient CA to mount the required equipment.

Put it All Together 

With knowledge comes confidence. Focus on the fundamentals of medium-duty chassis and suspensions, and you’ll spec a truck that offers improved vehicle safety, better performance, lower operational costs, and fewer crew complaints.

About the author
Sean Lyden

Sean Lyden


Sean Lyden was a contributing author for Bobit publications for many years.

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