The Car and Truck Fleet and Leasing Management Magazine

8 Mistakes to Avoid When Spec'ing a Medium-Duty Chassis

May 2007, by Sean M. Lyden

Proper medium-duty chassis and suspension specs contribute to improved vehicle safety, better performance, less downtime, lower operational costs, and fewer crew complaints.

The challenge, however, is defining the term “proper.” By offering dozens of medium-duty spec options, opportunities for error increase. With vehicle up-time and safety at stake, how do you determine which chassis and suspension specs are “proper” for your application? Start by evaluating what does not work. Here are eight common mistakes fleet managers make with chassis and suspension selection —and how you can avoid them.

1. Under-Estimating Payload Requirements
What exactly will your truck haul? How much do those items weigh?

A common mistake is to think in general terms; for instance, “The generators and compressors don’t weigh all that much.” Or, “I’m thinking maybe 4,000 lbs. or so for tools and equipment.” But in reality, you’re looking at, say, 6,500 lbs. Also, what about the weight of the fuel or other fluids in the equipment? Are you carrying extra fuel tanks on the bed? How many gallons? How much does the tank (including pump, nozzle, etc.) weigh when full?

If you’re not as precise as possible with payload projections, you risk under-spec’ing the truck, causing potential safety and maintenance issues.

Include the following key factors when projecting payload requirements:

  1. Bed weight. Your body manufacturer can provide this number.
  2. Equipment weights. Determine the curb weight if you’re hauling large pieces of equipment. This is the total weight of the equipment, including a full fuel tank and fluids. Consult your equipment representative for these amounts.
  3. Fluid weights. Suppose you’re hauling a 600-gallon water tank. How much does the water weigh when the tank is full? Use 8.4 lbs. per gallon as the multiplier. 600 gallons x 8.4 lbs. per gallon = 5,040 lbs. when the tank is full.
  4. Weight of driver and occupants in all seating positions. Truck manufacturers across the board attribute a nominal 150 lbs. per seating position in payload calculations. If your truck has five-passenger seating, multiply five by 150 lbs. This equals 750 lbs. — the minimum amount to factor in payload projections.
  5. Chassis curb weight. This is defined as the shipping weight of the cab and chassis (without body and aftermarket upfits), including all standard equipment, fluids, and full fuel tank. Consult your truck OEM or dealer representative to obtain curb weight estimates on chassis closest to your spec requirements.

Totaling these components helps accurately assess which gross vehicle weight rating (GVWR) truck best suits your needs. GVWR is the maximum allowable weight (of 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 without overkill, which would unnecessarily drive up costs.

2. Selecting a Chassis with the Right GVWR, but Wrong GCWR
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 trailer weight at maximum load capacity. This is the key number from which you calculate trailer capacities.

Take two chassis by the same manufacturer as an illustration. Both the GMC W-5500 HD cab-over (also known as low-cab forward) and a GMC C-5500 TopKick conventional cab offer the same GVWR: 19,500 lbs. But, which is a better fit for your application if you pull a heavy trailer on a regular basis?

This is where GCWR becomes important. The combined weight rating for the W-5500 is 21,000 lbs. and the C-5500 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, subtract that number from 5,000 and you arrive at 3,500 lbs. net trailering advantage for the C-Series.

Do you need that additional trailer capacity? If you use the truck to pull a trailer, factor GCWR in your spec requirements.

3. Not Matching Rear Suspension to the Right Application
Too much suspension rigidity for the wrong application may cause driver complaints and discomfort. Yet, if you intend the truck to handle constant, heavy loads, you may need to spec a stiffer, heavier-duty suspension or risk premature performance and maintenance issues.

Select the suspension that best following are three primary rear-suspension choices for most medium-duty truck manufacturers and when to spec each:

  1. Taper leaf. This best fits a shuttle bus or any other application that transports people. Spec taper leaf for a smoother ride — not maximum loads.
  2. Multi-leaf. This provides extra stability and rigidity for dump bodies or other applications that require heavier loads on the rear axle. To mitigate ride harshness, some truck manufacturers offer rear shock absorbers as a compatible option.
  3. Air suspension. This is typically available on Class 6 trucks and larger. Besides contributing to greater driver comfort without sacrificing stability under maximum loads, the air suspension lowers truck height for easier loading and unloading.

4. Overlooking Wheel & Tire Size
You've ordered a Class 6 (26,000 lbs. GVWR) truck from dealer stock, originally spec’d for a rollback application, but you intend to upfit it with an 18-ft. box for warehouse delivery. The rollback spec seems a good fit for your driver because it offers better comfort and convenience options with power windows and locks and air-ride seats. But when you take delivery, you notice a problem. The truck is not dock high. Instead of the 22.5-inch wheels, which your application requires to achieve the proper loading and unloading height, the truck sits on smaller 19.5-inch wheels. You now have a crew productivity and efficiency issue.

The opposite also holds true if you intend to upfit the truck as an industrial rollback to load and transport heavy equipment. Tire size directly impacts 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 up the equipment.

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

5. Considering Frame Strength in Selection
This is a common mistake for those new to medium-duty fleet management. For some applications, the standard frame strength may be sufficient. Why pay an additional several hundred dollars to more than $1,000 for a stronger frame if you don’t receive a corresponding benefit? Yet, if you use the truck for a heavier-duty application, you may compromise the frame’s integrity, contributing to increased maintenance issues and shortened truck life.

The following are terms and measures you’ll come across when reviewing frame strength options:

  1. Section modulus. This measurement, in cubic inches, is based strictly on frame siderail height, width, and thickness. As a frame of reference, the section modulus for the standard frame on a Chevrolet Kodiak C-7500 is 9.58 cu. in., while the strongest frame option for the chassis offers a section modulus of 17.93 cu. in.
  2. Yield strength. This refers to the maximum weight in lbs.-per-squareinch (PSI) that can be placed on the frame while 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.
  3. Resistance bending moment (RBM). This combines both section modulus and yield strength and seems to be a fair and accurate measure when comparing frame strengths between truck manufacturers. The formula: section modulus x yield strength = RBM. Go back to the previous example with the C-7500’s standard frame strength. Here’s how the numbers compute: 9.58 cu. in. (section modulus) x 50,000 PSI (yield strength) = 479,000 RBM. As comparison, the truck’s strongest frame option offers 2,151,600 RBM, based on a section modulus of 17.93 cu. in. and 120,000 PSI yield strength.

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

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

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

Avoid this mistake by matching fuel tank capacity with the intended truck application.

7. Choosing Fuel Tank Placement that Interferes with Body/Equipment Upfits
If your tank configuration gets in the way of the body company’s requirements, you must pay for modification. And it costs significantly more in labor and downtime to alter the configuration after the fact than to get it right from the factory. Ask your upfitter upfront which tank configuration your equipment requires.

The following are common terms when discussing fuel tank positioning:

  1. Side saddle. This tank is mounted outside the frame rails and is placed on the driver side, passenger side, or both.
  2. Midship. This refers to a tank inside the frame rails toward the middle of the chassis, before the rear axle.
  3. Aft axle. This tank is placed behind the rear axle.

8. Selecting Exhaust System Incompatible with Body/Equipment Upfits
Just as with fuel tank configurations, the wrong exhaust placement will drive up truck costs. There are two options when it comes to exhaust systems: horizontal and vertical. The horizontal exhaust is often standard and works fine for most applications.

When does it make sense to select a vertical exhaust? When exhaust heat becomes a safety issue to crew operating truck-mounted equipment, such as a crane, from outside the cab. The vertical exhaust diverts 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 for mounting required equipment.

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