September 2008, Work Truck - Feature
Medium-Duty Chassis & Suspension Fundamentals
By Sean Lyden
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. Calculating 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.
