After creating the layout, indicate on the drawing the weights and centers of gravity for each component, plus the payload, using the front end of the body and the frame mounting pads as reference points, he adds. Then, determine the total weight of the body and payload combination and locate the center of gravity by performing a weight distribution analysis.
Properly matching the chassis to the application to obtain the necessary gross payload and weight distribution needed to properly carry body and equipment and to provide the required towing capacity helps increase productivity and reliability, reduce maintenance and operating costs, extend vehicle life and improve safety.
A chassis that is too “small” for the intended application will be overloaded, resulting in poor reliability, says Johnson. If too “large,” there is unnecessary initial cost and a weight penalty. If the wheelbase is wrong, the vehicle will handle poorly and will be unable to carry the full gross vehicle weight rating (GVWR).
When making the chassis selection, he says there are a number of key factors to consider. Among them:
- Gross combination weight rating (GCWR).
- Gross axle weight rating (GAWR).
- Wheelbase needed to accommodate the selected body with a chassis OEM-specified back-of-cab-to-body clearance allowance.
- Frame rating. Be aware that frame selections may de-rate the GVWR on a long wheelbase chassis.
- Chassis style (conventional, set-back conventional, low cab forward, etc.)
The frame, axles and associated brakes and suspension carry the weight, provide the structural strength needed for durability and steer and stop the truck, says Johnson. In order to maintain a safe, certifiable truck, none of these components can be overloaded.
The manufacturers of some types of equipment, such as cranes, aerial devices and dump bodies, may specify minimum frame requirements over and above what the chassis manufacturer requires for a given GVWR, he points out.
While the individual axles and suspensions cannot be overloaded, it is wise to provide some margin of reserve, he says. Recommendations vary, but in general, the front axle of a truck should not be loaded to more than 90 to 94 percent of its rated capacity. Rear axles should be limited to around 95 percent of their capacity.
The next step in the design process is to decide upon the powertrain necessary to provide a truck that meets the established performance criteria while also offering optimum fuel economy. An improperly specified engine, drive axle ratio and transmission will not operate efficiently, resulting in poor performance, higher fuel costs and increased maintenance and operating costs.
Most chassis dealers have access to computer programs that will calculate the optimum powertrain for an application, Johnson says, or it can be done using available charts and tables.
When making these selections, be sure to establish realistic vehicle performance guidelines, he warns. If the guidelines are set too low, vehicle performance will suffer and you may end up with a high-maintenance unit. If the standards are too high, fuel consumption will be excessive.
Other elements that will help determine the correct selection of engine, transmission and other vehicle components are:
- How often the vehicle will be driven in the city, on the highway or off-road.
- Will it be used mostly in level or hilly terrain?
- Operational (drive and duty) cycles, including desired cycle time and daily hours of operation.
- Loading cycle, climate and maintenance requirements.
For example, Johnson says that if the vehicle will be used in temperatures above 90 degrees Fahrenheit for an extended period of time, it would be wise to upgrade the engine and transmission cooling systems, select high-temperature-rated tires and specify deeply tinted glass in the cab. In a very cold climate, it would be advisable to relocate air system tanks or use remote drain systems to facilitate manual draining, install upgraded air dryers and specify heated mirrors and windows.
Once the foundation and powertrain components have been selected, it is time to complete the chassis by choosing the rest of the truck’s components, says Johnson. These include the cooling, electrical, fuel braking and exhaust systems, along with power take-off provisions, the cab and working environment and steps, bumpers, fairings, etc..