How to develop better chassis specs

WHEN SPECIFYING THE CHASSIS for a work truck, the ultimate objective should be to ensure that the completed vehicle will be optimized for its intended application. Doing so will result in a safer, more productive vehicle while minimizing the overall life-cycle cost of ownership.

In his Work Truck Show presentation, “Top 10 Elements for Developing Better Chassis Specifications, Robert Johnson said that fleets too often select a chassis for a given application based on intuitive factors, including past experience, recommendations from others, or just a guess as to what size and chassis are needed.

“Instead of relying on this type of decision process, you should follow an orderly procedure for identifying your chassis requirements,” he said. “In order to properly spec the chassis, you must first identify a number of factors, such as the second unit (upfitting) requirements, completed vehicle performance criteria, regulatory, and other design constraints. These requirements will ultimately determine the final design of the completed vehicle and therefore drive the chassis specifications.

“The overall design and specification process can be broken down into 10 discreet steps. Following this process will help ensure that your completed vehicle will be the right vehicle for the desired application.”

Here are the 10 steps, with examples of the types of information that need to be identified in each step:

  • Identify basic application requirements.

    What will the vehicle be used for? What are the performance requirements for the second unit? What is the projected duration of the application? What type of second unit body and/or special equipment is desired?

    Johnson warned that customers who automatically buy the same model truck as the one they are replacing may be surprised to discover that capacities have changed.

    “Because trucks weigh over 1000 pounds more these days, if you buy the same truck you did 10 years ago, you have less payload,” he said.

  • Determine applicable design constraints.

    Dimensional limits (width, height, length, weight); operational restrictions, regulatory issues, contractual requirements, financial constraints.

  • Determine net payload requirements.

    Payload weight (delicate equipment or cargo?); payload volume, dimensional requirements (size of materials to be transported, etc).

    “Sanity Check 1: At this point, you should stop and determine if any of the various factors identified to this point are mutually exclusive,” he said. ‒For example, if you have determined that the completed vehicle cannot exceed 26,000 pounds total weight and the required payload is 24,000 pounds, you probably have a conflict. If a conflict is identified, go back and revise your requirements as necessary. If no conflicts are identified, go on to the next step.”

  • Identify primary special equipment/second-unit requirements.

    Size of special equipment and/or second unit to be upfitted to chassis; weight of upfitting components; cargo storage/securement requirements; component installation requirements (chassis frame requirements, clear cab to axle distance, exhaust system location restrictions, etc); operational requirements (power sources for equipment, equipment access requirements, etc).

    Capacities, however, are not always what they seem, Johnson said. Alternator output, for example is shown at a specified engine speed. But if peak electrical demand occurs at a time when the truck is operating at a lower engine speed, the alternator probably will not be producing at its rated capacity. He said snowplow applications frequently drain the truck battery because they usually operate at engine speeds well below those needed to reach peak alternator output — which may require the truck to travel at full highway speeds.

    The location of exhaust systems will be important when the 2007 diesel engines begin to appear, Johnson said. Chassis manufacturers are expected to restrict the use of U-bolts behind the cab because of the new regulations. And because of the extra heat associated with the new exhaust systems, distributors should evaluate what they mount near the exhaust systems.

  • Identify secondary special equipment/body requirements.

    Size of accessory items, weight, installation requirements, operational requirements. “Most chassis aren't set up for a fuel tank tap,” he said.

  • Determine gross payload requirements (total weight to be carried by chassis).

    Net payload, second-unit weight, special-equipment weight, trailer towing loads (tongue weight, total weight), crew weight, fuel requirements (weight).

    “Sanity check 2: At this point, you should again stop and ensure that there are no mutually exclusive requirements identified for the completed vehicle,” he said. “If so, they must be addressed before continuing.”

  • Identify vehicle operating conditions and environment.

    Duty cycle (on road/off road, operational cycle in terms of desired cycle times, daily hours of operation, etc); loading cycle (static or dynamic loading impacts selection of suspension components), climate, weather, terrain, maintenance considerations (including maintenance of existing vehicles). A static load, one that is never removed, causes long-term damage to springs because they can never relax — even on delivery trucks that are loaded overnight on Friday to be ready on Monday. “Consider upgrading the suspension,” he said.

  • Determine desired vehicle performance requirements.

    Starting gradability (on/off road), maximum gradability, reserve gradability, maximum road speed, braking requirements.

  • Preliminary chassis design.

    Availability of appropriate chassis (GVWR/GCWR, frame, dimensions, powertrain options), estimated weight of completed unit.

    “Sanity Check 3: Using the rough data from the preliminary chassis selection make a final determination that the completed vehicle will meet all of the criteria previously identified,” he said.

  • Final chassis specification.

Spec the chassis, ensuring that all critical design factors are addressed to include: gross axle weight loadings do not exceed GAWRs (including any trailer loadings); gross vehicle weight does not exceed GVWR (including any trailer loadings); gross combined weight does not exceed GCWR (with heaviest trailer); frame selection meets or exceeds chassis OEM and equipment requirements; suspension matched to other requirements; powertrain provides desired performance (engine/transmission/final drive); support systems such as electrical system, cooling system, etc, meet or exceed identified requirements; auxiliary equipment requirements are addressed (PTO provisions, exhaust system routings, etc); other non-functional considerations (employee retention/productivity issues, resale value, driver/operator skill considerations).

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