The significance of unloaded vehicle weight

From an NTEA member We are a truck equipment distributor, and we have noticed the term “unloaded vehicle weight” mentioned a number of times in body builder books and, especially, in the incomplete vehicle documents. What is so important about this weight? Unloaded vehicle weight is a key characteristic of a vehicle’s compliance to the Federal Motor Vehicle Safety Standards (FMVSSs). It is utilized in a couple of different ways, as related to these requirements, which is why it appears so many times in an incomplete vehicle document (IVD). Unloaded vehicle weight is used by the National Highway Traffic Safety Administration (NHTSA) as a threshold for the application of some FMVSSs, and it is also used by chassis manufacturers as a limit for compliance with a particular FMVSS. NHTSA defines unloaded vehicle weight in Code of Federal Regulations (CFR) 49 Part 571.3 as “the weight of a vehicle with maximum capacity of all fluids necessary for operation of the vehicle, but without cargo, occupants, or accessories that are ordinarily removed from the vehicle when they are not in use.” This may sound like an elaborate definition of what the industry would call “curb weight”. However, NHTSA actually defines curb weight in the same section as unloaded vehicle weight. According to NHTSA, curb weight “means the weight of a motor vehicle with standard equipment; maximum capacity of engine fuel, oil, and coolant; and, if so equipped, air conditioning and additional weight optional engine.” For the most part, both terms are synonymous as they are used throughout the FMVSS regulations in some of the test procedures. But unloaded vehicle weight is additionally used as part of the application of different FMVSS requirements, such as in FMVSS 208, which states in S4.2.6.2, “Trucks, buses, and multipurpose passenger vehicles with a GVWR of 8,500 pounds or less and an unloaded vehicle weight of 5,500 pounds or less manufactured on or after September 1, 1998….”[emphasis added], where vehicles with an unloaded vehicle weight above 5,500 pounds would be excluded from this particular requirement. You may also recognize the term as one of the ingredients in the value for which manufacturers must account in their assignment of the gross vehicle weight rating for a new motor vehicle on the certification label from CFR 49 Part 567.5(d)(2)(iii): “'Gross Vehicle Weight Rating' or 'GVWR' followed by the appropriate value in kilograms and (pounds), which shall not be less than the sum of the unloaded vehicle weight, rated cargo load, and 150 pounds times the number of the vehicle's designated seating positions. However, for school buses the minimum occupant weight allowance shall be 120 pounds per passenger and 150 pounds for the driver." Since weight so heavily influences the outcome of dynamic testing, unloaded vehicle weight is referenced greatly in the FMVSSs that involve such tests applicable primarily to vehicles with a GVWR of 10,000 pounds or less. From the chassis OEM perspective, unloaded vehicle weight is one of the characteristics of a vehicle as it goes through its FMVSS compliance testing and development. Since more mass means more crash energy that has to be managed for something like a frontal barrier test, the heaviest unloaded vehicle weight a manufacturer uses to successfully pass these tests becomes the “maximum unloaded vehicle weight” that the OEM will represent as one of the means for achieving pass-through compliance. By keeping the curb weight of the vehicles you complete at or under the maximum, you preserve traceability back to the successful test results of the OEM. Building vehicles heavier than this value goes outside of what the OEM is representing based on its testing, so you would need to rely on some other method of compliance besides the conformity statements in the IVD. Conversely, you may see a reference to a minimum unloaded vehicle weight in an IVD. These would typically apply to compliance with brake system requirements, where a vehicle that was completed with a weight below the minimum specified in the IVD potentially could have issues meeting requirements involving anti-lock brake system functions for a particular chassis. Notes referencing the importance of meeting these weight limitations can be found in the IVDs. Building vehicles within the limits, minimum or maximum, for unloaded vehicle weight is probably the most important element of complying with the FMVSS that apply to vehicles with a GVWR of 10,000 pounds or less.

The significance of unloaded vehicle weight

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