Maximum center of gravity limitations for trucks 10,000 pounds GVWR and below

From an NTEA member A customer recently brought in a new 2014 C/K 2500 HD pickup box delete chassis-cab and asked us to install a service body with a corner-mounted crane. We have done the calculations to ensure the finished work truck meets the maximum unloaded vehicle weight limitations. What is next? Every final-stage manufacturer needs to perform various compliance calculations based on the individual OEM’s incomplete vehicle documents (IVDs). Some sample Federal Motor Vehicle Safety Standards (FMVSS) that require compliance calculations from various IVDs include FMVSS 105: Hydraulic and Electric Brake Systems; FMVSS 121: Air Brake Systems; FMVSS 135: Light Vehicle Brake Systems; FMVSS 208: Occupant Crash Protection; FMVSS 212: Windshield Retention; FMVSS 219: Windshield Zone Intrusion; FMVSS 301: Fuel System Integrity; and more. Most chassis manufacturers require the completed vehicle horizontal and vertical center of gravity (CG) to fall within a specific range based on the chassis for FMVSS 105, FMVSS 121 and FMVSS 135. Many OEMs have a maximum vehicle weight and/or a maximum frontal area limitation for FMVSS 208, FMVSS 212, FMVSS 219 and FMVSS 301 based on the specific chassis involved. Vehicles 10,000 lbs. gross vehicle weight rating (GVWR) and below also require special care when determining FMVSS compliance. You need to review the OEM’s IVDs to see which FMVSS may be the most limiting in terms of CG and weight limitations. One such standard is FMVSS 126_ Electronic Stability Control. Many times, the horizontal, vertical and lateral CGs are lower than the same limitations for FMVSS 105_ Hydraulic Brake Systems. Both GM and Ford have an additional CG limitation for vehicles subject to FMVSS 126. For GM, a lateral CG calculation is added for the completed vehicle in addition to the normal horizontal and vertical CG calculations the final-stage manufacturer must complete for FMVSS 105 compliance. Ford limits the maximum vertical CG on vehicles subject to FMVSS 126 to a level that may be lower than the maximum vertical CG limitation for its FMVSS 105 compliance limit based on the specific vehicle. In reviewing GM’s IVD for a 2014 GM C/K, there are specific tables for regular cab, extended cab and crew cab incomplete vehicles, with each table covering the maximum horizontal, vertical and lateral CG allowed for the completed vehicle under FMVSS 126. To determine if the vehicle in question meets the limitations, a complete horizontal, vertical and lateral CG must be calculated. Previous issues of “Tech Connections” have addressed how to calculate the horizontal, vertical and lateral CG for individual components. The following example utilizes those formulas to determine the completed vehicle’s horizontal, vertical and lateral CG. They can then be compared to the FMVSS 126 horizontal, vertical and lateral CG limitations provided in the OEM’s IVD. Example Let’s look at a 2014 Chevrolet 2500HD regular cab 4x2 with pickup box deleted from the factory with a service body and a small corner-mounted crane as an example. C/K 20903_ Weight = 5,200 lbs., Service Body Weight = 1,300 lbs., Crane Weight = 250 lbs., Crane Support Weight = 75 lbs. and Crane Reinforcement Weight = 125 lbs. The chart below shows the horizontal, vertical and lateral CG as mounted on the chassis for the previously mentioned components. Note the chassis, service body and crane support are centered on the vehicle and have a 0" lateral CG. The crane and crane reinforcement are mounted to the right of the centerline of the vehicle at 31.75" for their lateral CG. Using the Formula W Completed Vehicle Horizontal CG = 5,200 x 51.57 + 1,300 x 133 + 250 x 155.4 + 125 x 165.2 + 75 x 165.2 / 6,950 = 73.88" Completed Vehicle Vertical CG = 5,200 x 29.25 + 1,300 x 42.60 + 250 x 74.8 + 125 x 45.8 + 75 x 36.3 / 6,950 = 33.6" Completed Vehicle Lateral CG = 5,200 x 0 + 1,300 x 0 + 250 x 31.75 + 125 x 31.75 + 75 x 0 / 6,950 = 1.7" From the horizontal, vertical and lateral CG restriction for a C20903 in the FMVSS 126 section of the 2014 GM IVD, for the completed vehicle noted above, with a horizontal CG of 73.8" and a lateral CG of 1.7", you can see the vertical CG of 33.6" just barely falls within the table’s limitations (between 36.4" for 0" lateral CG to 33.5" for 2" lateral CG). Upfitters may want to determine which limiting components will affect an FMVSS. In addition, upfitters may want to advise the customer that chassis GVWR choices can have a significant impact on which equipment can be mounted and that FMVSS 126 can be the limiting factor in determining which equipment can be mounted where.

Maximum center of gravity limitations for trucks 10,000 pounds GVWR and below

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