Center of gravity (CG) is the point at which the weight of the chassis, body/equipment and payload (if collectively or individually supported) would balance vertically, horizontally and laterally. This engineering concept finds the center of the mass of an object.
The weight of the body and attached equipment and their center of gravity locations (horizontal and vertical), as well as the positioning of the cargo load, are important to the stability, handling and braking of the vehicle.
Adhering to the CG recommendations provided by the chassis manufacturer and the second unit body weight limitations for compliance with Federal Motor Vehicle Safety Standards (FMVSS) is only part of the task of producing a completed or altered vehicle that performs well in service. The allowable CG limits (vertical and horizontal) for truck chassis are contained in the incomplete vehicle document (IVD) or manual and the body builder’s book.
Check your math
Normal truck body installations require the calculation of the horizontal and vertical centers of gravity. In a few instances the lateral or side-to-side CG will need to be calculated. When calculating the CG for a complicated piece of equipment, the CG for component pieces can be computed and then combined.
The first step in calculating the CG is to establish the reference points for the horizontal, vertical and lateral center of gravity measurements. For compliance calculations, the horizontal CG measurements from the centerline of the front axle can be used. If the CG of a body is being calculated, any point such as the outside surface of the front wall can be used. Measurements forward of the reference point are negative.
The vertical CG for a chassis can be measured either from the ground or the top of the frame rail at the back of the cab for compliance calculations, depending on the requirements outlined in the IVD. If the CG of a body is being calculated, any point such as the bottom of the long sills can be used.
Any point can be used to locate the center of gravity. For example, if you need to determine the vertical CG of a truck body using the ground as a reference point, but only know the CG of the body as measured from the top of the frame rail at the back of the cab, simply add the vertical CG of the body and the distance from the top of the frame rail to the ground.
Remember, component CGs below the vertical reference point and forward of the horizontal reference point are negative.
In order to calculate the horizontal and vertical center of gravity for a truck or truck body, the weight and CG for each component must be identified and then combined to find the total CG for the truck or body.
The basic formula for center of gravity is the sum of the weight times the distance from the reference point for each component divided by the total weight of all components. The basic formula is as follows:
CG = (W1 x D1) + (W2 x D2) + (W3 x D3) + (Wn x Dn)
W1 + W2 + W3 + Wn
CG = Center of gravity
W = Weight of component
D = Distance from reference point
n = Last item
The NTEA recently released a new software program called UltraMod that is designed to assist with what can be tedious center of gravity and weight distribution calculations. UltraMod is an Excel-based spreadsheet program that allows users to quickly determine the weight distribution of a truck, including wheelbase changes and additional axles. The program allows users to configure the vehicle for optimal payload while assuring the vehicle and/or individual axles are not overloaded.
A “WTCG” program also is available that allows you to calculate axle loads; percentage of body/equipment payload on each axle; horizontal, vertical and lateral centers of gravity (depending on input); and individual wheel loads. A “TRKTLR” program enables you to calculate axle loads and horizontal and vertical centers of gravity for a straight truck and trailer combination; and wheelbase modifications and adding axles.
Programs are available separately on a CD-ROM or 3.5-in. diskette by calling the NTEA at 800/441-NTEA (6832).
