Do you know how much truck weight matters matters? Performing a truck weight distribution analysis can prevent building trucks that are overloaded in normal use, causing problems for users and the equipment installer. Overloads can shorten the life of a vehicle and its components. Overloads can also prevent compliance with weight laws and federal safety standards.
A truck's payload capacity is an important component of understanding cargo carrying capabitilies. What is a payload capacity? The term payload capacity refers to the total, maximum weight a fleet can add to a truck’s cargo area, in addition to its curb weight, safely.
What & Why of Truck Weight Distribution
What is truck weight distribution, exactly? In simple terms, truck weight distribution is the amount of the total vehicle weight imposed on the:
- Ground at an axle
- Group of axles
- An individual wheel.
The weight on a truck must be distributed on the axles to comply with the chassis manufacturer's axle ratings and weight laws.
Having the correct wheelbase on the chassis and placing bodies and loads in the proper place will ensure that axles are loaded correctly and that payload capacity is not maxed out. Performing a truck weight distribution analysis identifies the proper axle loadings before building a truck.
Axle capacities are limited either by the axle capacity or legal weight limits, whichever is lower.
Both trucks in Image A have a front axle rated at 12,000 lbs., limiting the weight on those axles. The rear axle on the single-drive truck is limited by the axle capacity or legal weight restrictions to 20,000 lbs. The two rear axles on the tandem-drive truck are limited either by axle capacities or legal weight restrictions to 17,000 lbs. for each axle, or a total of 34,000 lbs. for the pair of axles.
What is the Impact of a Truck's Center of Gravity?
While the concept of an object's center of gravity is something many of us feel confident in understanding, do you know how to figure out the center of gravity on an odd-shaped object? What about something like a truck, considering the weight of the engine, axles, and loaded or unloaded trailer?
The center of gravity of an object does not have to be in the material of the object (Image C). When considering a solid ball, such as a billiard ball, it is obvious the center of gravity is the center of the ball and in the material that makes up the object.
In the case of a hollow ball, the center of gravity is also at the center, but it is not in the material. If the center of gravity of a truck body is in the material of the body, it is coincidence. Most of the time, the center of gravity of a truck body is in the air, similar to the hollow ball example.
Each truck component, including passengers and cargo, have a center of gravity. For example, the center of gravity of the truck chassis is shown at the top of the frame rail at the back of the cab and the body and cargo centers of gravity are shown with a single symbol at the center of the body about a third of the body height above the floor.
For the process of calculating weight distribution on the axles, the horizontal centers of gravity are used. If individual wheel loadings are calculated, the lateral centers of gravity are also used. Unless a lateral center of gravity is given for the chassis or a component, it is assumed to be on the centerline of the chassis.
Lateral centers of gravity (CGs) are measured from the vehicle centerline toward the right or passenger side as positive, left as negative. These are used to calculate individual wheel loadings and incompliance calculations.
"Moments" are a force or weight times a distance: One pound of force times one foot is 1 ft.-lb. One pound of force times 12 inches is 12 in.-lbs. They are both the same moment.
Moments can be used to calculate:
- Weight distribution.
- Frame loads.
- Hoist capacities.
- Effective wheelbase.
Moments provide a way to combine a number of components or items to calculate a CG for them as a group (Image F).
When the CG distance is known for all items combined, it is possible to calculate the weight on each axle. Moments in one direction must match the moments in the other. The down moment of the chassis plus the down moment of the body must equal the up moment on the rear axle.
The Truck Weight Distribution Analysis Process
If you want to know how to calculate truck weight distribution, use the following four steps:
- Determine the truck weight and center of gravity location for all of the components and items to be considered
- Multiply the center of gravity distance times the weight to get the moment for each component and item.
- Add all of the moments and divide by the wheelbase to get the weight on the rear axle.
- Subtract the rear axle weight from the total weight to get the front axle weight.
Glossary of Truck Weight Terms
AF - (After Frame). The distance from the center of the back axle to the rear end of the chassis frame.
BA - Bumper to axle
BL - (Body Length). Overall length of the body.
CA - (Cab to Axle). Distance from back of the cab to center of the rear axle.
CB - (Cab to Body). Distance between back of cab to front of body.
GVWR - Gross vehicle weight range
WB - (Wheelbase). Distance from center of the front axle to center of rear axle.
Why Does Truck Weight Matter?
Bottom line: the weight of a truck matters in truck fleet management and operation. A heavier truck with a 5,000-lb. GVWR cannot carry as much payload as a lighter truck with the same 5,000-lb. GVWR, it's simple math. Additionally, when unloaded, a lighter truck utilizes less fuel than a heavier counterpart.
Some additional ways to reduce weight include the materials your truck is made out of. As previously reported here in Work Truck: Reducing the weight of the truck lowers the vehicle’s center of gravity, leading to increased maneuverability and a tighter turning radius. Less weight also means superior acceleration and braking. This not only leads to a comfortable driving experience but also better fuel mileage, as the truck is getting up to speed and stopping faster.
A lighter truck allows for greater hauling capacity. Because work trucks must abide by weight restrictions, a heavier truck means lighter loads, less efficiency, and lost dollars.
Curious about how to lightweight your truck fleet? Or what impact the added weight of an electric battery has on a truck's suspension? Read more now and register today for Bobit connect, the secure and easy all-access connection to tips and challenges related to lightweighting your truck and work truck fleet management.
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