Longitudinal Stability

A forklift's longitudinal stability is from front to back.

When a forklift tips forward, the axis of rotation (or fulcrum) is the front wheels' points of contact with the pavement. When a truck is stable, the vehicle-moment must exceed the load-moment. As long as the vehicle-moment is equal to or exceeds the load-moment, the vehicle will not tip over.

On the other hand, if the load-moment slightly exceeds the vehicle-moment, the truck will begin to tip forward, thereby causing the rear to lose contact with the floor or ground and resulting in loss of steering control. If the load-moment greatly exceeds the vehicle moment, the truck will tip forward, and serious injury is often a result.

To determine the maximum safe load-moment, the truck manufacturer normally rates the truck at a maximum load at a given distance from the front face of the forks. The specified distance from the front face of the forks to the line of action of the load is commonly called the load center.

Because larger trucks normally handle loads that are physically larger, these vehicles have greater load centers. Consider the following:

• Trucks with a capacity of 30,000 pounds or less are normally rated at a given load weight at a 24-inch load center.
• Trucks with a capacity greater than 30,000 pounds are normally rated at a given load weight at a 36- or 48-inch load center.
• To safely operate the vehicle, the operator should always check the data plate to determine the maximum allowable weight at the rated load center.

Although the true load-moment distance is measured from the front wheels, this distance is greater than the distance from the front face of the forks. Calculating the maximum allowable load-moment using the load-center distance always provides a lower load-moment than the truck was designed to handle.

When handling unusual loads, such as those that are larger than 48 inches long (the center of gravity is greater than 24 inches) or that have an offset center of gravity, a maximum allowable load-moment should be calculated and used to determine whether a load can be safely handled. Consider these examples:

• If an operator is operating a 3000-pound capacity truck (with a 24-inch load center), the maximum allowable load-moment is 72,000 inch-pounds (3,000 times 24).
• If a load is 60 inches long (30-inch load center), then the maximum that this load can weigh is 2,400 pounds (72,000 divided by 30).

To learn more about Forklift Safety visit our Forklift Safety Online Training web page.