what three points make up a forklift's stability triangle

For safe operation, it is essential for forklift operators to understand the stability triangle, a foundational concept in forklift training. The stability triangle is a principle of physics that explains how a forklift maintains its balance and what actions can cause it to become unstable and tip over. This concept is a cornerstone of OSHA regulations for powered industrial trucks because forklift tip-overs are a leading cause of accidents, injuries, and fatalities in the workplace.

The Three Points of the Stability Triangle

The stability triangle is an imaginary triangle formed by three specific points of a forklift's suspension system.  These three points are:

The two front wheels (Points B and C). These are the two points at the front of the forklift where the load is carried. They form the base of the triangle and are also the fulcrum or the axis of rotation for forward and backward tipping.

The pivot point of the rear axle (Point A). The rear axle of most counterbalanced forklifts is not rigidly attached to the frame. Instead, it is connected by a central pivot pin. This pivot point allows the rear axle to oscillate, ensuring that all four wheels stay in contact with the ground even on uneven surfaces. This point is the apex of the stability triangle.

When these three points are connected with imaginary lines, they form a triangle. As long as the combined center of gravity of the forklift and its load remains within the boundaries of this triangle, the forklift is stable and will not tip over.

Understanding the Center of Gravity

To fully grasp the stability triangle, you must also understand the concept of the center of gravity. The center of gravity is the theoretical point where the entire weight of an object is concentrated.

Unloaded Forklift: When a forklift is empty, its center of gravity is located near the center of the machine, usually low to the ground and well within the stability triangle. In this state, the forklift is at its most stable.

Loaded Forklift: When a load is placed on the forks, the center of gravity shifts. The combined center of gravity of the forklift and the load moves forward and upward, closer to the front wheels. The heavier the load and the higher it is lifted, the more the center of gravity shifts. If the combined center of gravity moves outside the stability triangle, the forklift will become unstable and tip over.

This principle is why forklifts have a large counterweight at the back. The counterweight is designed to offset the weight of the load, keeping the combined center of gravity as far back as possible and within the triangle.

Factors That Affect Stability

Several factors can cause the combined center of gravity to shift and move outside the stability triangle, leading to a tip-over. A skilled operator must be aware of these risks and know how to mitigate them.

Excessive Speed and Sudden Movements: Rapid acceleration, sudden braking, or sharp turns can cause the load to shift, altering the center of gravity and potentially moving it outside the stability triangle. This is particularly dangerous when carrying a raised load.

Uneven Surfaces and Inclines: Driving on a slope, a pothole, or uneven ground can change the relationship between the forklift's wheels and the ground. This can shift the center of gravity laterally or longitudinally, making the forklift prone to tipping sideways or forward. OSHA recommends that operators never turn on a ramp or an incline.

Improper Load Handling: Placing an oversized or irregularly shaped load on the forks can make it difficult to determine the center of gravity. If the load is not properly centered, the center of gravity can shift sideways. Additionally, carrying a load with the forks raised too high will cause the center of gravity to rise, making the forklift significantly less stable.

Exceeding Load Capacity: Every forklift has a data plate that specifies its maximum load capacity at a given load center. Exceeding this limit will inevitably shift the combined center of gravity outside the stability triangle, guaranteeing a tip-over. A forklift's capacity rating is not just about weight; it's about the maximum moment, which is the product of the load's weight and the distance from the front wheels.

The Importance of Training

The stability triangle is more than just a theoretical concept; it's a practical safety guide. Proper forklift operator training focuses on applying this principle to real-world scenarios. Operators are taught to:

Keep loads as low as possible (4-6 inches from the ground) and tilted back while traveling.

Travel with the heaviest part of the load against the carriage.

Avoid sudden stops, starts, or turns.

Inspect the work environment for hazards like uneven surfaces.

Always check the data plate to ensure the load is within the forklift's capacity.

By understanding what three points make up a forklift's stability triangle and how to keep the combined center of gravity within its boundaries, operators can significantly reduce the risk of accidents and ensure a safe working environment for everyone.

This video provides an excellent 3D animation to help visualize and understand the concept of the forklift stability triangle.