Stability, load centre and capacity

Most lift truck tip-overs and dropped loads come down to one thing: the combined centre of gravity of the truck and load moved somewhere it should not be. This lesson explains how a counterbalanced truck stays upright, why the load centre and lift height control its capacity, how to read a data plate, and why turning, braking and slopes can tip a truck that is well within its rated load. The calculations here are for understanding. The manufacturer's data plate and manual always govern.

The seesaw principle

A counterbalanced truck works like a seesaw with the front axle as the pivot. The load on the forks creates a turning moment forward, equal to its weight multiplied by its distance ahead of the front axle. The truck and counterweight create a turning moment backward, equal to their weight multiplied by the distance of their centre of gravity behind the front axle. As long as the backward moment is larger, the rear wheels stay on the ground.

This explains why a short, heavy load may be fine while a long, lighter load of the same weight is not: the long load's centre sits further forward, so its moment is larger. It also explains why lifting a load off the ground is the moment of truth. If the rear wheels lift or the steering goes light as the forks rise, the truck is overloaded for that load centre.

The stability triangle

Most counterbalanced trucks are supported at three points: the two front wheels and the pivot pin in the middle of the rear steer axle. The rear axle can rock about that pin, so the truck behaves as though it stands on a triangle. Lines joining the front wheels to the rear pivot form the stability triangle. The truck is stable as long as the combined centre of gravity of the truck and load stays inside that triangle, as seen from above.

Several things move the combined centre of gravity:

• Picking up a load moves it forward, toward the front axle.
• Raising the load moves it upward, which makes the truck more sensitive to anything that pushes it sideways or forward.
• Tilting the mast forward moves it forward; tilting back moves it toward the truck.
• Turning, braking and accelerating add inertial forces that act at the centre of gravity and push it sideways, forward or back.
• Slopes tilt the triangle and the force of gravity relative to it.

Because the triangle is narrow at the rear pivot, an empty truck, whose centre of gravity sits near the rear, can tip sideways more easily than a loaded one when turned sharply. Many operators are surprised to learn that a fast turn with an empty truck is a classic tip-over scenario.

Load centre

The load centre is the horizontal distance from the vertical face of the fork shanks to the load's centre of gravity. Lift trucks are rated at a standard load centre, commonly 500 mm for small and medium trucks and 600 mm for larger ones in metric ratings, or 24 inches on trucks rated in imperial units. A standard pallet loaded evenly has its centre of gravity at about half its depth, so the rating assumes a typical pallet sitting fully back against the fork face.

If the load is longer than standard, sits forward on the forks, or is heavier at the front, the actual load centre is greater than the rated one and the safe capacity falls. Loads with a high centre of gravity, such as tall stacked cartons, also reduce stability when raised.

Reading the data plate

The data plate, also called the capacity plate or nameplate, is the operator's primary source of capacity information. It typically shows the truck model and serial number, the truck weight, the battery or fuel type, the attachment fitted, and the rated capacity at one or more load centres and lift heights. Capacity usually falls as lift height rises, because the mast deflects and the centre of gravity rises.

Combined centre of gravity

Combining the truck and load into one centre of gravity shows how close a lift brings the truck to tipping forward. Take moments about the front axle, counting distances behind the axle as positive and ahead of it as negative.

Dynamic stability: turns, braking and slopes

The data plate rating assumes the truck is on a level, firm surface, the mast is vertical and the truck is moving slowly. Real operation adds forces. A turn pushes the centre of gravity sideways with a force that grows with the square of speed and falls with turn radius. Hard braking with a raised load pushes the load forward. A slope shifts the centre of gravity downhill relative to the triangle.

Here \( d \) is the sideways distance from the combined centre of gravity to the nearest edge of the stability triangle and \( h \) is the height of the centre of gravity. When the sideways acceleration in a turn, \( v^{2}/r \), reaches this value, the truck starts to tip. The formula is a simplification, but it shows the two things that matter: a narrow base and a high centre of gravity both lower the speed at which a turn becomes a tip-over.

What keeps a truck upright in practice

The physics translates into a short list of habits that operators are trained in and supervisors should watch for:

• Know the load's weight and centre before lifting; never exceed the plate capacity for the load centre and lift height.
• Keep the load against the fork face, centred between the forks, and tilted back slightly for travel.
• Travel with the load low, just high enough to clear the floor and any obstacles.
• Slow down before turns, and never turn on a ramp or slope.
• Raise and lower loads only when the truck is stopped, and keep the mast vertical or tilted back while raising.
• Avoid sudden starts, stops and direction changes.
• Never add counterweight or have people ride on the back to lift a heavier load.

If a truck tips

Operator restraints exist because operators thrown from a tipping truck are often crushed by it. If a sit-down counterbalanced truck begins to tip sideways, the trained response is to stay in the seat with the seatbelt fastened, hold the steering wheel firmly, brace the feet and lean away from the direction of the tip. Stand-up trucks with rear entry are different: the operator is usually trained to step out the back if the truck tips or goes off a dock. Follow the manufacturer's instructions for each truck type.