Forklift Stability: The Stability Triangle, Load Centre & Capacity Manufacturer requirement

Forklift Classes, the Stability Triangle, and Capacity De-Rating

The 7 forklift classes (Industrial Truck Association classes, used by the Manitoba code of practice)

• Class I — electric motor rider, counterbalance, cushion or pneumatic tire
• Class II — electric motor narrow aisle (reach truck, order picker)
• Class III — electric motor hand truck (pallet jack, walkie stacker)
• Class IV — internal combustion, cushion tire (indoor)
• Class V — internal combustion, pneumatic tire (outdoor / rough)
• Class VI — electric/IC tractors
• Class VII — rough terrain (telehandler, masted rough terrain)

Operator certification is class-specific AND machine-model-specific within the class. A Class I certified operator is NOT automatically certified on Class V — even if both are "forklifts."

The stability triangle

Conventional sit-down counterbalance forklifts have a triangular stability footprint:

• Two front wheels (load axle)
• Single rear pivot (steering axle)

The combined centre of gravity of forklift + load + operator must stay INSIDE this triangle. If it moves outside, the truck tips.

Conditions that move the centre of gravity outside the triangle:

• Load too heavy (exceeds capacity)
• Load centre too far forward (long or shifted load)
• Mast raised high (raises combined centre of gravity)
• Mast tilted forward (shifts centre of gravity forward)
• Turning at speed (centrifugal force adds load to outside wheel)
• Uneven floor (one wheel drops)

Capacity de-rating

Rated capacity is at a SPECIFIC load centre (typically 24 inches / 600 mm) and a SPECIFIC lift height (typically ground or short). Real loads de-rate:

• Long load shifts the load centre — capacity drops proportionally
• High lift raises combined centre of gravity — capacity drops
• Attachment (drum clamp, fork extension, push-pull) adds weight ahead of the load axle — capacity drops

A 5,000 lb rated truck may safely lift only 3,200 lb of a long load at full mast extension. Operators must consult the de-rating chart on the data plate for every non-standard load.

Load size, pallets, and the load centre

The load centre is how far the load's centre of gravity sits from the face of the forks. A forklift's rated capacity assumes a standard load centre of 24 inches (600 mm) — the figure you get from a standard pallet sitting squarely on the forks:

• North American standard (GMA/CPC): 48 × 40 in (1219 × 1016 mm) — the pallet you'll see most on a Manitoba site.
• ISO / Euro: 1200 × 1000 mm and 1200 × 800 mm (Euro).

When the load matches that assumption, the data-plate number holds. When the load is bigger or off-centre, it does not:

• A deeper (longer front-to-back) load pushes its centre of gravity past 24 inches — the load centre grows, the safe capacity drops sharply, and the combined centre of gravity moves forward toward the front axle (closer to a forward tip).
• A wider load changes the load centre little, but it cuts visibility, fouls aisles and racking, and can hide a pedestrian — and an off-centre or unevenly packed load shifts the centre of gravity sideways, eating into the stability triangle.
• A double-deep / oversized pallet, a long load on fork extensions, or two pallets at once all move the load centre out. Every one is a non-standard lift that must be checked against the de-rating chart first.

Rule of thumb: the plate rating is for a tidy, standard pallet. The moment a load is longer, wider, off-centre, or stacked beyond that, you are below the rated number — consult the de-rating chart before you lift, and you'll feel it in the stability model above (slide the load centre out and watch the margin shrink).

Common mistake

"Cargo is light — the truck will easily lift it."

Even a "light" load on an over-extended attachment can exceed the de-rated capacity. Lift weight is only one variable. The data plate publishes the de-rating curve — consult before lifting non-standard loads.