Controls and Stability Principles
Controls and Stability Principles tests the physics behind the seat: why a counterbalance truck tips, which way it tips, and how the numbers on the plate relate to the load actually being carried. Expect questions on the stability triangle, load moment and rear-wheel steering.
The stability triangle
A counterbalance truck rests on three points: the two front wheels and the centre of the steer axle, which pivots. Those three points form a triangle, and the combined centre of gravity of truck plus load must stay inside it. The truck is far more stable fore-and-aft than side-to-side, so most overturns happen in a turn, on a slope, or with the load raised.
Load moment
Load moment = weight × distance from the front wheel line to the load's centre of gravity. The counterweight's moment opposes it. Anything that increases the load's moment - a heavier load, a load centre further out, an attachment, mast flex at height, braking, cornering, lowering too fast, uneven ground - reduces the usable capacity.
Rear-wheel steering
The rear wheels steer while the front wheels carry the pivot. That gives a tight turning circle, but the rear end, counterweight included, swings out in the opposite direction to the turn. It is the rear corner that hits racking, door frames and people, not the forks.
Worked example
A truck is rated 2,500 kg at a 500 mm load centre. Your load's centre of gravity sits 700 mm from the fork face. Effective capacity ≈ (2,500 × 500) ÷ 700 = 1,786 kg. If the load weighs 2,000 kg it is over capacity even though it is under the headline 2,500 kg, so the lift must not be made: reduce the load, bring the load closer to the fork face, or use a bigger truck.
Traps
- Treating rated capacity as valid at any load centre or height.
- Believing a heavier truck is automatically safer - it is heavier, not more stable.
- Forgetting dynamic forces: speed, braking and turning all add to the load moment.