Stability & the Stability Triangle

What does the 'stability triangle' of a counterbalance forklift refer to?

• AThe triangular base formed by the two front wheels and the centre of the rear axle✓
• BThe three warning lights on the dashboard
• CThe triangular shape of the overhead guard
• DThe triangle formed by the three load forks fitted to the carriage of a heavy-duty truck

Answer: On a counterbalance truck the stability triangle is formed by the two front wheel contact points and the pivot point at the centre of the steered rear axle. The truck stays stable only while the combined centre of gravity remains inside this triangle.

A counterbalance forklift is most likely to tip over sideways when the combined centre of gravity moves where?

• ATowards the centre of the front axle
• BDirectly over the rear wheel
• CLower towards the ground
• DOutside the stability triangle✓

Answer: Lateral tip-over happens when the combined centre of gravity of the truck and load moves outside the stability triangle, for example when turning too fast or travelling across a slope. Keeping loads low and turning slowly keeps it inside the triangle.

How does raising a load to a high position on the mast affect the stability of a counterbalance forklift?

• AIt shifts the rear counterweight forward along the chassis and adds grip
• BIt raises the combined centre of gravity and reduces stability✓
• CIt lowers the centre of gravity and improves stability
• DIt has no effect on stability at all, because the counterweight compensates fully

Answer: Raising a load lifts the combined centre of gravity, making the truck more top-heavy and easier to tip, especially sideways. This is why loads should be carried low while travelling and only raised when ready to stack or de-stack.

Why is the counterweight fitted at the rear of a counterbalance forklift truck?

• ATo add extra mass to the rear so that the drive tyres grip better on icy floors
• BTo provide a mounting point for the overhead guard and the operator's access step
• CTo offset the weight of the load on the forks and keep the truck balanced✓
• DTo house the fuel tank and hydraulic oil reservoir away from the engine bay

Answer: The rear counterweight balances the load carried on the forks ahead of the front axle, which acts as the fulcrum. Without enough counterbalance, a heavy or distant load would tip the truck forward.

On a counterbalance lift truck, which part acts as the fulcrum or pivot point?

• AThe front axle, with the load ahead of it and the counterweight behind it✓
• BThe rear axle, which steers the truck and carries the counterweight
• CThe mast, which pivots as the tilt rams extend and retract
• DThe centre of the operator's seat, which sits midway along the chassis

Answer: Think of the truck as a see-saw balanced on the front axle. The load sits on one side of it and the truck's own weight plus the counterweight sit on the other. Overload the forks, or move the load further out, and you increase the turning effect on the front axle until the back wheels lift — which is also why steering goes light just before a forward tip-over.

Why is turning a corner especially dangerous with a raised load?

• AThe stability triangle becomes a square as soon as the truck begins to turn
• BTurning transfers almost all of the truck's weight onto the rear steered axle and its small tyres
• CThe hydraulic system cannot hold a raised load completely steady while the truck is turning a corner
• DTurning throws the combined centre of gravity sideways while the load has already raised it✓

Answer: Two effects stack. Raising the load lifts the combined centre of gravity, and turning pushes it outwards — so the point you are trying to keep inside the stability triangle is both higher and moving toward the edge. Add any speed and it leaves the triangle. Travel with the load low, and do your turning before you lift, not during.

What is meant by the 'combined centre of gravity'?

• AThe single balance point of the truck and its load taken together✓
• BThe point midway between the front and rear axles of the unladen truck
• CThe centre of gravity of the load alone, measured from the fork heel
• DThe point where the counterweight's mass is concentrated at the rear

Answer: The truck has a centre of gravity and so does the load, and once the load is on the forks what matters is where the two combine. Adding a load moves it forward; raising the load moves it up; tilting forward moves it further forward still. The truck stays upright only while that combined point stays within the stability triangle.

What is the most likely consequence of driving a laden truck across a side slope?

• AThe load slides forward off the forks because the mast is no longer vertical
• BThe combined centre of gravity shifts downhill and the truck may tip sideways✓
• CThe rear steered wheels lose contact and the truck loses its steering entirely
• DThe truck becomes more stable, as its weight is pressed into the slope

Answer: Crossing a slope moves the combined centre of gravity toward the downhill side of the stability triangle, and a lateral tip-over gives almost no warning before it happens. This is why gradients are driven straight up and straight down, never traversed — and why the laden and unladen rules exist for which way the forks should point.

What effect does tilting a raised load forward have on stability?

• AIt shifts weight onto the rear wheels and improves the truck's steering
• BIt lowers the combined centre of gravity and makes the truck more stable
• CIt has no effect, because tilt only changes the angle of the forks
• DIt moves the combined centre of gravity forward and towards the front axle✓

Answer: Forward tilt pushes the load out beyond the front axle, which is the last thing you want with the load already high. Tilt forward only when the load is over the rack and about to be set down, and keep the mast as near vertical as you can while it is up. Travelling with a raised load tilted forward is how trucks tip nose-first.

How does a rough or pot-holed surface affect a laden truck?

• AIt only matters when the truck is unladen and the rear end is light
• BIt has no real effect, because the tyres and the seat absorb any unevenness in the floor
• CIt improves grip on the surface and therefore makes the truck considerably harder to overturn
• DA sudden jolt can throw the combined centre of gravity outside the stability triangle✓

Answer: Lift trucks have little or no suspension, so the surface goes straight through to the load. A pothole taken at speed with a raised load can shift the combined centre of gravity far enough, fast enough, to tip the truck or drop the load. Slow down, keep loads low, and get surface defects repaired rather than learning to drive around them.

What can cause a counterbalance truck to tip forward rather than sideways?

• ADriving the truck unladen with the forks pointing downhill on a steep, slippery gradient
• BTurning too sharply at speed while carrying a load close to the ground on a smooth level floor
• COverloading, an excessive load centre, or braking hard while travelling downhill laden✓
• DCarrying a load that is lighter than the rated capacity on the data plate

Answer: Longitudinal tip-over comes from too much turning effect about the front axle: too heavy a load, a load centre further out than the plate allows, or the momentum of hard braking on a downward slope throwing the load forward. Sharp turning causes lateral tip-over instead — the two failure modes have different causes and both are avoidable.

Why must a load be carried low while the truck is travelling?

• AIt keeps the combined centre of gravity low, which is what keeps the truck stable✓
• BIt stops the hydraulic oil overheating while the truck is being driven
• CIt reduces wear on the mast chains and the lift rams while the truck is being driven
• DIt is only a convention, as the carrying height makes no real difference to stability

Answer: Around 100 to 150 mm clear of the floor, mast tilted back. Low keeps the combined centre of gravity low so the truck can absorb turns, slopes and surface defects without the centre of gravity leaving the triangle. Raise the load only when you are at the rack and ready to place it, then lower it before moving off again.