Stabilitetstrekanten og tyngdepunkt

A counterbalance forklift rests on three points and stays stable as long as the combined centre of gravity of truck and load remains within the stability triangle. Here is how the moment balance about the front axle works, and what makes the truck tip over.

Introduction

Stability is all about balance. A forklift weighs more than the load it lifts, and the counterweight behind the front axle balances the load out in front. The assessment is a moment balance about the front axle, which acts as the pivot point.

The moment balance

The farther the load's centre of gravity lies from the front axle (greater load centre), the greater the load moment becomes for the same weight — and the less the load may weigh before the counterweight at the rear begins to lift.

Fig. The lever principle: the front axle is the pivot point — the load in front creates a tipping moment, the counterweight behind creates a stabilizing moment.

The stability triangle

A counterbalance forklift effectively rests on three points: the two front wheels and a point at the centre of the rear axle at the steering. The centre of gravity of truck + load must be kept within the triangle formed by these three points. With a tilted load, sharp turns or hard braking, the centre of gravity can slide outside the triangle — and then the truck tips over, often sideways in a turn.

Fig. Viewed from above: the two front wheels and the rear axle's midpoint form the stability triangle — the center of gravity must stay within it.

01Carrying a heavy load low and close to the mast keeps the centre of gravity to the rear.
02Tilt the mast back while driving, and lower the forks — a high-raised load raises the centre of gravity.
03Drive slowly through turns and brake gently — dynamic forces shift the centre of gravity.
04Overload or a displaced centre of gravity makes the counterweight lift = the forklift tips over.

Longitudinal and lateral stability

Stability is not one phenomenon, but two. It is important to keep them separate, because they are triggered by different errors and countered differently.

01Longitudinal stability (lengthwise): the risk of the truck tipping FORWARD over the front axle. Triggered by too heavy load, too large load centre, too high lift height or hard forward braking. It is the one that the load diagram directly protects against.
02Lateral stability: the risk of the truck overturning SIDEWAYS. Triggered especially by turns (centrifugal force), skewed/displaced load, driving across a slope or a wheel in a hole. It is often the most dangerous because it comes suddenly.

Industrial truck stability is verified precisely along both axes in the DS/EN ISO 22915 series, where the truck (typically on a tilting table) must remain stable at defined angles both longitudinally and laterally. The series has specific sections for counterbalance trucks, reach trucks, narrow aisle trucks etc., because their geometry is different.

Dynamic stability — braking and turning

The diagram's figures apply to a truck at rest or in calm driving. In motion, dynamic forces are added, which shift the total centre of gravity:

01Braking: the vehicle's inertia pushes the centre of gravity forward — like making the load 'heavier' and the load centre larger for a moment. Therefore: brake gently, especially with raised or heavy load.
02Cornering: centrifugal force pulls the centre of gravity outward towards the outside of the turn — the main source of lateral overturning. The force grows with the square of speed, so double speed gives four times as much lateral force. Therefore: slow down significantly BEFORE the turn.
03Acceleration and uneven ground: sudden movements and holes/edges can momentarily move the centre of gravity outside the stability triangle.

Calculation example: moment balance with numbers

The moment balance can be made concrete from the load diagram's nominal point. Assume a truck whose diagram specifies 1,600 kg at 500 mm load centre (Toyota example). The allowed load moment at that point is:

Worked example

Tilladt lastmoment = Q_nom · D_nom = 1.600 kg · 0,50 m = 800 kg·m. Har din faktiske last et lastcenter på 700 mm, bliver den størst tilladte vægt ved uændret højde: Q = 800 kg·m / 0,70 m ≈ 1.143 kg. Ved 1.000 mm lastcenter: 800 / 1,00 = 800 kg. Bemærk: dette er momentbalancens princip regnet på det citerede nominelle punkt — løftehøjdens fald er IKKE med her, så den bindende værdi er stadig lastdiagrammets aflæsning ved din faktiske højde. Armen (D) regnes her som lastcenteret målt fra gaffelforvæggen, og denne forenklede model sætter aksel-offsetten a = 0; regneeksemplet i opslaget »Sådan læser du et lastdiagram« føjer a til som en fysisk raffinering — begge modeller bruger altså lastcenteret fra gaffelforvæggen som arm. Tallene gælder kun eksempel-trucken.

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The stability triangle and the centre of gravity