Test themes
Forklift stability: why trucks overturn and how to prevent it
The practice set below draws on the R489 technology and stability theme, giving priority to questions on balance, load centre and overturning.
Independent site, not linked to the French Health Insurance (Assurance Maladie), the INRS, testing bodies or training centres.
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Questions written by Radif Partners from the French Health Insurance recommendations. This is not the official test, which is sat at a certified testing body and is in French. Your answers stay in this browser.
A forklift stays upright as long as the combined centre of gravity of truck and load falls inside its base of support. On a counterbalance truck with a pivoting rear axle, that base is a triangle: the two front wheels and the central pivot of the rear axle. As soon as the vertical line through the centre of gravity leaves the triangle, the truck rolls onto its side or tips forward. In the R489 theory test, the technology and stability theme is worth 25 points out of 100, of which 11 deal directly with load mass, load centre, balance, sideways overturn (3 points) and forward tip-over (3 points). You need at least 13 points in this theme to pass. The operation theme adds one more syllabus point on preventing overturns. R485 for pedestrian stackers and R482 for construction machinery ask questions of the same kind, with the same physics behind them.
Is this load within the load chart?
Handling
Allowed
| Maximum load read | 2,050 kg |
| Maximum height for this load | 4.5 m |
| Rule | row and column equal or above |
Questions, explanations and calculations by Radif Partners · Editorial policy · Method
The stability triangle
Picture three points on the ground: the left front wheel, the right front wheel and the pin around which the rear axle pivots. The truck, counterweight included, has its own centre of gravity inside that triangle. Picking up a load moves it forward; raising the load moves it up; turning pushes it sideways through centrifugal force. While the vertical line stays inside the triangle, the truck stays up. The rear of the triangle is narrow, which is why a counterbalance truck rolls sideways more easily than people think.
Two ways to lose balance
| Situation | Sideways overturn | Forward tip-over |
|---|---|---|
| Bend | Tight or fast turn, especially with a raised load | – |
| Load | Load off-centre on the forks | Load too heavy or centred too far out |
| Mast | Travelling with the load raised | Mast tilted forward with the load raised |
| Ground | Pothole, kerb, flat tyre, side slope | Driving forwards down a slope when laden |
| Driving | Sudden steering | Hard braking when laden and moving forwards |
Sideways overturn is the most frequent and the most serious, because it happens on the move, at speed, often without warning. Forward tip-over tends to occur when picking up or setting down, when the load exceeds what the plate allows: the load chart page shows how to check the cell before lifting.
What the test checks in this theme
| Marking theme | Syllabus point | Points |
|---|---|---|
| Technology and stability | Estimating the load mass | 2 |
| Technology and stability | Locating the load centre of gravity | 2 |
| Technology and stability | Load balance and stability | 1 |
| Technology and stability | Causes of sideways tip-over | 3 |
| Technology and stability | Causes of forward tip-over | 3 |
| Operation | Preventing truck or load tip-over | 4 |
The stability points weigh 11 of the theme’s 25 points. The rest cover energy sources, safety devices, controls and the driving position. A practice set therefore mixes both, but the one on this page favours balance questions. In the French test, look out for renversement (sideways overturn) and basculement (forward tip-over).
Speed and braking
Stability in a bend depends on the square of the speed: double the speed and the force pushing the truck outwards is four times greater. Braking distance follows the same law. The mini tool below works out the total stopping distance for a speed, reaction time and deceleration you choose; the traffic and signs page covers speed rules on site.
How far do I travel before stopping?
Stopping distance
7.0 m
| While reacting | 3.3 m |
| While braking | 3.7 m |
Double the speed and the braking distance is four times longer.
The same laws on other machines
The pedestrian stacker under R485 has a narrower base and an operator on foot, which changes the habits but not the physics. Telehandlers under R482 add boom outreach: the further it extends, the lower the capacity, as the telehandler CACES page explains. The R489 recommendation remains the reference for ride-on trucks.