The Forklift Stability Triangle, Explained

A forklift balances on three points of contact, and the combined center of gravity of the truck and its load has to stay inside the triangle those three points make. The sliders below show how load weight, fork height, mast tilt, and turning push that center of gravity toward the edge.

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Stable

At 2,500 lb and 0 in of lift, level tilt and 0% turning, the combined center of gravity has about 13.4 in of room before it reaches the edge of the triangle.

Simplified model of a 5,000 lb capacity sit-down counterbalance truck. Your truck's data plate and the manufacturer's rating govern, this diagram teaches the idea, it does not rate a load.

Load handling and the stability triangle are a full module of the Easy Forklift Certification online course, which takes about an hour and costs $59.

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What the stability triangle is

A sit-down counterbalance forklift touches the ground at three points: the two front wheels, and the single pivot point at the center of the rear steer axle. Three points of contact define a triangle, not a rectangle, because the rear axle steers by pivoting around its own center rather than sitting on two fixed wheels the way the front axle does.

The truck's own center of gravity sits inside that triangle when it's empty. Add a load on the forks and the combined center of gravity, truck plus load, shifts. As long as that combined point stays inside the triangle, the truck stays on all three points of contact and stays upright. The moment it crosses one of the triangle's edges, the truck tips toward that edge.

This is not a minor point on the exam. OSHA's operator training rule lists vehicle capacity and vehicle stability as required training topics, at 29 CFR 1910.178(l)(3)(i)(H) and (I). Every forklift certification course, including ours, covers the stability triangle for that reason.

Three things move the center of gravity

Load weight

A load on the forks sits forward of the truck's own center of gravity, so adding weight pulls the combined center of gravity forward, toward the front axle line. A heavier load always moves the combined center of gravity closer to the front edge of the triangle than a lighter one, even before you raise or tilt anything.

Fork height

Raising the forks raises the combined center of gravity's height above the ground. On its own, that does not push the center of gravity outside the triangle, a load sitting straight up and down on raised forks is still supported the same way. What it does is make every degree of tilt and every bit of cornering force move the center of gravity farther sideways or forward for the same input. Try the sliders above: a 5,000 lb load at 0 in of lift is stable, the same load at 200 in with just 5 degrees of forward tilt is not.

Tilt

Forward tilt, especially with the forks raised, swings the load's center of gravity forward and can push the combined center of gravity past the front axle line. Back tilt does the opposite, it pulls the load's weight back over the truck. That's why the rule is to travel with the mast tilted back and the forks 4 to 6 in off the floor, never with the mast tilted forward while driving.

Turning

Cornering throws the combined center of gravity sideways, toward the outside of the turn. Because the triangle is narrow at the back and wide at the front, a sideways shift that would be harmless near the front axle line can be enough to cross the triangle's edge farther back, especially with the forks raised.

Why the triangle is narrow at the back

The stability triangle is widest at the front axle line, where the two front wheels sit about 36 in apart on a typical 5,000 lb truck, and it narrows to a single point at the rear axle's pivot. That shape means the truck has the least sideways margin near the back of the triangle, which is also where a raised load's sideways shift matters most.

This is why an empty forklift can still tip sideways. With nothing on the forks the combined center of gravity sits well back, near the point of the triangle, where the diagram above leaves it under 10 in of sideways room instead of the 16 in it has with a 5,000 lb load on low forks. Set the load weight slider to 0 and turn hard: the margin drops to about 4 in, on a truck carrying nothing at all.

What to do if the truck starts to tip

Stay in the seat. Hold the steering wheel firmly, brace your feet, and lean your body away from the direction the truck is falling. Do not try to jump clear. That is the guidance in OSHA's Powered Industrial Trucks eTool for sit-down counterbalance trucks, the type this page covers. Stand-up rider trucks with no door are the exception: OSHA's advice there is to step backward off the truck.

Jumping is what kills people in tip-overs. The overhead guard, the metal frame above the operator's head, comes down with the truck. An operator who stays belted in and stays low is protected by that frame. An operator who jumps lands on the ground next to the truck, in the path of the guard and the mast as they come down, and that is how most tip-over fatalities happen.

Rules of thumb operators actually use

  • Keep the load pulled back against the carriage, not out on the tips of the forks.
  • Tilt the mast back before you lift, then only tilt forward briefly to place a load.
  • Travel with the forks 4 to 6 in off the floor and the mast tilted back.
  • No turns with the load raised. Lower it first.
  • Slow down before you turn, not while you're in the turn.
  • On a grade over 10 percent, drive a loaded truck with the load pointing uphill, which means backing down the ramp (OSHA 1910.178(n)(7)(i)).

The rest of the OSHA forklift rules, from speed to parking, are in our plain-English guide.

Want the number? Our load capacity calculator applies the same moment idea to your data plate.

Frequently asked questions

What is the forklift stability triangle?
It's the triangle formed by a sit-down forklift's three points of contact with the ground: the two front wheels and the single pivot point of the rear steer axle. The combined center of gravity of the truck and its load has to stay inside that triangle, or the truck tips toward whichever side the center of gravity crosses.
Why is it a triangle and not a rectangle?
A forklift's rear axle steers and pivots at its center, so the rear of the truck only ever contacts the ground at one point, not two. Three points of contact, two in front and one in back, define a triangle. A rigid four-wheel rectangle would be more stable, but it couldn't steer the way a forklift needs to.
What happens when the center of gravity leaves the stability triangle?
The truck tips in that direction. If the combined center of gravity moves past the front axle line, the truck tips forward and the rear wheels lift off the ground. If it moves past one of the triangle's side edges, the truck tips sideways.
Does raising a load make a forklift less stable?
Yes. Raising the load raises the combined center of gravity's height, which does not by itself move the center of gravity out of the triangle, but it multiplies the effect of every degree of tilt and every bit of cornering force. A load that is stable on the ground can tip the same truck at full mast height with a small forward tilt or a moderate turn.
Should you jump if a forklift tips over?
Not on a sit-down forklift. Stay in the seat, hold the steering wheel firmly, brace your feet, and lean away from the direction the truck is falling. Jumping puts you on the ground in the path of the overhead guard as it comes down, which is how tip-over fatalities happen. OSHA's Powered Industrial Trucks eTool teaches staying in the seat on sit-down counterbalance trucks. A stand-up rider truck with no door is the exception: step backward off that one.

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