Most forklift tip-overs don't come from one big mistake. They come from a load that's a little too far out, a turn taken a little too fast, or a raised mast on a slight slope. Each of those moves the truck's balance point toward an invisible edge. Cross that edge and the truck goes over.
That edge is the stability triangle. Every operator needs to understand it, and it's one of the topics OSHA expects operator training to cover. The truck-related topics in 29 CFR 1910.178(l)(3)(i) include vehicle capacity and vehicle stability. OSHA explains the principles in a non-mandatory appendix to the standard, Appendix A: Stability of Powered Industrial Trucks. This post walks through it in plain terms.
A few terms first
- Center of gravity: the point where all of an object's weight is concentrated. For a symmetrical load, it's in the middle of the load.
- Line of action: an imaginary vertical line through the center of gravity.
- Counterweight: the weight built into the back of the truck to offset the load and resist tipping.
- Fulcrum: the axis the truck rotates around when it tips.
- Load center: the horizontal distance from the vertical face of the forks to the load's center of gravity.
- Moment: weight multiplied by its distance from the fulcrum.
The see-saw principle
A counterbalanced forklift works like a see-saw with the front wheels as the pivot. On one side is the load. On the other is the truck and its counterweight.
Each side's "turning force" is its moment: weight times distance from the front wheels. As long as the truck's moment is greater than the load's moment, the truck stays level. If the load's moment gets bigger, because the load is heavier or sits farther out on the forks, the rear of the truck lifts. First the operator loses steering, because the steer axle is at the back. Then, if the load moment is much greater, the truck tips forward.
The stability triangle
OSHA's appendix explains that almost all counterbalanced forklifts rest on a three-point suspension, even with four wheels. The two front wheels are two of the points. The steer axle is attached to the truck by a single pivot pin at its center, and that pivot is the third point.
Connect those three points and you get the stability triangle.
The rule is simple: as long as the line of action of the combined truck and load stays inside the triangle, the truck is stable. If it falls outside, the truck can tip over.
The combined center of gravity isn't fixed. It moves every time the operator does something:
- Picking up a load moves it forward, toward the front wheels.
- Raising the load moves it up, which makes the triangle feel smaller. A small shift at height takes the balance point out of the triangle faster.
- Tilting the mast forward moves it forward.
- Turning throws it sideways.
- Braking hard throws it forward.
- Driving across a slope shifts it toward the downhill side.
Forward tips and side tips
OSHA's appendix describes two kinds of stability.
Longitudinal stability is the truck's resistance to tipping forward or backward. Forward tips happen when the load moment beats the truck's moment. Common causes are an overloaded truck, a load carried too far out on the forks, braking hard with a raised load, and tilting forward with the load high.
Lateral stability is the truck's resistance to tipping sideways. OSHA lists the factors as where the load sits on the truck, how high the load is, and how far the truck is leaning. Side tips often happen when an operator turns too fast, especially unloaded. With no load, the truck's center of gravity sits farther back, near the narrow point of the triangle, where it takes less sideways force to push it outside. Driving across a ramp, or turning on one, is another classic cause.
OSHA's appendix also points out dynamic effects. Moving, braking, cornering, lifting, tilting and lowering all shift weight while the truck is in motion, so a truck that's stable standing still can become unstable when it moves. For a load near the truck's limit, OSHA advises carrying it as low as possible, accelerating slowly and evenly, and tilting forward cautiously.
Reading the data plate
The data plate, or nameplate, tells the operator what the truck can safely lift. OSHA requires nameplates and markings to be in place and legible (1910.178(a)(6)). It limits operators to loads within the truck's rated capacity (1910.178(o)(2)).
The key point: a truck's capacity is rated at a specific load center. According to OSHA's appendix, trucks rated at 30,000 pounds or less are normally rated at a 24-inch load center. Larger trucks are normally rated at 36 or 48 inches. A 24-inch load center matches a standard 48-inch pallet with its weight centered.
The capacity on the plate is only good at that load center. If the load's center of gravity sits farther out, the truck can safely lift less.
If an attachment is added, such as a clamp, rotator or sideshifter, the capacity changes too. OSHA requires the truck to be marked for non-factory attachments (1910.178(a)(5)). Modifications that affect capacity need the manufacturer's written approval, and the plates have to be updated (1910.178(a)(4)).
Worked example: a longer load
OSHA's appendix gives this example. Take a truck rated for 3,000 pounds at a 24-inch load center.
- Maximum load moment = 3,000 lb × 24 in = 72,000 inch-pounds
- Now the operator needs to carry a load 60 inches long. Its center of gravity is 30 inches from the fork face.
- Maximum weight for that load = 72,000 ÷ 30 = 2,400 pounds
Moving the load center out just 6 inches cuts the safe weight by 600 pounds, even though nothing about the truck changed. OSHA notes that this calculation is conservative, because the true pivot point is the front wheels, which sit behind the fork face. For unusual loads that are longer, taller or off-center, operators should work out the maximum allowable load this way rather than trusting the plate number.
Everyday habits that keep the truck inside the triangle
- Check the data plate, and know the weight of what you're lifting.
- Center the load, and keep the heaviest part toward the front wheels, against the backrest.
- Travel with the load low and tilted back.
- Slow down before turning, and more so when the truck is empty.
- Take ramps slowly, with the load tilted back and just high enough to clear the surface. On grades over 10 percent, drive loaded trucks with the load upgrade (1910.178(n)(7)). Don't turn on the slope.
- Raise and lower only when stopped, and tilt forward only over the stack.
- Wear the seat belt, if the truck has one.
If the truck tips anyway
OSHA's eTool gives this procedure for a sit-down counterbalanced truck that is tipping over:
- Don't jump. Stay in the forklift.
- Hold tight to the steering wheel.
- Brace your feet.
- Lean away from the impact.
- Lean forward.
OSHA notes the procedure differs for other trucks. For example, operators of stand-up forklifts with rear-entry access should step backward off the truck. Training should cover the procedure for the exact trucks your operators use.
How BHG Forklift Training teaches stability
Stability isn't something operators absorb from a slide. In our training, operators read the data plates on your trucks, work out the safe weight for the loads you actually handle, and practice on your floors, ramps and docks with loads that matter. That applies whether they drive Class 1 electric riders, Class 2 reach trucks or Class 5 yard trucks. For telehandlers, where capacity changes with boom reach, see our telehandler certification guide. Training is available in English and Spanish.
See our forklift classes or schedule training at your facility.