The load never moves — the fulcrum decides. A counterbalanced forklift is a see-saw. The front axle is the fulcrum. Everything behind it holds the truck down; everything in front of it tries to tip the truck over. Move a load further out, lift it higher, or put the machine on a slope, and the combined centre of gravity walks toward that fulcrum — and then past it. Below you can move it yourself and watch exactly when the truck lets go.
Every counterbalanced lift truck is built around a single balance: the truck's own weight sitting behind the front axle against the load's weight hanging out in front of it. Multiply each weight by its distance from the front axle and you get its moment. As long as the truck's moment is bigger than the load's moment, the truck stays down.
That front axle is the fulcrum — the F-word this whole course turns on. It never moves. What moves is the combined centre of gravity of truck plus load. Picture a plumb line dropped straight down from that combined centre of gravity: while it lands between the wheels, you are driving a forklift. The moment it lands outside the wheels, you are riding a falling object.
Load moment = load weight × distance from the fulcrum. Truck moment = truck weight × distance from the fulcrum. When load moment beats truck moment, it goes over. Nothing else in this course is more important than that sentence.
The load is using 40% of the moment the truck can hold against it.
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Only the front wheels are fixed. The rear axle hangs on a single pivot pin so the steer wheels can follow uneven floors — which means the back of a forklift is one point of support, not two. Join that pin to the two front wheels and you get the stability triangle. The combined centre of gravity has to stay inside it.
The triangle is widest at the front axle and narrows to nothing at the back. So as the combined CG moves forward, it also loses sideways room. That is why a heavy load out in front makes a truck feel twitchy in a turn, and why a truck can tip sideways on a slope you would happily walk across.
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On level ground with the mast vertical, lifting a load straight up does not move the centre of gravity forwards or sideways — it only moves it up. That is the part most operators get wrong. Height is not dangerous by itself; height is dangerous because it multiplies every tilt. Lean the truck over and the centre of gravity swings sideways by roughly its height × the slope. Carrying 3,500 lb at 6 inches, the combined centre of gravity sits about 23 in off the floor and a 4% cross-slope nudges it under an inch. Lift the same pallet to 15 feet and the combined centre of gravity climbs past 6 feet, so the same 4% moves it three inches — while the load up top swings more than seven, and the triangle underneath has not got any wider. Try it on the sliders.
A capacity rating is never just a weight. It is a weight at a stated load centre, and usually below a stated height. "5,000 lb at 24 in" means the engineers balanced 5,000 lb sitting 24 inches out from the fork face. Push the same 5,000 lb out to 36 inches and you have not added a pound — but you have added 29% more moment at the fulcrum, and the plate no longer applies.
| Load centre | Safe capacity | Lost |
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The quick field formula operators are taught — rated × rated LC ÷ actual LC — ignores the distance from the front axle to the fork face, so it comes out lower than the true moment calculation. That is fine: it errs towards safety. The table above uses the full moment method, measured from the fulcrum.
Watch the bar, not the badge. In the simulator above, the vertical marker on the moment bar is the plate limit and the end of the bar is the actual tip-over point. The gap between them is your engineered safety margin — and grade, height, tilt, a swinging load, potholes and hard braking all spend out of it.
Run the simulator on level ground, set the mast vertical, and drag the fork height from 6 inches to 15 feet. Watch the stability used number: it does not move at all. The combined centre of gravity goes straight up, not forward, so the moment about the fulcrum is unchanged. Operators who have been told "lifting high shifts the weight forward" are often surprised by this — and then stop trusting the rest of the lesson. (Leave the mast tilted back and the number actually falls as you lift, because tilting back pulls a high load in towards the truck. That is precisely why you tilt back.)
Here is what actually happens. Raising the load raises the combined centre of gravity, and a high centre of gravity converts every small tilt into a large horizontal shift. Now go back and add a grade, a cross-slope, a bit of forward tilt, or a pothole under one front wheel, and the same height that looked harmless becomes the dominant term.
On a slope, gravity stops pulling straight down through the truck and starts pulling down the hill. The rule that falls out of that is short enough to shout across a warehouse: the heavy end goes uphill. Loaded, the load is the heavy end — so the load points up the grade, whichever way you are travelling. Empty, the counterweight is the heavy end — so the forks point down the grade. Click any case to load it into the simulator.
Every inch of gap is an inch of extra load centre, and an inch of extra load centre costs you capacity at the fulcrum. Push the forks all the way under, then tilt back.
Lowest possible combined centre of gravity, shortest possible moment arm. This single habit removes most of the risk from most of the driving you do.
Forward tilt is for placing and picking, over the stack, at the moment of transfer. It is never a travel position.
Loaded: load points up the grade. Empty: forks point down the grade. Turn before the ramp, never on it.
Capacity is a weight at a load centre below a height. Long load, offset load, unbalanced drum, attachment fitted — all of them move the real number down.
A turn pushes the centre of gravity toward the narrow rear of the stability triangle. Do your braking in a straight line.
An empty truck has a high, rearward centre of gravity and no load damping it. Empty trucks tip sideways more often than loaded ones do.
Hold on, brace your feet, lean away from the fall. The overhead guard and the seatbelt are designed to keep you inside the survival space. Jumping is how operators are crushed.
How this simulator is modelled. A generic 5,000 lb sit-down counterbalance truck: 8,000 lb service weight, centre of gravity 30 in behind the front axle and 22 in high, front axle to fork face 17 in, 58.5 in wheelbase, 40 in front track. The load is treated as a solid block resting on the forks, its centre of gravity one load-centre out from the fork face and 20 in above the tines. Moments are taken about the front axle, with gravity resolved onto the slope, so grade, height and mast tilt all feed the same calculation. It is a teaching model, not your truck. Always operate to the data plate and the manual on the machine in front of you.
Training and certification requirements in Canada are set provincially and are built on CSA B335, the national safety standard for lift trucks. It requires classroom instruction, practical hands-on training on the class of truck the operator will actually use, a documented evaluation, and refresher training — typically at least every three years, and immediately after an incident, a near miss, or any change of equipment or conditions. This page is a teaching aid. It is not training, and it does not certify anyone.