Load Capacity & Stability
1.5 hours
Learning Objectives
- •Read a forklift data (capacity) plate and state the truck's rated capacity at its rated load center
- •Explain the stability triangle and how the center of gravity must stay within it
- •Explain how load weight and load center distance combine to create a load moment
- •Explain how attachments and load center changes derate a truck's safe capacity
Topics Covered
- •Data / capacity plate: required by OSHA to be in place and legible; lists rated capacity, rated load center, mast height, fuel type, attachments
- •Rated capacity stated at a rated load center — commonly 24 inches for trucks under 30,000 lb
- •Load center: the horizontal distance from the face of the forks to the center of gravity of the load
- •Stability triangle: the three support points — the two front wheels and the center pivot of the rear (steer) axle
- •Center of gravity must stay inside the stability triangle or the truck tips; the combined CG shifts toward the load when loaded
- •Load moment = load weight x load center distance; the truck stays stable only while the vehicle moment exceeds the load moment
- •A load whose center of gravity is farther out than the rated load center reduces the safe capacity even if the weight is under the rated number
- •Attachments add weight and often move the load center forward — both derate capacity; the plate must be updated for attachments
- •Raising a load raises the combined center of gravity and reduces lateral stability
Resources
Self-Check Questions
Question 1: A forklift data plate reads "Rated capacity 5,000 lb at 24 in load center." You need to lift a load whose center of gravity is 36 inches from the fork face. What is true?
- A. You can still lift 5,000 lb because the truck is rated for 5,000 lb
- B. The safe capacity is reduced because the load center is greater than the rated 24 inches(correct)
- C. The load center has no effect on capacity
- D. You can lift more than 5,000 lb because the load is spread out
Show Explanation
Explanation:
Rated capacity is only valid at the rated load center. A load center farther out than 24 inches increases the load moment, so the truck's safe capacity drops below 5,000 lb. Exam tip: both weight AND load center matter — never read the rating as a flat weight limit.
Question 2: The forklift stability triangle is formed by which three points?
- A. The two forks and the mast
- B. The two front wheels and the center pivot point of the rear axle(correct)
- C. The operator seat, the mast, and the counterweight
- D. The four wheels
Show Explanation
Explanation:
A counterbalanced forklift has a three-point suspension: the two front (drive) wheels and the pivot at the center of the rear (steer) axle. Connecting those three points forms the stability triangle. As long as the combined center of gravity stays inside it, the truck will not tip.
Question 3: What does a forklift's "load moment" depend on?
- A. Only the weight of the load
- B. Only the height the load is raised to
- C. Both the weight of the load and its load center distance from the fork face(correct)
- D. Only the fuel type of the truck
Show Explanation
Explanation:
Load moment = load weight x load center distance. A lighter load held far out can create the same tipping moment as a heavier load held close in. That is why rated capacity is always stated together with a load center.
Question 4: You install a fork-mounted carton clamp attachment on a forklift. How does this affect the truck?
- A. It has no effect on capacity or the data plate
- B. It increases the truck's capacity
- C. It adds weight and typically moves the load center forward, derating capacity — and the data plate must be updated(correct)
- D. It only matters if the attachment is hydraulic
Show Explanation
Explanation:
Attachments add weight and usually shift the load center forward, both of which reduce safe capacity. OSHA requires the capacity plate to be updated to reflect the truck-and-attachment combination.
Question 5: Why does raising a load high on the mast reduce the truck's stability?
- A. It increases the truck's rated capacity
- B. It raises the combined center of gravity, reducing lateral (sideways) stability(correct)
- C. It shifts the counterweight forward
- D. It has no effect on stability, only on speed
Show Explanation
Explanation:
Raising the load lifts the combined center of gravity. A higher CG is easier to push outside the stability triangle sideways, so an elevated load makes the truck much more prone to a lateral tipover, especially while turning.
Question 6: OSHA requires that a forklift's capacity (data) plate be:
- A. Removed when the truck is sold
- B. In place and maintained in a legible condition(correct)
- C. Painted over to prevent tampering
- D. Replaced every year regardless of condition
Show Explanation
Explanation:
OSHA 1910.178 requires that all nameplates and markings be in place and legible. If the plate is missing or unreadable, the truck must not be operated until it is restored — the operator cannot safely judge capacity without it.
Question 7: When a forklift picks up a load at its rated capacity and load center, where does the combined center of gravity move?
- A. Toward the rear of the truck, behind the rear axle
- B. It does not move
- C. Forward, toward the front axle line — to the edge of the stability triangle(correct)
- D. Straight up out of the triangle
Show Explanation
Explanation:
An unloaded truck's CG sits between the axles. Picking up a rated load shifts the combined CG forward toward the front axle line — at rated capacity it sits near the front edge of the stability triangle, which is why overloading or a long load center pushes it past the edge and tips the truck.