Forklift Attachment Capacity Loss: How to Recalculate Permitted Load
A 2,000 kg truck does not lift 2,000 kg once you bolt a clamp or a rotator on the carriage. This article shows how forklift attachment capacity loss is calculated from the load centre, and how to derate the truck against its load chart.
1. Why the number on the data plate stops applying
Two things change at the same time when an attachment goes onto the carriage.
The attachment has its own weight. A side-shifter, a carton clamp, a bale clamp, a rotator, a push-pull or a fork positioner is steel and hydraulics sitting in front of the mast. A single-pallet handler can weigh 80 kg. A heavy-duty rotator on a 3-tonne truck can weigh 400 kg or more. That weight is now part of the load the mast has to carry, and the load chart does not know about it.
The attachment also pushes the load further forward. A standard carriage holds the fork shank close to the mast. An attachment adds thickness โ the body of a clamp, the frame of a rotator, the extra plate of a positioner. If that body is 200 mm thick, the centre of the load moves 200 mm forward. The load centre is the horizontal distance from the fork face to the centre of gravity of the load, and it is the single number every load chart is built around.
Move the load centre forward and you increase the lever arm. The mast, the carriage, the tilt cylinders and the drive axle all see a bigger moment, not just a bigger weight. That is why the permitted load falls faster than most buyers expect.
The rated capacity on the data plate is valid only under the conditions printed with it. For a counterbalance forklift those conditions normally include a stated load centre โ 500 mm on most Asian and European trucks in the 1.5 to 3.5 tonne class, 600 mm on some models and on many US-market machines โ plus a standard carriage, standard forks, a vertical mast, and a lift height at or near the top of the rated range. Change any of those and the rating changes.
2. A worked example, step by step
Take a counterbalance forklift rated 2,000 kg at 500 mm load centre, with a standard carriage. A 100 kg attachment with a 200 mm body is fitted.
Step one: the new load centre. Fork face to load centre moves from 500 mm to 700 mm.
Step two: read the chart at the new load centre. On this example truck, the chart at 700 mm gives roughly 1,500 kg.
Step three: deduct the attachment. The attachment is carried by the same mast, so its 100 kg comes off the permitted figure: 1,500 โ 100 = about 1,400 kg.
| Item | Before attachment | After attachment |
|---|---|---|
| Load centre | 500 mm | 700 mm |
| Chart capacity at that load centre | 2,000 kg | approx. 1,500 kg |
| Attachment weight | โ | 100 kg |
| Usable load | 2,000 kg | approx. 1,400 kg |
| Capacity lost | โ | approx. 600 kg (30%) |
That is the method. The numbers above are an illustration only, not a specification for any particular truck. Actual permitted loads come from the manufacturer's load chart for the exact truck, mast, carriage and attachment combination. Some manufacturers publish a separate chart for a specific attachment; where they do, that chart overrides the standard one.
Two details are easy to miss. First, a lighter attachment is not automatically the better answer โ a 60 kg clamp that is 300 mm thick can cost more capacity than a 120 kg clamp that is 150 mm thick, because thickness drives the load centre and weight only adds to it linearly. Second, the attachment weight is deducted at the load centre of the attachment, not at the fork face, so on long attachments the true penalty is slightly worse than this simple subtraction suggests.
3. How to choose: what to check before you order
- Ask for the derated capacity in writing. Request the permitted load at your actual load centre โ say 600 mm or 700 mm, not the catalogue 500 mm โ and at the lift height you really work at.
- Measure the attachment body thickness before you commit. A 200 mm-thick body on a 500 mm-rated truck pushes you to 700 mm; a 100 mm body keeps you at 600 mm, where the chart is kinder.
- Compare usable capacity, not attachment weight. A 100 kg attachment that costs 500 kg of capacity may be worse value than a 150 kg unit that costs 300 kg.
- Check the residual capacity against your heaviest real load. If your heaviest pallet is 1,200 kg and the derated figure is 1,400 kg, you have 200 kg of margin โ thin once you add a wet pallet or an off-centre load.
- Recheck at high lift. On a 4,500 mm mast, capacity at 6,000 mm can be 30 to 50% below the floor-level figure on the same chart. If you stack to 5 m, use the 5 m column, not the top-line number.
- Confirm hydraulic flow and pressure. A clamp or rotator needs a fourth function and enough flow; a truck that lifts the derated load but cannot clamp it is still the wrong truck.
- Ask whether a heavier counterweight or a different mast is an option, and what that does to the chart.
- For explosion-proof or cold store applications, confirm the derating is applied on top of the protection-related derating, not instead of it.
4. FAQ
Does the attachment weight reduce capacity by exactly its own weight?
No. The attachment weight is a direct deduction, but the bigger loss usually comes from the load centre moving forward. In the example above the attachment weighed 100 kg, yet roughly 600 kg of capacity disappeared.
What is load centre and why does 500 mm matter?
Load centre is the horizontal distance from the fork face to the centre of gravity of the load. Load charts are published at a specific load centre, commonly 500 mm or 600 mm. Reading the chart at a longer load centre always gives a lower permitted load.
Can I just use the truck's rated capacity minus the attachment?
Only if the attachment does not change the load centre, which is rare. Clamps, rotators and positioners all add thickness. Use the load chart at the new load centre and deduct the attachment weight from that figure.
Does high lift reduce capacity further?
Yes. Most charts show capacity falling as lift height rises, because the mast and carriage have to hold the load higher and further from the support. On a typical 4,500 mm truck, capacity at full height can be well below the rated floor figure, before any attachment is considered.
How do I read a forklift load chart?
Find the column matching your load centre, then the row or curve matching your lift height. The value at the intersection is the permitted load for that condition, with the standard carriage and forks. Attachments are not included unless a separate chart is supplied.
Who should confirm the final figure?
The manufacturer. Ask your supplier for the approved capacity plate or a written derated capacity statement for the exact configuration โ truck, mast, carriage and attachment โ and keep it with the machine's documents.
Does this apply to electric stackers and reach trucks?
Yes. Reach trucks and stackers have their own load charts, and attachments or non-standard fork spreads change the permitted load the same way. Reach trucks are more sensitive to load centre because the load is carried out on the reaching mast.