Two things change at once
The first is mass. An attachment is dead weight hanging off the front of the truck, and every kilogram of it is a kilogram the truck is no longer available to lift.
The second, and usually the larger effect, is load centre. A forklift is rated at a stated load centre — commonly 500 mm — measured forward from the fork face. An attachment pushes the load further forward, and because the truck is a lever pivoting on its front axle, moving the load away from the mast multiplies its effect. A modest forward shift can cost more capacity than the attachment’s own weight.
Why the counterweight cannot simply absorb it
The truck’s counterweight was sized for its plated rating at its plated load centre. Nothing about fitting an attachment changes the counterweight, the wheelbase or the tipping line — so the reserve that existed at the original load centre is what gets consumed. On a truck already specified close to its working limit, there may be very little of it.
What to ask for, and what to do afterwards
- Ask for the derated capacity in writing, at the load centre your work actually uses.
- Have the truck re-plated for the truck-and-attachment combination once fitted.
- Brief operators that the number on the original plate no longer applies.
- Re-check the arithmetic whenever the attachment or the fork set changes.
The practical consequence for specification
This is why we ask for the truck’s rated capacity at the start of an enquiry rather than at the end. Occasionally the honest answer to "which attachment fits this truck" is that the attachment fits but the truck is too small for the work, and a larger machine — or a different family with less overhang — is the real recommendation.
Fork extensions deserve a specific warning. They are cheap and they feel harmless, but they move the load centre a long way forward. They are a way of reaching a load, never a way of lifting more.