Key points
- The rating is per cup. A 120 kg rating on four cups is 480 kg static — but only if the load is shared evenly, which on a real floor it is not.
- Static is not dynamic. Pianos get wheeled. A cup rated for static load can crack under a dynamic pass over a threshold or a tile joint.
- Material decides failure. PP and ABS creep under sustained load; glass-filled nylon and die-cast zinc hold their shape but transmit shock to the floor.
- The floor is half the product. A hard cup on a soft floor spreads load; a soft cup on a hard floor grips. Specify the pair, not the cup.
A caster cup is a small dish that sits under a piano wheel. It costs a few dollars, it has no moving parts, and it is the only thing standing between a 300 kg instrument and a hardwood floor. Buyers ask one question about it: "How much does it hold?" The answer is more complicated than the number on the box.
Static load is not the number that matters
A piano at rest puts its weight through four wheels. A 300 kg upright is roughly 75 kg per cup — well inside a 120 kg rating. So why do cups crack?
Because the load is never static. Someone leans on the case while tuning. The piano is pushed across a floor and a wheel drops off a threshold. A child climbs on the keyboard lid. The peak load in normal use is two to three times the resting load, and it arrives on one cup, not four.
Rate a caster cup for the worst single moment, not the average afternoon.
What the rating should cover
| Load case | Multiplier | 300 kg piano, per cup |
|---|---|---|
| At rest | 1.0× | 75 kg |
| Leaning / tuning | 1.5× | 113 kg |
| Pushed over a threshold | 2.5× | 188 kg |
| Grand piano, 3 wheels loaded | — | up to 200 kg |
A 120 kg cup on a 300 kg upright is adequate at rest and marginal in the third row. A 120 kg cup on a 400 kg grand, loaded on three legs, is not adequate at all — which is exactly why the failures cluster around grands and not uprights.
Material decides how it fails
Solid brass deforms rather than shatters. Die-cast zinc cracks at the rim under a point load. ABS takes the load but creeps under sustained heat — a cup under a piano in a south-facing shop window in August will flatten over a summer.
None of these is "better". They fail differently, which means they suit different customers:
- Brass — restoration, heritage instruments, anyone who will pay for the look.
- Die-cast zinc — the volume distributor's standard. Good load, good price, cracks if abused.
- ABS with a TPE base — schools and churches, where the floor matters more than the piano and the cup gets kicked.
The floor is half the product
The cup exists to spread load, not to hold it. A 65 mm cup spreads 75 kg over about 33 cm². An 80 mm cup spreads the same load over 50 cm² — a third less pressure per square centimetre, which is the difference between a mark and no mark on a lacquered floor.
So the question is not "how much does it hold". It is "how much does it hold, on what floor, under whose piano". That is a three-part question, and a supplier who answers only the first part has not thought about it.
