What size top box do I need?

Capacity in litres is measured inconsistently between makers, so the useful test is the one people actually apply: which helmet fits inside, and whether it is yours. The figure that limits you is elsewhere — the published maximum load.

Why litres do not compare well

Makers measure internal volume in different ways, and a shape that holds forty-five litres of loose objects may not hold one bulky one. Two boxes quoting the same number can differ noticeably in what they swallow.

The helmet test, and its catch

Most listings say a helmet fits. Which helmet is the question: a large adventure helmet with a peak is a very different object from a compact jet, and a box that takes one may not take the other. If the point of the box is leaving a helmet with the machine, measure your own helmet before ordering.

The figure that actually limits

Maximum load, published in kilograms. A top box sits at the end of a lever behind the rear axle, and the maker’s figure accounts for that — it is not a suggestion, and exceeding it changes how the machine behaves before it damages anything.

The two parts nobody prices in

A mounting plate, specific to the box maker, and a rack or fitting kit specific to the machine. Three references, and a listing that quotes only the box has quoted about half the cost. The plate also decides which boxes can ever be fitted afterwards, so it is worth choosing the system before the box.

The choice that affects a pillion

Opening direction — top-hinged or side-hinged — decides whether the box can be opened with somebody on the seat. And a backrest is a common accessory that changes how a pillion sits, which is worth deciding at the same time rather than later.