October 1, 2025

Double Buckle

A backpack sitting loose on a rolling suitcase slides, tips at kerbs, and needs a hand on it — which defeats the point of the wheels. What it needed was a strap running across its harness to pull it tight against the case.

The hardware for that was already on the bag, except that the two ends which had to meet were the same half of the buckle. Two tongues, no receiver.

Two black 3D-printed versions of the part on a wooden surface, a thin first attempt and a thicker revision

The black printed part seen at an angle, showing the waisted bar shape with a flanged slot at each end

The same part printed in white filament, showing the layer lines across its top face

The first print was PLA, and laid out on the bed so that the layer lines ran across the direction of the load — weak exactly where the strap pulls. Reprinting it in a stronger material, rotated so the layers run with the load instead of across it, fixed it.

Drag to rotate · scroll to zoom

The final geometry, roughly 47 × 27 × 12 mm

Close-up of the white printed part joining two black webbing straps across the padded shoulder harness of a red backpack — Joining the two strap ends across the back of the harness

With that strap closed, the pack can be cinched onto the handle of the rolling case and stays put over kerbs and platforms — and comes apart in two clicks when it has to go on a back again.

The red backpack mounted on the back of a rolling suitcase by its shoulder straps, with the white part joining a strap across the middle — Pack and case travelling as one object

Two cubic centimetres of plastic, letting hardware that was already there reach each other.