A helical vertical loop for GraviTrax-compatible marble runs. The ball enters at standard track level, does a full 360° loop and returns to the grid axis over a 3-hex-cell footprint (181 mm). Fully parametric OpenSCAD source included.

Design highlights

• True helix: the track shifts 20 mm sideways during the loop so entry and exit never collide, then an S-curve brings the ball back to the cell axis.

• Gothic-arch (90° V) channel profile: the V always faces the outside of the loop, where centrifugal force presses the ball. It self-centres the ball, gives two clean rolling contact lines, and keeps every internal overhang at ≤45° — the channel prints with no internal supports.

• Full GraviTrax-style integration: three hex feet sit on the grid, underside grooves accept the included baseplate anchor rings, and both mouths have official-style rail sockets (nose recess + clip slots with retention lips) so standard rails snap in.

• Viewing slot along the loop (9 mm — the 12.5 mm ball cannot escape).

Physics: for a rolling steel ball in a 90° V-groove the minimum ideal drop is 2.9·R. With R = 35 mm and real-world losses, feed the loop from ≥160 mm above the entry (≈16 height tiles). If your tower is shorter, lower R in the .scad and re-render (OpenSCAD nightly with Manifold renders it in under a second).

Printing

• looping.stl: as oriented, 0.2 mm layers, brim recommended. No supports needed inside the channel; optionally tree supports from build plate only for the outer crown of the loop. Walls ≥ 2.4 mm (6 × 0.4 perimeters).

• anillos.stl (2 anchor rings): print flat as oriented, flip and press into the baseplate holes under cells 1 and 3, then seat the loop on top (the middle foot needs no anchor). For PETG set petg = 1 in anillos.scad.

• Supports: none — the channel (gothic arch), the base cavities (45° ceilings) and the weld tunnels are all self-supporting.

Assembly: occupies 3 cells in a row; leave the two cells on the +Y side of cells 1–2 free (the helix overflies them at low height). Ball enters at cell 1 edge, exits at cell 3 edge; both accept standard rails or direct adjacent tiles.

Interface dimensions (hex pitch, track height, rail socket, anchor ring) taken from the community-measured library mkaschub/gravitrax-extensions — thanks! All code written from scratch; fully parametric.

GraviTrax is a trademark of Ravensburger. This is an unofficial, compatible part.