Tri-Star Stair-Climbing Wheelchair 2D: Phase-Matched Pivot Kinematics

Assistive wheelchairs that climb ordinary stairs don't use legs or tracks — the most common mechanical solution is a tri-star (planetary) wheel cluster: three wheels mounted 120° apart on a rotating hub, sized so the chord between successive step edges exactly matches one third of the cluster's own circle. This 2D side-view simulator builds that geometry from real step height and depth, drives the rigid three-wheel assembly through a genuine 120°-per-step pivoting climb, and lets you toggle the self-leveling seat linkage on and off. It also corrects a phase-alignment bug found in the 3D original: that engine's fixed −90° starting angle only lands each new spoke exactly on the next step for a 60° stair, so for typical 30°-ish stairs its hub visibly teleports at every hand-off. Here the phase is derived from the stair's own pitch angle, so the hand-off stays continuous and the "Landing error" readout converges to zero at every hand-off for any step height/depth you choose. Drag to pan and scroll to zoom around the climb, and switch the self-leveling seat off to see why real designs like the iBOT need that counter-rotating linkage to keep the rider upright.