Scientists in Paris come up with unexpected answer to the age-old problem of running to the departure gate with a two-wheeled suitcase

Half a century after the American businessman Bernard D Sadow shocked travellers with the invention of “rolling luggage”, scientists have worked out why suitcases tend to to rock violently from one wheel to the other until they overturn on the race through the airport.

This most pressing of modern mysteries was taken on by physicists in Paris, who devised a scale model of a two-wheeled suitcase rolling on a treadmill and backed up their observations with a pile of equations and references to holonomic restraints, finite perturbations and the morphing of bifurcation diagrams.

Fortunately for non-physicists, the findings can be reduced to simpler terms. For the suitcase to rock it had to hit a bump or be struck in some other manner; the faster the suitcase was being pulled, the more minor the bump needed to set it off. So far, so obvious.



But Sylvain Courrech du Pont wanted to know more. Why did a rocking suitcase swerve and make such violent movements that it might eventually topple over? After more treadmill tests and more equations, the answer popped ou: because a suitcase’s handle pulls from the middle and the wheels are at its sides, the suitcase swerves inwards whenever it tilts up on one wheel. If the rocking overcomes the dampening effect that happens when each wheel touches the ground again, the suitcase will keep on rocking or eventually flip over.



The physicists had solved part of the puzzle, but the case was not yet closed. More experiments in the lab at Paris Diderot University revealed that the amount a suitcase rocked decreased when it was pulled at higher speeds. According to Courrech du Pont, that is because the energy used in pulling the luggage along makes the suitcase swerve faster from side to side, rather than making it tilt up more as it rocks.

Play Video 1:29 Toy suitcase tested on treadmill – video

The finding led the researchers to a radical conclusion. When a suitcase starts to rock out of control, the correct response is not to slow down but to pull it faster. “One should rather accelerate than decelerate to attenuate the amplitude of the oscillations,” the scientists write in the Proceedings of the Royal Society journal. With what appears to be a mix of sniffiness and despair, they add: “A non-experienced suitcase puller would not react this way.”



But there are times – many of them involving dragging a suitcase – when speeding up is a fanciful option. Fortunately for travellers with a tendency to overpack, Courrech du Pont has another solution that works just as well. He advises to decrease the angle of tilt with the horizontal in order to reduce the coupling into sideways motion. In short: lower the handle.

