Researchers from the Cornell University have cracked the decades-old problem which plagues travelers everywhere — why won’t our wheelie suitcase just stay put and not wobble?

Hiker with luggage.

Image credits Pixabay.

If you’ve ever had to pull a roller luggage for any meaningful length you know how wobble-prone these things are. For us muggles, the bag’s fishtailing only amounts to a nuisance and a secret sense of shame at not being able to control the things, but for physicists, it’s an actually intriguing problem — much like it is with shoelaces.

To get to the bottom of things, a team of researchers from the Cornell University took a model suitcase to the treadmill and observed its motions. Now, they report that the luggage’s side-to-side motions at any point in time are directly related to its forward motion, its tilt, and the distance between its center of weight and the line of travel.  In essence, because the two wheels are fixed together on a single rod and can’t move independently, they create a link between the bag’s forward movement and its rotational motion — in short, the wheeled bag as we know it today is almost designed to wobble.

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Inherently unstable

Let’s imagine you’re pulling one such suitcase straight ahead, then something happens (such as a change of direction or a bump in the pavement) which causes the right wheel to lift. The now-tilted suitcase will turn right initially, but when the wheel falls down to the ground and the left one goes airborne your suitcase (which is now tilted and orientated slightly rightwards) banks left.

It’s this process, repeating again and again, that makes your suitcase wobble more intensely instead of steadying down. It’s kind of like resonance, only much more annoying — or, as study coauthor Sylvain Courrech du Pont, a physicist at Paris Diderot University puts it, “a bit funny and counterintuitive.”

If your first instinct is to slow your pace and give the wheelie some time to settle down then you’re doing it wrong, it seems. Du Pont says that the wobbling should actually decrease as the suitcase rolls faster. Lowering the angle of the suitcase (bending down and lowering the handle closer to the ground) also helps, and can even stop the rocking altogether.

The stakes, as they say, are higher than the frustration the wheelies inflict on tourists every day. Understanding how the bags behave in motion can help us design more stable two-wheeled carriers like car-pulled trailers

“The suitcase is a fun way to tackle the problem but the study would be the same for any trolley with two wheels or blades,” Courrech du Pont said.

“In the near future, maybe we will have a car without a driver. It would be a good thing if the car knows how to stop this kind of motion.”

The full paper “The rolling suitcase instability: a coupling between translation and rotation” has been published in the journal Proceedings of the Royal Society A.