Researchers from the MSC laboratory at Université de Paris have cracked the decades-old problem which plagues travelers everywhere — why won’t our wheelie suitcase just stay put and not wobble?

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 Université de Paris 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.

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.”

How to Repair Wobbly Suitcases

You’ll find suitcases with built-in wheels, giving you the option to pull or push your luggage. While it’s such a convenient feature, the wheels of your luggage undergo wear and tear, eventually needing repair. You can fix the wheels yourself or take your luggage to a repair shop.

Here’s how to repair wobbly suitcases:

• Buy replacement wheels for your wheelie suitcases. Most suitcase manufacturers sell the spare wheels or buy them from a suitcase store that’s selling your brand. Make sure to check the replacement wheels, so they’re all identical to your old ones to ensure stability.
• Open your luggage and unzip the liner. This is done to expose the nut bolts that hold the screws in place.
• Using a wrench, grip the nut bolt from the inside and remove all the screws around the luggage wheel using a screwdriver by turning them counter-clockwise.
• Find the small clip, and push it out to remove the bolt inside the wheel. Next, remove the damaged wheel and discard it. Once broken, luggage wheels cannot be repaired.
• Insert the replacement wheel and place a washer on both sides of the wheel.
• Secure the wheel by attaching the clip to the axle slot.
• Properly position the luggage wheel to its original position, tucking in any material.
• Screw the wheel in place to secure it. Test your suitcase to ensure that the wheels are not wobbly or crooked.

If your suitcase is old and needs replacement, you might want to consider buying and using one of the best rolling backpacks, which makes carrying your things more convenient and flexible. A durable rolling backpack prevents travel problems, like ripped fabric and broken zippers. An ideal rolling backpack is made from durable fabric, with high-quality zippers, reinforced stitching, and an additional layer of protective lining.

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.