Un­pack­ing the mys­tery of wob­bly suit­case syn­drome

China Daily (Hong Kong) - - FASHION -

You are rush­ing to catch a train or plane and sud­denly the suit­case you’re pulling starts to rock from wheel to wheel, threat­en­ing to flip over.

Tens of mil­lions of trav­el­ers suf­fer this an­noy­ance ev­ery day, but up to now, no­body could say ex­actly why.

Poor de­sign? Sub­stan­dard con­struc­tion? Faulty tech­nique?

Per­haps a bit of each, but the un­der­ly­ing cause lies else­where, ac­cord­ing to a study pub­lished re­cently in the no-non­sense Pro­ceed­ings of the Royal So­ci­ety A: Math­e­mat­i­cal and Phys­i­cal Sciences. Call it wob­bly suit­case syn­drome. “The ten­dency of a two-wheeled suit­case to os­cil­late from one wheel to an­other is due to an in­her­ent me­chan­i­cal in­sta­bil­ity,” ex­plained se­nior au­thor Syl­vain Cour­rech du Pont, a re­searcher at the Com­plex Ma­te­ri­als and Sys­tems Lab­o­ra­tory of Paris-Diderot Univer­sity.

More pre­cisely, the in­built un­steadi­ness arises at the in­ter­sec­tion of types of mo­tion, sim­u­la­tions with toy-scale mod­els and the­o­ret­i­cal cal­cu­la­tions showed.

“The prob­lem is the in­ter­ac­tion” — the clash, in other words — “of ro­ta­tional and trans­la­tional mo­tion,” Cour­rech du Pont told AFP.

“It may seem para­dox­i­cal, but it is pre­cisely be­cause the suit­case is de­signed to roll in a straight line that it moves side­ways.”

Ro­ta­tional mo­tion oc­curs when an ob­ject, such as a wheel (or two), spins around an axis in a con­tin­u­ous way.

Stop or speed up

Trans­la­tional mo­tion in­volves slid­ing an ob­ject — in this case, the whole suit­case — in any di­rec­tion.

“The wheels con­strain the suit­case to roll in a di­rec­tion that is per­pen­dic­u­lar, or at a right an­gle, to the axis on which the wheels are turn­ing,” Cour­rech du Pont ex­plained.

There is lit­tle tol­er­ance for de­vi­a­tion: a sharp change in di­rec­tion while walk­ing briskly, or a wheel hit- ting a bump and lift­ing off the ground, is enough to make the suit­case tip from wheel to wheel.

The same kind of rock­ing os­cil­la­tions can oc­cur with any twowheeled ve­hi­cle in which the wheels are on the same axis, such as a handtruck or a trailer pulled by a car.

What to do? There are two op­tions, ac­cord­ing to the study.

One is to stop, sta­bi­lize, and start again.

But the suit­case — in the­ory — will also re­gain sta­bil­ity if wheel ro­ta­tion speeds up, though run­ning in an air­port may carry risks of its own.

How did this in­ter­play of two ba­sic physics ax­ioms go un­no­ticed for so long?

“I re­ally don’t know,” said Cour­rech du Pont, who set up the ex­per­i­ments as a class­room pro­ject for his third-year univer­sity stu­dents.

“Ear­lier ef­forts to study this prob­lem only looked at the mo­tion of a solid block, so they missed the cou­pling,” he said.

Only when he set about pre­par­ing a bib­li­og­ra­phy did he re­al­ize that no one had cracked this par­tic­u­lar nut.

“The world is full of sim­ple prob­lems like this that are not solved,” he added.

It is pre­cisely be­cause the suit­case is de­signed to roll in a straight line that it moves side­ways.” Syl­vain Cour­rech du Pont, Paris-Diderot Univer­sity re­searcher

PHO­TOS BY PAOLO ROVERSI

One of the first de­sign­ers to truly chal­lenge the no­tions of clas­sic tai­lor­ing, Rei Kawakubo’s cri de coeur has be­come de­con­struc­tion, bias cuts and all sorts of ex­per­i­ments in shape, seams and asym­me­try.

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