They're pretty tough. I have a Heys polycarb 20" four-wheel that I mostly carry on, but it's gotten quite a bit of abuse being dragged over curbs/up and down steps/through muddy medinas. After 3-4 years it's just starting to show a little friction wear at the bottom edge, but for the price I paid at Marshall's ($50?) it's been a great purchase and my favorite piece of travel gear.