Under normal rolling conditions, the forward motion of the suitcase is perpendicular to the wheels' axis of rotation.

在正常滚动的情况下，行李箱的前进运动与轮子的转动轴垂直。

If you put heavy stuff close to the axis of symmetry, or close to the wheel axis, then it's stable, too.

“如果你把重物放在箱子对称轴或轮轴附近，那行李箱也会稳定。”