崎岖不平的

These rough roads soon wore the tread of motorcycle tires off.

这些不平的路很快就把摩托车轮胎磨平。

So the bumpiness, the rising high altitude, and the long duration of these two things made this short distance of the road the toughest.

不平的道路，逐步升高的海拔，和很长的持续时使这段路程成为最艰难的一段。