英语翻译The tyre is normally fitted with so called envelope that

英语翻译
The tyre is normally fitted with so called envelope that can be applied externally around the tread so as to extend down over the sides of the tyre casing,said envelope then being sealed at the bead part by so-called open rim.
在下是这方面的苦手,怎么也看不明白,请大侠出手相助