Normaly placed around the edges of the trolley (between corner buffers) to prevent damage to door jambs etc.
atted by pusteig a (off-cut) metal strip the center of the section and screwing through into the side of the trolley. The Head of the screw will pop through the outer rubber leaving a small hole.
If the strip is to be on a rounded corner. This should be of at least 50mm rodius to prevent bucking of the strip.