I had the same problem on my mtn bike. The donuts are a nice idea but in practice dont do the job. I use a short piece of electrical tape and wrap it around the bare cable a few times essentially making an elongated "donut" but with sticky glue inside. I sandwitched the existing donut between two of the tape donuts before with good results. This technique does leave glue residue but its unimportant in the middle of the cable and can be removed with isopropyl .