Originally Posted by
Steve B.
Nonsense. Housings can always get dirt in them, is likely the single biggest reason for deteriorating shifting performance, mt. bikes more than road, but it happens to both. Its usually the first thing recommended as a fix when a newbie on here complains about poor shifting performance. Mechanical is also a slow process, you don't realize sometimes that your shifting sucks as it happens slowly over time. Then you switch housing and cables and are like "I should have done that a while ago".
Quality housings have rubber pieces that prevent dirt ingress where the cable exits the outer.
Apart from this, lots of mountain bikes use a one piece outer from the shifter to the RD, which also helps.
Then there's the big mistake done people make: lubing the cables. That only attracts dirt and is totally useless when using quality cables and outers.