What is this rubber thing on my gear cable and what does it do?
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You can remove them by careful cutting with a sharp knife or scissors - they're soft rubber. Avoid damaging or nicking the metal inner cable though. – Criggie♦ May 16 '19 at 1:43
Such rubber rings are supposed to keep gear and brake cables from rubbing against frame tubes. That is, when the cables are mounted openly outside the frame.
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10And when they're not all gathered together like that but spaced out more or less evenly along the cable. – Daniel R Hicks May 15 '19 at 18:38
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7They appear to be quite social though, i.e., they do not care very much for separation. – gschenk May 15 '19 at 20:09
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2Actually, the cables wear them to avoid getting a "social disease". – Daniel R Hicks May 15 '19 at 23:42
There are three rubber rings, also known as "cable donuts" in your picture. With a little effort you can separate them and space them out along the cable. As gschenk says, they are supposed to prevent rubbing or slapping damage to paint or decals and prevent cable slapping noise.
They get mixed reviews.
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2If the cable slaps against the frame, especially the brake cable, it has too much slack and the rider is giving away precious pulling power.. With gear cables, the indexing might lack precision. – Carel May 15 '19 at 19:42
