Bicycle Tutor

Adjusting derailleur cable tension, I am curious why compressing the housing tightens the cable and therefore moves the derailleur. Please explain the mechanics.

The term "adjusting the cable tension" isn't really accurate. When you turn the adjuster barrels, you're actually making the housing longer or shorter (not by compressing the housing, but by making the adj barrel itself longer or shorter). In effect, you're making the path that the cable has to follow to get to the derailleur longer or shorter. The length of the path of the cable will change where the derailleur sits for a given shifter position. The actual tension on the cable is just a function of the strength of the spring in the derailleur. I'm not sure why this became the conventional term for the adjustment.

(Sep 24, 2012 09:11 AM)DaveM Wrote:  The term "adjusting the cable tension" isn't really accurate. When you turn the adjuster barrels, you're actually making the housing longer or shorter (not by compressing the housing, but by making the adj barrel itself longer or shorter). In effect, you're making the path that the cable has to follow to get to the derailleur longer or shorter. The length of the path of the cable will change where the derailleur sits for a given shifter position. The actual tension on the cable is just a function of the strength of the spring in the derailleur. I'm not sure why this became the conventional term for the adjustment.

Thank you. Please clear it up a little more. I understand you are saying when the barrel adjuster is turned counterclockwise it lengthens the barrel, so the cable has further to travel. But how does it travel further and tension more, when it seems as though the cable is just passing through the barrel? Does the shape of the housing stay the same?
Sorry for all of the questions, just trying to understand.

The housing itself does not change. On most adjusting barrels, there are two parts with one threaded inside the other. When you turn it, the inner part threads out of the outer part making the part's overall length longer. Look closely as you turn it, you should be able to see what changes.

(Sep 24, 2012 10:15 AM)DaveM Wrote:  The housing itself does not change. On most adjusting barrels, there are two parts with one threaded inside the other. When you turn it, the inner part threads out of the outer part making the part's overall length longer. Look closely as you turn it, you should be able to see what changes.

So the housing does not change, the cable tension does not change, but making the barrel longer, the cable pulls on the derailleur, making it move. Do I have it, and thank you?

(Sep 24, 2012 11:09 AM)Bob K Denver Wrote:  So the housing does not change, the cable tension does not change, but making the barrel longer, the cable pulls on the derailleur, making it move. Do I have it, and thank you?

Yes