Originally Posted by
FBinNY
Both of the above are wrong. You adjust the RD trim in any except the high gear position (usually one or two in form high), since the high gear trim is usually determined by the limit screw. Then the tension or slack of the cable in high is whatever it is, which varies by brand. For example with Campagnolo, there will be noticeable slack in high.
When I teach repair, I have no problem teaching without saying tension. You start by pulling up the slack, then adjust the trim with the adjusting barrel, shortening or lengthening the cable as needed. Of course You're not shortening the wire, but changing the length of the run by lengthening or shortening the rear housing loop, but I explain how the cables work in the preliminary overview, so don't have to be as pedantic throughout the rest of the course.
IME- folks with a clear understanding of how things work fare better in the learning process and are less likely to go astray later.
BTW- this thread is a perfect example of how speaking of tension instead of length/position leads people astray.
Regarding your statement that a Campy rear gear system has significant slack in the cable when the chain is on the smallest cog. That would mean the shifter moves the RD less to shift to the second cog than any of the succeeding shifts leftward on the cassette because some of the first shift is lost to taking the slack out of the cable. If not, the shifter would have to have more "throw" built into the first shift than the rest. Is that the case? Otherwise how does that work?
And about adjusting the trim on a lower gear than the smallest cog. Sure, but it is while on the smallest cog that you have to attach the cable. As I said, you won't likely make that connection with the perfect cable "length". The trim on a larger cog fixes that. But if you then shift back to the smallest cog, if the high limit screw is just right, shouldn't the cable now be very close to the zero tension point, just a tiny bit away from slack (except for what you said about Campy)? I understand the correct manipulations that you described. I am just trying to understand the underlying principles so it won't just be a rote process.