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Probably one of the first maintenance one might want to do at home, unlike fixing a flat tire which is the first maintenance that you'll have to do outside of home. Adjusting the alignment of the derailleur can make riding more comfortable.
This page is not meant to be a fully detailed explanation of how to perfectly adjust the rear-mech but more a list of trick and observation I have been using to make my life easier. As such it assumes that you a familiar with limit-screws, the pinch bolt, and the general operation of the rear-mech. Maybe I'll make a full guide derailleurs at a later date.
The first thing I have been using is a trick mentioned in a GCN video that is supposed how Mr. Shimano wanted derailleur to be indexed. I have not found any other references to this trick so I doubt it is still valid, if it ever was. In my experience it does get you to a close starting point, where the rest can be adjusted using just the barrel adjuster (cf. next section).
For this technique I like to put the barrel adjuster towards the middle of its travel so we a free to adjust both ways once the trick gets us in the ballpark of correct. You also want to make sure that your lower limit screw is set correctly and that you shifter command is on the lowest gear.
* Without Any other factors at play the derailleur should align with the smallest cog.
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We then move the chain to to the second smallest cog by turning the pedals while gently pushing the derailleur sideways. Once the chain is on the second cog, stop the pedals from moving and then release the derailleur. You'll get a situation were the chain is sitting on the second smallest cog while the derailleur is trying to align with the smallest cog.
1. The derailleur and chain are on the second cog.
2. Without spinning the chain the derailleur is released, falls back towards the smallest cog.
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| | | [|]DD | | | [|]DD
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In this position if we were to turn the pedals the chain would naturally move to the smallest cog. Which makes it the perfect starting position cable tension. Without touching the pedals, set your shifter to it highest position, tension the cable through the pinch-bolt, and tighten the bolt.
And there you have it, legend says that once you do that the shifting in perfect across the whole range. In my experience its not, but it's close enough that I only need a half-turn on the barrel adjuster to finish it.
If you want to do minor adjustment on cable tension you'll need to use the barrel adjuster. By turning the barrel you get to add or remove a bit off length in the cable's travel thereby adjusting the tension in the cable. The problem being which turn direction does what.
Sadly I don't have a cool mnemonic to share, I'm not good with mnemonics and I prefer to remember how the different component work together to reason the way to turn the adjuster to get what I want. Here's what you need to know.
In the end, turning the barrel adjuster counter-clockwise pushes the derailleur towards bigger cogs, while turning it clockwise moves the derailleur towards the smaller cogs.
One of the big problems I often get is that when testing speeds at home, with the bike off the ground or upside down. I can't seem get the speeds to work in both direction. I would turn the barrel adjuster counter-clockwise until downshifting works perfectly, but then upshifting wouldn't work. So I'd start adjusting the barrel clockwise until upshifts work but now downshifts do not work anymore.
If that happens to you, you probably also noticed that if you actually the bike there is a comfortable range of adjustment in which downshift and upshifts work, so why not on the bike stand.
My understanding is that shifting, and upshifting especially, relies not only on derailleur position but also on chain tension. All the cog are turning at the same rate, so naturally the teeth on the smaller cogs are moving slower than those on the bigger ones. Because of that it's easier to downshift, once the chain grabs a tooth on the bigger cog the difference in speed will create the tension necessary for the shift. But on such luck on upshifts.
So, when testing your shifting with the bike off the ground, you need to accelerate de pedals as you upshift to maintain the tension in the chain that would naturally be there if you where riding the bike.