First it needs to be understood that every upshift, the inner cable not only moves, but also the spring at the rear derailleur compresses, leading to a larger cable tension. This cable tension tends to compress the outer housing and seat the end caps a bit better. How much the outer housing compresses per every shift is dependent on many factors: (1) the type of housing, (2) the type of end caps, (3) the number of housing segments, (4) the rear derailleur spring constant. So, if a particular system has a 4.8mm spacing in the cassette and a rear derailleur ration of 1.7, then the shift detents in the shift lever aren't 4.8 mm / 1.7 = 2.823529 mm apart, but rather a bit more apart. The shift lever, cabling system and rear derailleur spring form a system where every component affects each other.
For example, let's consider that you buy an MTB shift lever and pair it with the correct MTB rear derailleur according to Shimano's instructions. Then you don't install the components on an MTB but rather a touring/road bike frame. Shimano intended those parts to be used in an MTB that has three housing segments: (1) loop from handlebars to top tube, (2) segment from top tube to seat stay, (3) rear loop from seat stay to rear derailleur, but your bike only has two: (1) loop from handlebars to downtube, (2) rear loop from chainstay to rear derailleur, and the transition between downtube and chainstay is handled by a bottom bracket cable guide. Thus, your bike with a given amount of cable tension compresses the housing a bit less than what Shimano intended due to the smaller number of housing segments. The end result is that the rear derailleur moves a bit too much per every shift. The effect is marginal, and Shimano Centeron upper guide pulleys can tolerate some mismatch. So the use of MTB parts on a road bike frame doesn't eliminate the system from working, but it causes slight mismatch which affects how the rear derailleur indexing should be adjusted.
Also creating problems is the fact that today Shimano sells dual 10/11 speed "MTB" rear derailleurs even though the 10-speed and 11-speed systems have a different pull ratio, so strictly speaking the rear derailleur always moves a bit too little or a bit too much because its pull ratio is the average of 10-speed and 11-speed pull ratio.
According to Shimano instructions, the procedure is as follows:
- Shift to second smallest sprocket (Shimano doesn't specify which chainring to use)
- Adjust tension so that the chain slightly rubs the third smallest sprocket when pedaling
- Turn barrel adjuster clockwise until the rub goes away when pedaling
Let us analyze this procedure. Firstly, in a perfectly matched system due to cassette being wider than chainring set, the smallest sprocket is the sprocket at which rub is most likely. However, the smallest sprocket is affected by the high limit screw. By adjusting the tension on the second smallest sprocket, the chain angle is still such that rub is likely, but the high limit screw no longer eliminates freedom of adjustment. Based on this, the optimal chainring to use during this procedure is the smallest chainring because it creates a chain angle where rub is the most likely.
My procedure to adjust is as follows:
- 1: Shift to smallest chainring and second smallest sprocket
- 2: Adjust tension so that the chain slightly rubs the third smallest sprocket when pedaling
- 3: Turn barrel adjuster clockwise until the rub goes away when pedaling
- 4: Shift to second largest sprocket
- 5: Check if chain rubs the largest sprocket when pedaling, A: no, B: yes
- 6B: If it rubs, turn barrel adjuster clockwise until the rub goes away when pedaling
- 7A: You are now in the second largest sprocket now (from step 4). Shift down to largest sprocket while not pedaling (pre-shift) and start pedaling, checking that the chain goes to the largest sprocket (only if answer to 5 was "no")
- 7B: Shift up to smallest sprocket while pedaling, then shift down to second smallest sprocket while not pedaling (pre-shift) and start pedaling, checking that the chain goes to the second smallest sprocket (only if answer to 5 was "yes")
Let us analyze this procedure. The start is exactly what Shimano described, with the addition that small chainring should be used. If the rear derailleur moves exactly the right amount or moves too little per shift, the rub in step 5 will not happen. In this case, the upshift that's most likely to not work is the upshift from second smallest sprocket to smallest sprocket, but since the indexing was adjusted in second smallest sprocket already according to Shimano's instructions, the upshift need not be checked. The downshift that's most likely to not work is the downshift from second largest sprocket to largest sprocket. Especially if the rear derailleur moves too little per shift, this can happen. Also the most likely way the downshift fails is that it was a pre-shift, i.e. shift when not pedaling. Thus this shift is checked when not pedaling.
If the rub in step 5 happened, then it is eliminated away in Shimano instruction style. This is a sign of rear derailleur moving too much per shift. Thus, the upshift that's most likely to fail is the upshift from largest sprocket to second largest sprocket, but since the indexing was adjusted on the second largest sprocket in Shimano style, we already know the upshift doesn't fail. The downshift that's most likely to fail is the downshift from smallest sprocket to second smallest sprocket. Therefore this downshift is checked in a pre-shift style, shifting while not pedaling and then starting pedaling.
If the downshift check in step 7A or 7B failed to execute, it is a sign of problems in the cabling system, and thus cables should be replaced with authentic Shimano parts.
