Sprocket replacement and chain length. Seems like I rarely saw this back in the day.
Recently, it comes up quite often.
Shimano also presents two methods depending on the time period.
If you understand everything from why those two methods came up at each time
you don't need to read this article.
The chainring and rear derailleur stay the same,
does the chain length have to change when the sprocket changes?
Answer: If it's within the rear derailleur's capacity range, you should not change it.
The answer is very simple.
To understand, let me give a reverse example.
Using a 50-34 chainring with an 11-30 sprocket
and assuming the chain is set perfectly.
To this bike, if you just add two chain links, what happens?
Yes, when you put on 34-11 (small chainring, small sprocket), the chain becomes slack.
Then if you change to an 11-34 sprocket, when you put on 34-11 (small chainring, small sprocket), won't it be slack?
Of course it will be slack.
No matter what, when small gears are engaged front and back, chain tension drops extremely.
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So if you use 34 instead of 30 sprocket, isn't the chain length insufficient?
Hmm...
Then if you go from 27 to 30 on an 11-30 sprocket, will the chain length be insufficient?
That can't be the case, right?
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Starting at some point, as larger cogs than 28 came out on bikes that only used 12-23 and 12-25,
shifting is handled entirely by the rear derailleur.
If the GS rear derailleur's capacity is 28-34 (min-max),
no matter what sprocket you set it up on, that derailleur can handle 28-34 with the same chain length.
(It means the derailleur can accommodate by reducing by the amount of the increased sprocket size)
If you're concerned at all, it's just about setting the 0.8cm B-tension screw on the largest sprocket.
That's it.
Bonus section.
However, there are times when you do change the chain length.
This happens not so much related to the sprocket, but due to the ambiguous fit of the chainstay length.
The chain can fundamentally only be shortened or lengthened by two links at a time,
and the chainstay length may fall ambiguously in between.
In such cases, it can change depending on shifting needs or preference.
I didn't know this before, but the rear derailleur turned out to be surprisingly sophisticated.
The oblique curved structure to overcome lateral pressure coming from inside and outside of the tension pulley and transmit it to the guide pulley, and the shifting range of motion and such.
The tightness and clearance of the guide pulley depending on the number of chain links and such.
Thank you for reading.
