I have bike with 2x8 compact chain ring in front 50/34. If I change the 50 to 46 do I need to cut the chain shorter? I know I will adjust the front derailleur lower, right?
2Just be aware that changing the chain rings (or cassette) changing the chain may actually be needed regardless. Drive train components wear together meaning new ring and old chain might not be the smoothest of experiences– HurseyMar 16 at 19:45
Agree with Hursey. Worn chain on new chainrings can lead to chainsuck which is pretty nasty.– MichaelMar 17 at 12:42
Short answer: no.
You would need to check if it needs to be shortened if you used a smaller small chain ring. Or you might need a longer chain if you used a larger large chain ring. Your chain should be long enough to allow the big-big combination, and short enough so in the short-short-combination, your rear derailleur still creates some tension and the chain doesn't slide over other parts than the pulleys.
Thanks, the cassette is 32-11– EmadnessMar 16 at 15:46
As long as you don't change the cassette, that has no impact on your question.– BurkiMar 16 at 15:59
3If the chain works with 50/34, it will work with 46/34. No need to shorten. Shortening could improve shifting, though. The only case where the chain may need to be shortened is if the smallest cassette sprocket is swapped for something smaller. But that's impossible since usually the smallest cassette sprocket is as small as the mounting allows, like 11-tooth for HyperGlide.– juhistMar 16 at 17:29
I know I will adjust the front derailleur lower, right?
Maybe, but not really.
A 50T chainring is 208.8mm in diameter and a 46T is 192.7mm So you might think that the front mech needs to move down by half the difference, which is about 8mm.
However your smaller chainring(s) haven't changed. Thus, if the tail of the FD's cage worked where it was, then it will continue to work without adjustment.
If you were going the other way, from 46T to 50T then you would probably need to raise the front derailleur. But I think you can try it without changes, and this gives an easy path back should you decide to refit the 50T.
1Chainring measurements came from wolftoothcomponents.com/pages/chainring-diameter-by-tooth-count– Criggie ♦Mar 17 at 1:42