Originally Posted by
yankhor
so I need to buy another chain to get two extra links, right?
I should say that if I need to sacrifice any gear combination, would be big x big. small x small will be used, but not the other.
That said, it's still necessary the extra two links?
The chain
absolutely must be able to loop the big/big combination with enough to spare for the RD cage and the ability to shift in or out of that combination. 1" or two links is considered the minimum, but some times you can pinch this down to a bit over 1/2" instead of going to 1.5" or so.
If the RD doesn't have enough capacity for the range, sacrifice it at the small/small combination, where the chain will be too long and sag in the middle.
The reason is the question of consequences of you accidentally shift into the wrong combination. A too-long chain will droop in small/small, no big deal, you can even ride that way. OTOH a too-short chain shifted to the big/big, will try to pull the cassette and chainrings closer. Since the chain is the strongest part of the system, this results in a bent rear axle, bent chainrings, damaged RD, or damaged chainstay (rare), or some combination of the above.
So big/big PLUS 1" is the minimum, but like CXwrench, I prefer to measure for the longest chain that works. This gives me extra links in case I ever have to cut out damaged ones (never happened yet, but Murphy's Law dictates that it will when I don't provide for it).