Originally Posted by
Psimet2001
that's so incorrect that it's sad. You either don't know what youre doing or you are completely running incompatible setups. Small small results in the absolute LONGEST chain that you can run on that bike. If it results in a chain that is too short in the big big then you are running a combo that is outside of feasible wrap. Period.
Then I suggest that you take an elementary school reading course since I clearly stated that on that bike the rear derailleur has insufficient chain wrap capacity for the current rings and cogs - so yes, it's obviously 'outside of feasible wrap'. [Getting the gear range I wanted for the wheel size on that bike required going outside the wrap capacity of available derailleurs.]
Given that condition then there will either be a loose chain in the small-small combination (if the chain is sized using large-large) or a chain that's too short to go into the large-large combination (if the chain is sized small-small). The latter is far more likely to cause serious problems so on that bike it's better to use large-large for sizing the chain and live with the fact that the chain will be loose if I ever accidentally shift into the smallest cogs while still on the 'granny' smallest chain ring.
So, as I said before, chain sizing using small-small is safe if (and only if) one is sure that the rear derailleur has sufficient chain wrap capacity for the rings and cogs that will be used.