Originally Posted by
RCMoeur
As others have stated, setting up the chain length based on minimal sag in the small / small combination can result in a chain too short to shift onto the large/large (or even several large rear cogs when on the large chainring). In the least-worst case, the chain simply refuses to shift, whereas other scenarios involve potential damage to the chainring or derailleurs.
And you could be wrong if that was used with a rear-suspension bike or 1x. If compression is not accounted for, both would be in the wrong. You should be able to figure out the 1x for yourself. Small-small basically comes up with the same chain length as long as your rear derailleur b-screw is adjusted correctly and within the manufacturer's limits on derailleur capacity.
Originally Posted by
RCMoeur
Another thing I've learned through experience is to never presume the components on a bicycle are the ones it was sold or speced with.
It wouldn't make a difference. The controlling parameters are the front rings, cogs, and derailleur. Either method applies
Originally Posted by
RCMoeur
I also don't always trust the "lay out the old chain and compare" method, as the previous chain may have been too installed short or too long by some other owner or mechanic. In addition, excess wear can make a worn chain measure out to 3 links longer than optimal/original. Using the large/large method avoids this.
Agreed