Originally Posted by
HillRider
There may be something quite different with SRAM rear derailleurs but I've used the big-big+1" w/o going through the rd sizing technique with literally dozens of Shimano road and MTB rear derailleurs and a couple of Campy rear derailleurs and it always works. I have no problems shifting into the big-big gear combination after the chain is joined and there is no hesitation or interference with the derailleur pulleys. I've used it with cassettes with largest cogs from 24 to 30T so that isn't an issue either.
Hi HillRider,
Perhaps you're right about SRAM. I've used the big-to-big estimation for years (as well as the equation) and it has only failed once.
But the conditions that I'm describing require a rear derailleur that is being used at the precise upper limit of its spec with regard to max cog size. 24 vs 30t are irrelevant if they were not the largest cogs that their respective derailleurs are designed to use because it is only at the extreme upper limit of the derailleur where its b-screw must be used aggressively.
Besides, there are so many other factors to consider. The only way we could make a fair comparison between different manufacturer's rear derailleurs is if we removed all other variables. The bikes would need to have the same chainstay length, same hanger, same cage type (short, medium or long), et cetera. These other factors could have mitigated your observations. (For example, my hanger may be a little shorter than most---I don't know---which caused me to use the b-screw adjustment more than you.)
Regarding the OP, I'm not sure what the max cog size is for his Shimano XT rear derailleur with a medium cage. But 40t sounds very large for a medium cage length---I would have expected such a large sprocket to require a long cage. Be that as it may, it sounds to me like Paul may be working near the upper limit of his rear derailleur so it would be prudent to use a little extra care when determining the optimum chain length.
Kind regards, RoadLight