Originally Posted by
ymee
What puzzled me about SRAMs approach of looping it over the two largest cogs and then adding an inch or so is that once you loop it thru the back deraileur jockey wheels...that's adding more than an inch of extra travel for the chain...how does that possibly work. I guess I should try it rather than visualizing it but it's a $50+ chain and I don't want to screw it up.
The extra inch in B/B +1" isn't for the derailleur, it's to provided the necessary slack for the chain to lift off the sprocket and allow shifting. If you simply looped the two sprockets without the added slack, the chain couldn't come off, which was fine for your 20" coaster bike, but not for one that depends on that ability.
You don't later add any more when threading nit through the RD. Modern derailleurs off all brands have the ability for the idler wheel to rise above the straight line distance on the lower loop.
FWIW - Sram derailleurs are 100% insensitive to chain length and/or cage angle because the jockey (upper) wheel is the center of cage rotation and therefore never moves as the cage swings through it's range. That contrasts with Shimano and Campagnolo where the cage pivots between the pulleys and the jockey wheel rises and falls with cage angle changes. That rise and fall makes the RD more sensitive to chain length to achieve good balance for jockey wheel height with all chainrings.