Originally Posted by
Litespud
I guess it depends on what you mean by "modern". My ~2003 Chorus 10 RD has no such screw, and I have seen as many Campag RDs without a B-screw equivalent as those that have one. My point being that many RDs function perfection well without a B-screw. It seems to me that if the large cog size and chain wrap are within spec for the RD, and the chain is the correct length, is there really a need for a B-screw?
There are a few other variables in play, minor though they may be. Derailleur hanger length and BB drop (and possibly another one or two very small things, like the stop tang position and slot angle, or the relationship of the axle to the hanger) can each factor in a small way in derailleur adjustment, at least in theory.
Keep in mind there are two different kinds of "B" screws.
One is a B-tension screw, like Shimano derailleurs have. Its purpose is to balance the A-tension spring, which is set by the chain length. But given the variables (the above two, plus chainstay length) that can slightly affect the A-tension setting, a B-tension screw's function is usually fairly subtle.
The other is a B-position screw, used mainly on derailleurs without a sprung upper pivot. It directly sets the chain gap after the chain length is set, and its effect is generally far more noticeable than a B-tension screw. Some derailleurs have used variable-position blocks that perform the same function.