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?
This really doesn't make much sense.
If a derailleur has no B or A screw, then we can assume that the "B" setting is already at maximum to clear the largest designed cog.
But if it has a setting, then we can assume that the setting is anywhere between the max cog and something much smaller, and must be adjusted for more clearance if it isn't already at that setting.
Many Campy derailleurs have an A screw adjustment instead (though it is labeled "H" on their diagrams, despite the separate H/L stops). All derailleurs have an A spring in the front knuckle that tensions the derailleur pulley cage. The B knuckle tension pulls against the A tension and they balance each other out. On Campy derailleurs with A tension, you "increase" B tension by decreasing A, and then the derailleur swings down and back.
And then there are all the derailleurs that have no B tension spring, but still have a means of adjusting clearance. '80s Campy derailleurs had an indexed stop that you set when you installed the derailleur, and SRAM has a screw that pushes directly on the B knuckle without going through a tension spring.
A or B tension and direct adjustments clearly change the angle of the derailleur and the amount of cog clearance.
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