Kontact 

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.


https://forums.thepaceline.net/attac...1&d=1442884383

Litespud

yes my RD has a H screw, although I confess that through multiple combinations of chainrings, cassettes and chain lengths over the years, I have never touched it

evan326

https://si.shimano.com/pdfs/dm/DM-RARD001-04-ENG.pdf
Page 18 begs to differ

Ronsonic 

Campy traditionally produced only road racing gear with a fairly narrow range of cassettes. Back then you'd lose all your cool-points if you had anything bigger than a 25 on your cassette. Now with options ranging from corn-cob to 11-29 Campy has a knuckle screw.

My experience with an unadjusted B-screw is that on bumpy terrain that chain will wander and even walk itself up the cassette. Which is really weird to see, riding along and watching the gear indicator climb up from 5 to 9. Bike was new and I was dumb enough to ride it as shipped without checking that. Got home, adjusted the B and all was well for the next year or so until I killed that der.

Kontact 

Campy has used alternatives that accomplish the same thing as a B-screw since '85. People just aren't aware of them.

Ronsonic 

Good to know. I've only been using Campy in this century.

alexdi

Less B-screw means more chain wrap and a sharper chain angle when you initiate a shift. That makes for faster shifting. More B-screw pivots the derailleur backwards, moving the upper jockey away from the cassette and modestly increasing chain tension. More chain tension also pulls the lower pulley forward and pivots the upper jockey wheel away from the cassette, so adding B-screw has a double-effect on jockey distance (though this is really modest; if your chain doesn't have enough tension, take out a link, don't "fix" it with B-screw).

There's rarely a single correct setting. The right B-setting is the smallest amount sufficient to prevent the upper jockey from grinding the chain against the the cassette, but that often depends on how many gears you want to change at once. Particularly with wide-range cassettes, chain tension is pivotal to the correct position of the upper jockey, and it doesn't equalize until the chain is fully seated on the next gear. A B-screw offset that allows smooth shifts one gear a time can be inadequate to prevent grinding when downshifting multiple gears at once.

The most conservative way to set B is to shift to the smallest ring in the front (to create the least chain tension) and 3 down (or however many your shifter can do at once) from the biggest sprocket in the back. Let the B-screw out entirely, then downshift the RD all at once to the biggest sprocket. If it grinds, upshift back to where you started, add some B-screw, and try again. With full-suspension mountain bikes, you may need to do this riding the bike so the linkage (and therefore the chain tension, because most FS bikes have chain growth through the travel) is at the correct sag point.

The key takeaway with B-screw is that it's a fine-tuning adjustment. Derailleurs move at a slope intended for a specific set of sprocket configurations. If you add B-screw to make your max-28T derailleur fit a 32T cassette, you'll find the added offset compromises shifting in the smaller sprockets. You're not changing the slope of the RD's travel (much), just offsetting it.

If you try the opposite and use a RD intended for a big cassettes with a small cassette (e.g., fitting a 12-25 on a bike that came with an 11-34), you'll find that even with no B-screw at all, the derailleur moves too far from the larger sprockets for optimal shifting. It's not that it doesn't work entirely, it's just not as good as it could be.

I listed most all of Shimano's derailleurs here:

https://docs.google.com/spreadsheets...6D1DRbfS59EDrE

If you know the cassette and the front chainring setup you want to use, it's not hard to narrow exactly which RD will shift best with them.

Kontact 

This seems like a complicated procedure when almost every time the B screw adjustment is entirely about clearing the largest cog. You would have to have a rather oddly stepped cassette for an adjustment that clears the largest cog by the proper amount to run too close to sprockets on the rest of the cassette.


Also, some derailleurs, like SRAM road, locate the upper pulley together with the pulley cage pivot, so changes in pulley cage angle do not affect the upper pulley distance from the cassette.

alexdi

It isn't and it's not. The procedure reduces to 'shift multiple sprockets.' The reason for it, again, is that the chain doesn't reach full tension for a given sprocket until after the shift is completed. In the initiating stage, there has to be enough B-screw to prevent rub, and the amount differs with how many sprockets you want to shift at once.

Interference doesn't require an 'oddly-stepped' cassette, though there are plenty of variants that make it more probable. SRAM in particular often eschews even jumps in favor of larger ones moving into the big sprockets. I can't speak to the nuances of their (or Campagnolo's) road derailleurs.

Kontact 

Larger jumps are going to make the surface of the cassette even more concave, making the jockey pulley position prior to the largest cog even less critical. I can't see how the number of sprockets shifted at a time has any bearing on whether the derailleur is positioned to make the last jump cleanly onto the largest sprocket or not.

Only on a humped, convex cassette (like a 12-23 10 speed) would setting the B adjustment just for the largest sprocket fail to make the best possible adjustment for all the sprockets.

alexdi

I can't explain it any better than I have. This isn't a hypothetical; I can video the difference, or you can just walk into the garage with a screwdriver and find out yourself.

Kontact 

I'm not getting. In part, because some of the things you say aren't true:

The chain is under more tension before the shift is completed, if you're talking about rear derailleur tension. This is because the chain is riding a larger diameter before it settles into the new sprocket. Once it settles, then the new total chain wrap decreases.

I've worked as a shop mechanic for years and have adjusted a lot of B screws. What is it I'm supposed to be looking for when I pick up a screwdriver for the millionth time?