^This^ If you're using the b screw to make a pulley cage vertical in the big ring/small cog position you're definitely confused.

When using a drivetrain specced as compatible by the manufacturer I agree; do as the manufacturer says. When using unspecced mixes then some other guidelines are needed (big-big seems the foundation of these). I've a drivetrain with components from Shimano, SunTour, Campag, KMC, Stronglight and Simplex. The Shimano RD-M953 is (I think) specced with a wrap of 42 and max sprocket of 34. With larger jockey wheels, reversed and lengthened B-screw and the chain just the right length this is pushable to a wrap of 53 and max sprocket of 40 (no drop-link).

I did not say that the B screw adjustment has anything to do with sizing a chain. I only said that it affects the angle between the ground and the line between the two RD pulleys, especially since (as you correctly pointed out) the Shimano Dealer Manual calls for them to be lined up "perfectly vertical."

B screw adjusts the distance between the guide (i.e., top) pulley and the cassette. Depending on the chain tension, it also affects the aforementioned angle between the ground and the line between the two RD pulleys.

And if the cassette is not within the official specifications of the RD (e.g., GS model with a 12-25 cassette) but still shifts fine, it may not be possible to have the two RD pulleys perfectly vertical.

Then I don't know why you'd bring up the B adjustment in a thread specifically about sizing chains, in response to a post specifically about sizing chains for a specific drive train.

My delay in responding to this post is because I wanted to be absolutely sure about this before I called out BS, so I had to wait until I got home before working on my bike. Now that I've done so, and run the B adjustment screw--all 11 mm of it--all the way in and all the way out, with my chain on the big ring and smallest cog, I can safely say, and without reservation, BS! Moving the screw those miniscule 11 mm one way or the other does not move the derailleur cage at all off the vertical.

You're correct on the function of the B adjustment screw. Please see my "BS" comment above concerning the rest of this statement.

I only specifically referenced the Dura Ace 7700 drivetrain. Of course, this same setup was the same for all my Shimano road bike drivetrains going back to 1986.

As I had already explained above, solely in response to your prior comment about having both RD pulleys "perfectly vertical."

That is because you have the appropriate chain length (and therefore chain tension) AND a cassette within the specifications of the RD. I don't, because I am using a GS model to shift a 12-25 cassette.

But please feel free to insist that the B screw does not affect the angle of the derailleur cage under any circumstances whatsoever.

What’s your specific derailleur model number? I want to look up the manual.

I have a Shimano RD-5800-GS. This is the dealer manual: DM-RD0003-09-ENG.pdf (shimano.com)

I am familiar with the diagram you are referring to, which is on page 8, right column "Assemble with sprocket max 27T or less".

With a 12-25 cassette, I have the B screw all the way out, to minimize the gap between the guide pulley and the big cog.

Thanks. So contrary to your previous post, you are running everything within spec. The B adjuster should have zero effect on your derailleur cage angle if set up correctly with chain on large ring, small cog.

I run my B screw all the way out too.

No, a RD-5800-GS has a minimum large cog size of 28T. RD-5800-GS (shimano.com). But I am using a 12-25 cassette, forcing me to have the B screw all the way out. I can get the two RD pulleys pretty close to vertical but not perfectly vertical. It shifts fine so I am OK with that. Next year, before I replace the RD cable, I am also installing a RD-6800-SS and a 11-28 cassette, so maybe then I can get the two RD pulleys perfectly vertical then.

Minimum large cog?? There’s such a thing?
Well, the manual you linked to, for a 2x, on page 8, specifies “Max 27T or less.”

Yes. I understand that page to be instructions, not specifications.

A 2X drivetrain with a "sprocket max 27T or less" should use a RD-5800-SS according to Shimano specifications: RD-5800-SS (shimano.com).

You need to read more manuals.

Show me some.

Here's one, it's not even a manual, just a product description:

Low sprocket_Max. 28T Low sprocket_Min. 23T

Or this, from the contents of the 5800 etc derailleur dealers manual:Chain length.. „ Assemble with sprocket max 27T or less ... „ Assemble with sprocket max 28T or more

Or page 8 of this, the actual manual:

Derailleur installation: Chain length

That enough?

Um, most of what you typed, and page 8 of the Shimano manual, says “MAX.” As in “maximum.” As in the highest number of teeth. So no. Not enough. But thanks anyway.

What about page 9? „ Assemble with sprocket max 28T or more
or „ For front triple, assemble with sprocket max 30T or less

or even page 10 „ For front triple, assemble with sprocket
max 32T or more

Though to be honest, I have no idea what you are even disputing.

Your reading comprehension can't be that poor. No wonder you argue about stuff like this.

Same. Apparently you don’t understand the difference between Maximum and Minimum.

You are obviously trolling this thread. You can't be this thick. Examples...again. „Assemble with sprocket max 27T or less (This is the one that's big/small w/ the cage vertical)
„Assemble with sprocket max 28T or more (This is the one where you size big/big not through the derailleur and add links) Obviously it means the smallest big cog you can use is a 28t. These last 2 examples are for a double.

For a triple:
„For front triple, assemble with sprocket max 30T or less OR
„For front triple, assemble with sprocket max 32T or more

From the new 105 product description:

Low sprocket max 36t (pay attention now, this is the part you refuse to acknowledge)
Low sprocket min 34t (understand?)

Remember in post #35 you asked: Minimum large cog?? There’s such a thing?
All of this is in answer to that question.