Redbullet

I was wondering whether B screw should be re-adjusted for different cassettes. For example: if I correctly adjust B screw for a cassette with the largest cog at the maximum limit of the derailleur, should I readjust B screw if I use another cassette with the largest cog at the lower limit of the derailleur?

soyabean

Supposed to release the B-screw to the point where the upper jockey wheel almost touches the 2nd biggest cog.

If you swapped a bigger cassette, you may need something like a Wolf Tooth derailleur drop adapter.

Redbullet

Not so radical.
Recommended gap for 10-36 cassette (36 is maximum accepted by derailleur) is 5 mm, whilst for 10-28 (still accepted by the derailleur) is 12 mm. I was wondering whether B screw adjustment for the 36 teeth cog will still match the 10-28 cassette - which means without re-adjusting B screw.
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FBinNY 

This is a case where the proof of the pudding is in the eating. If the largest sprocket clears properly and shifting seems OK, then there's no need to change anything. OTOH- if you notice issues adjust accordingly.

Redbullet

That is correct.
But I am lazy and I was wondering whether they simply deigned the system to work with any cassette accepted in the range with only one adjustment made at the extreme combination.

FBinNY 

There are two considerationsz basic functionality and optimization.

There's plenty of latitude in RD height, on most bikes so resetting is often unnecessary. The amount of latitude will depend on variables like the front chainring size gap, but as a rule anything that clears the largestsprocket passes, and will work on smaller cassettes.

However, if.you bring the RD higher to get better shift response on a smaller cassette, then it may no longer accept a larger one.

So it's a user choice to either have th RD freedom to change cassettes without worry, or get best shifting on the smaller one.

Bill Kapaun

You can't get much lazier than doing NOTHING!

Freerojo

There is indifference.

Redbullet

But that is what I'm trying to do, is it not? I just applied it for a specific situation.

smd4

My B screw is adjusted so that the jockey wheel is as close to the small sprocket as possible. Which in my case is all the way out. Of course I'm using a rear derailleur that's almost 30 years old.

Kontact 

Impossible to say, because what the B screw does and how the derailleur will react to it are variable depending on derailleur design and chain length.

If you set up a derailleur to work with a big low cog, it is likely to work okay with smaller cogs because modern shifting does work even when everything isn't perfect. Which takes us back to FBinNY's "try it".

Redbullet

My question doesn't touch an important practical topic, but just for the sake of discussion:


Chain length should not affect B screw adjustment because if you modify chain length, the cage of the derailleur will only rotate around the center of the upper pulley. It means that the center of the upper pulley will not change its position, thus, the distance between the biggest cog and the pulley will not change.

Derailleur design should not influence the answer to my question because I assume I don't change B screw adjustment, thus, I don't change derailleur position - the question is whether shifting is still ok in this situation..


Finally, I think that if you adjust B screw to (let's say) 5 mm distance to the biggest cassette (say max 36 teeth), then you will always have the same distance to the smaller cassette (say 12 mm to max cog of 28), regardless the type of derailleur and chain length. Whether that distance of 12 mm in my example is still ok for good shifting - this can only be answered by practical testing.

But, if it happens that the producer of the system recommends the same combination of 5 mm for max 36 teeth and 12 mm for max 28 teeth, then we can simply assume that he did the test for us and found out that changing cassettes within the recommended interval can be done without readjusting B screw, if this adjustment was done once at one of the extremes (5 mm for 36 teeth or 12 mm for 28 teeth).

Complicated...

Kontact 

That's not correct. For Shimano and Campy derailleurs until recently, the B screw was not an angle adjustment but a tension adjustment of the sprung mounting pivot. So when you shift, the whole derailleur pivots fore and aft. This was a Shimano invention from the '70s, still used on Claris.

And then there are all the derailleurs that don't have the upper pulley co-located with the pulley cage pivot. Many derailleurs have the upper pulley above the pivot, so when you shift to a lower cog in back it rotates toward the cogs and down, and when you downshift in front the pulley moves forward. All 1x derailleurs still work like that.

But if we were talking about a typical SRAM derailleur, you'd be right.

hokiefyd 

This is not a universal truth and was, in fact, usually not true until somewhat recently (the past decade or two). Many "traditional" derailleurs had the cage pivot somewhere between the two pullies so that rotating the cage would indeed change the "height" of the upper pulley, changing its position relative to the cassette or freewheel. This can definitely help the upper pulley more closely track the cassette as the effective chain length changes with shifting.

A side effect of this, though one that has never troubled me to any great degree, is the upper pulley would change height as the cage moved to wrap more or less chain with a front chainring shift, potentially causing a problematic change in upper pulley wheel clearance with the sprocket at the back. Again, I've never had a problem with that in practice, but I understand that's one of the reasons that some manufacturers, Shimano in particular, went to this design for their modern 2x and 3x rear derailleurs (such as RD-M3100 as an example).

I personally prefer the "traditional" design approach, where the upper pulley changes height as the cage rotates. I find this type of derailleur will track the cassette much more closely as I shift, though it does indeed sometimes require minding that B-screw adjustment. I've used deraillers where the upper pulley's axis is coincident with the cage pivot, and I've found they don't track the cassette well, especially wider range cassettes. You have to really crank the derailleur down with the B-screw to clear the large sprockets, which jeopardizes shift quality near the middle and small end of the cassette, at least in my experience (because the upper wheel doesn't rotate upward as you shift down the cassette).

Fortunately, manufacturers seem to have taken note of that and the newest stuff, at least the stuff designed for wider range cassettes, seems to have gone back to "traditional" design where the cage pivot is on its own axis, where the upper pulley will change height as the cage rotates, as in Shimano Deore M5100, SRAM NX Eagle, and Microsoft Advent. You can see that Shimano's more road-oriented units still use the design where the upper pulley mount and cage pivot are on the same axis, presumably because road cassettes don't typically have as much size difference (range), as in Shimano Ultegra R8000.