corrado33

I've almost never adjusted my "B" screw. I read that you set it so that it's X mm away from your biggest cog when you're shifted to that gear, but that's it. (I don't remember what X is at this point, it's been so long since I've had to adjust it.)

So in order to prevent me from simply going outside and doing it, what would happen if say, you had the pulleys adjusted too close to the biggest cog? Too far? What are the symptoms of a misadjusted "B" screw?

I'd imagine if it was too close it wouldn't want to shift to smaller cogs...

FBinNY 

Did you ever use a clothesline, or can you picture a rope on a pulley in your head?

You have the rope running in the groove of the pulley and want to derail it (assuming there's another pulley next to it). If you push from too close to the pulley the flanges resist you and it takes decent force to get the rope to jump. OTOH - if you hold your hand too far away you need to move the rope over a great distance to knock it out of the groove.

So the object is to push from as close as possible, so you don't have to overshoot so far, but not so close that you get "flange resistance".

Got it? exactly the same rules apply to derailleur systems, and the B-screw is about optimizing that distance.

dsbrantjr

After setting the screw with the chain on the large cog, shift it along the cassette/freewheel to verify that it maintains adequate clearance from all of the other cogs. Depending upon the cog sizes and the design of the derailleur you may need to readjust it slightly. This may happen when using a closer-ratio cassette with a derailleur designed for one with wider-spaced cogs, like a 13-25 road cassette on a mountain derailleur optimized for 12-34, for example.

ThermionicScott 

With the RDs and cassettes I use, I've found that I can remove them entirely.

70sSanO

I am not a B-screw expert at all. But from my limited experience in pushing the max cog limits on derailleurs, if you have the top pulley too close it can interfere with the large cog. This is easy to see if you backpedal the chain hangs up. Some derailleurs just have weak B springs and that will limit the max cog. XTR 950 series were notorious for weak springs and you could hardly run a 32t on them after a while. Adjusting the pulley too far from the cogs will slow down the shifting a bit. Therefore if you back it out completely you will supposedly get the best shifting.

For years I hardly tinkered with a B-screw adjustment, but more recently I have tried to dial it in to get the best shifting for the gearing.

John

FBinNY 

If you observe what is happening, especially on upshifts (to smaller sprockets) and think about my pulley analogy above, it'll all come together in your mind's eye, and you'll never "tinker" with it again, and will dial it in as easily and reliably as you set other adjustments.

Basically, you have it --- as close as possible, without interference or delayed upshifts. As a starting place, it seems that about 1-1/2 links free on the tangent between the pulley and sprocket is pretty close most of the time.

As mentioned earlier, be sure to check the clearance in ALL combinations, because what seems right on the larger ring may be too close when you shift to the inner ring.

rydabent

The "B" screw adjustment is probably the most misunderstood of all the bike adjustments. It is NOT a chain tension adjustment as many seem to think. It is adjusted so the jockey wheel misses the largest sprocket on the cassette by an amount specified by the mfg. On my Sram 7 that distance is 6mm. BTW the best way to measure that adjustment is with the end of an allen wrench. It is much easier to get into the space than trying to use a feeler gage.

When adjusted right your shifts will be quick and quiet without any binds.

Jed19

Yep. I run seven wheelsets with cassettes that are (12-25)t and (12-28)t. I adjusted my B-screw using a (12-28)t cassette for the 28 cog to be as close as possible to the pulley without it touching the pulley. That results in quick and quiet shifting. When I put on a (12-25)t cassette, I do a little minor adjustment. The goal being quick and quiet shifting always.

Thank you Sheldon Brown!

corrado33

Great analogy. Thank you. So optimally you want it as close as possible without the chain getting caught on upshifts (to harder gears.) (Or I could just look at the recommendations from the manufacturer.)

EDIT: Just looked at the manual for my RD. Man, that thing does a GREAT job explaining how to not only adjust your limit screws and tension, but also how to get it so that you need as minimal movement as possible to shift. What a great manual.

https://www.shimano.com/media/techdoc...9830606897.PDF

FBinNY 

There are many ways to teach mechanics, and no one is perfect. I'm of the old school which links causes and effects. If you know the effects of adjustments to either side of ideal, you can use that to know the right corrections based on how an issue manifests. Whether it's adjusting trim according to whether it upshifts or downshifts more crisply, or understanding the effects of cable friction, or what a misadjusted B-screw might cause. Then it's a question of observation and correction rather than following some sort of rote process.

If you understand the systems, the bike itself will usually tell you the problem. When I used to work on peoples bikes for special events, I always hated it when people would try to tell me the cause, and greatly preferred them to describe what it does or doesn't do.

rydabent

Great advice!!

To be a really good technician, the first thing that you have to do is understand how something works. Then and only then will you be able to make quick and accurate adjustments.

I was a tech specialist for years, and when other tech ran into something they couldnt fix, it was because they didnt understand what they were working on. Example --- after a customer complaint because someone had been there 4 times and threw $3000 of parts at the machine, I was able to fix it in 5 minutes because I knew the basic adjustments.