Does the "B Screw" really do anything?
 

I periodically see references in threads about rear derailleur adjustments to adjusting the B Screw. I understand basically what it is supposed to do with regard to relative position of pulleys to casette. However, not once in my early attempts at things mechanical did an attempt at B Screw adjustment have any effect on derailleur function. Then I spent some time at UBI in a bike mechanic class. The guy who taught the section on derailleurs commented that he had never found tinkering with B Screw to have a meaningful effect on derailleur function, validating my view that it is basically pointless. So, what am I missing. Under what circumstances does the B screw actually effect derailleur function in a meaningful way?

If the derailleur's upper pulley is bumping the largest cog, it can definitely have an affect. By screwing it in, you can change the derailleur's angle so you can make the pulley stop bumping into the cog. For the most responsive shifting (in theory, anyway), you should have the screw unscrewed as much as you can without allowing the pulley to hit the largest cog. Small gap between the cog and the pulley, in other words. I have certainly had to tighten the screw to make the pulley stop hitting the cog. It's a simple adjustment that's there if you need it. And again, for optimum setup, leave as small a gap as possible between pulley and cog.

Because he has no idea what he's talking about.

The closer the upper pulley is to the cogs, the quicker the shifts will be. Best setting is the minimum amount of b-tension without rubbing/hitting any cogs. This is usually the case in the small chainring/big cogs. As #2 has pointed out.

Another reason to crank up that screw is to add a bit of chain tension if it's too slack despite being the right length.

As in he was saying it would not effect the derailuers performance? because I could understand saying that. But if my teacher at UBI had said the b-tension screw had no function I would have asked for a refund and left

as in adjusting it had minimal if any noticable effect on shifting assuming all other factors (cable tension, chain length primarily) were correctly set. The maximum adjustment possible with the screw was so small that in his experience adjustments did not generally make a difference. I'll take the word of folks here (well biked, operator, etc) that smallest possible gap between pulley and cogs makes shifting better but I am not sensitive enough a rider to notice the difference a couple of mills would make.

That has nothing to do with it. If it's too slack then the capacity of the derailleur has been exceeded. Granted you can alleviate borderline capacity problems by using the b-tension screw in that fashion but it's really not what that screw was meant for.

Try this.

Set B-tension to optimal, then max out b-tension like you don't know what you're doing. The shifting will be noticeably crappier. How much depends on the range of b adjustment and the specific derailleur. To get 100% shifting performance, b-tension cannot be ignored.

I've tried experimenting with the extremes. On my 12-25 road cassette, I notice nothing. On my 11-30 MTB cassette, there's a difference. On my road setup, the tension of the chain kicks in and dictates spacing of the pulley from the large cog when I let out the B-screw. I think they've figured out modern derailleurs well enough at this point that these small adjustments make little difference unless you deliberately try to mess it up or really don't know what you're doing.

I know that's not what it's meant for.

But if the springs in your derailleur are a bit shagged out and you keep dropping your chain, it's worth a shot.

If I understand it correctly, the difference would be most noticeable when you're running a derailleur with sloppy pivots, weak return spring, or worn chain. If you take a length of a new quality chain and flex it to one side, you'll get a certain amount of movement. If you take an equal length of a worn or poorly spec'd chain and flex it to one side, you'll get more movement.
On a smaller scale, this movement is occurring between the upper pulley and cog in the length of chain that is not contacting the pulley or cog, maybe two or three pin's worth. If a b-adjustment can decrease this free chain length by a pin or two, that slop is decreased or eliminated. The derailleur movement is now working to move a stiffer length of chain instead of wasting movement to take up the small slop. In many cases, this can mean the difference between a chain hesitating or even refusing to make the shift.

Not true, it affects performance regardless of the state of wear of drivetrain components.

Please read page 32-9 chapter 32 of barnetts for a more and thorough explanation of how b-tension and chain length affect shift performance. And why it is *essential* to shift performance that both be set as optimal as possible (for the latter it's the shortest chain length).

32-25 of the "demo" chapter here: http://www.bbinstitute.com/dl/dx_demo_chapter_32.pdf

The demo chapter only has a small subset of that section that appears in the full version - but that gives you a small taste.

If your RD starts making complaints (noise), a simple tweak of the 'B' screw can solve it fast. It works. And is there for this very reason: Cables stretch and angles can change. Keep up on your maintenance and check the RD-hanger for bends - you'll never need it.

The function of the "B" screw is to have the correct distance between the largest cassette cog and the jockey wheel on the derailer. On the Sram derailer I use it is recommended to be 6mm. That adjustment keeps the jockey wheel from hitting the cogs on the cassette, and yet assures good shifts. I would use the adjustment that the manuf of the derailer recommends.

Great info, guys. I'm trying to tune in the large cog shifting on my recently acquired 1987 Schwinn Prelude and I'm having trouble keeping it from going off into the spokes versus not shifting at all. So far I've ony played with the cable barrel nut and the travel limit. I'll take a look at the B screw as well but from what y'all are saying I almost think the issue may be chain wear and too much compliance side-to-side. I learned a lot on this thread.

Other than the low limit screw adustment, I would look at derailleur hanger alignment, or even a bent derailleur cage. We often see bikes with hangers bent enough that the derailleur cage hits the spokes (even with the low limit screw unscrewed just enough to allow shifting to the largest cog).

It's an old thread, but I'll throw in my 2 cents anyway...

As several have already discussed, you don't want the derailleur pulley hitting the largest cog on the cassette, so that is an obvious, useful adjustment. In my experience on my mountain bikes, shifting will hesitate or stall completely in 1 or 2 of the middle gears if the B-tension is adjusted with the derailleur pulley too far away from the cassette. The chain will hesitate a few seconds or stall until I shift ahead another gear, then it will suddenly jump 2 gears, effectively catching up with the shifter position. When I adjust the B-tension where the derailleur pulley is as close as possible to the largest cog without hitting it during the shift to that cog, then shifting becomes precise again without hesitations or stalls. This is the B-tension's purpose. It is NOT there for adjusting the "tension" in the chain. That is accomplished with the correct chain length (removing appropriate number of links) and the spring pulling the "swing arm" of the derailleur.

I rate the B-screw as right up there with the ability to tweak the brake pad toe-in and front derailleur cage angle and shape. (Though, many front derailleurs are no longer "tweakable" with a crescent wrench like the old plate steel cages of the 1970s.) In a perfect world. none of these are needed, In the world I live in, these tweaks often turn my bikes from ridable to quiet joys.

Ben

I confess that I'm a bit of a "B screw skeptic" - I can see what it does, but I question the real necessity. Campagnolo RDs have gone without a B-screw equivalent forever, to no obvious performance detriment.

Seems like if you can adjust to min gap and still have adjustment range then it is clearly doing something right? As in if you back out the b-screw even farther then you won't be able to shift into the large cog at all. That is the main purpose of the b-screw. On shimano setups with the pivoting upper cage mount this is less of an issue, with sram its a lot more noticeable since the cage doesn't move substantially.

Modern Campagnolo derailleurs have an "H" screw which performs the same function as a B screw.

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?

False. The "B" screw is NOT a chain tension adjustment. It is to adjust the clearance between the jockey wheel and the largest sprocket on the free wheel.

To The OP: So tell us the truth. You've lost your B-screw, haven't you? ;)

There are a few other variables in play, minor though they may be. Derailleur hanger length and BB drop (and possibly another one or two very small things, like the stop tang position and slot angle, or the relationship of the axle to the hanger) can each factor in a small way in derailleur adjustment, at least in theory.

Keep in mind there are two different kinds of "B" screws.

One is a B-tension screw, like Shimano derailleurs have. Its purpose is to balance the A-tension spring, which is set by the chain length. But given the variables (the above two, plus chainstay length) that can slightly affect the A-tension setting, a B-tension screw's function is usually fairly subtle.

The other is a B-position screw, used mainly on derailleurs without a sprung upper pivot. It directly sets the chain gap after the chain length is set, and its effect is generally far more noticeable than a B-tension screw. Some derailleurs have used variable-position blocks that perform the same function.