Bicycle Mechanics - 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.

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?

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.

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

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 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.

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.

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.

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.

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.

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.

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.

but I am not sensitive enough a rider to notice the difference a couple of mills would make.
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.

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.

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).