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Old 06-06-11, 06:41 PM   #1
TurbineBlade
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b-tension screw -- instances where it doesn't do anything?

Hi, I've worked on my own bikes for a while now and am not a great mechanic, but I had a b-screw question. I know (from reading Sheldon, etc.) what it is supposed to do, but I've had 2 bikes where it seems like adjusting it does absolutely nothing, or pretty close to nothing.

In both cases, I've run a long cage mtn-type RD, a wide range cassette (12-32 or 11-34) with a double crank (38-52).

Does my b-screw not really do much because I'm not running gears that exceed the RD chain wrap? (i.e. - not running super slack gears like 22-34t or something).

Does it mostly matter when you have smaller chainrings and more slack in gearing?
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Old 06-06-11, 07:45 PM   #2
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It mostly matters when you're using a big sprocket in the back (32,34, etc.) In general, it might seem like the b-tension screw does nothing. But it does have a purpose.

Try backing the b-tension screw all the way out. Then shift into your lowest gear (small/large.) Spin the pedals and see if the upper pulley rubs the sprocket. If it does, you've discovered the screw's use. Now thread the screw in until your RD no longer interferes with the biggest cog.

Once you complete this you might (theoretically) find that your shifts are smoother and quicker, due to the RD being closer to the cassette.
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Old 06-06-11, 08:06 PM   #3
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Originally Posted by FastJake View Post
It mostly matters when you're using a big sprocket in the back (32,34, etc.) In general, it might seem like the b-tension screw does nothing. But it does have a purpose.

Try backing the b-tension screw all the way out. Then shift into your lowest gear (small/large.) Spin the pedals and see if the upper pulley rubs the sprocket. If it does, you've discovered the screw's use. Now thread the screw in until your RD no longer interferes with the biggest cog.

Once you complete this you might (theoretically) find that your shifts are smoother and quicker, due to the RD being closer to the cassette.
Also note that some derailleurs have multiple positions for the end of the upper derailleur pivot spring thus allowing you to adjust the B-tension screw's starting point at the all-the-way-in position.
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Old 06-07-11, 06:53 AM   #4
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The B-tension screw doesn't affect too much with Shimano derailleurs, it allows adjustment to keep the pulley from rubbing the large sprocket on the cassette.

On Suntour index shifting systems from the late 80's and early 90's, the B adjustment was a little more critical. The best shifts were achieved when the derailleur body was parallel with the chainstay. Some Suntour systems would barely work if this wasn't adjusted properly.
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Old 06-07-11, 05:52 PM   #5
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In my case, it appears that with the smallest chainring and largest cog (38 X 32t) the b-screw is backed all the way out and the cage still doesn't rub on the cog -- so in my case maybe it has no effect?

Thanks for your responses --
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Old 06-07-11, 06:07 PM   #6
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It winds up a return spring , if there is one around the mounting bolt.
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Old 06-07-11, 07:41 PM   #7
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Originally Posted by TurbineBlade View Post
In my case, it appears that with the smallest chainring and largest cog (38 X 32t) the b-screw is backed all the way out and the cage still doesn't rub on the cog -- so in my case maybe it has no effect?
Correct
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Old 06-07-11, 11:41 PM   #8
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Seriously ******** information on this thread.

Proper b-screw adjustment is INTEGRAL to shift performance. Both theory and practice back this up. Go read barnetts and educate yourselves.
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Old 06-08-11, 02:02 AM   #9
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Seriously ******** information on this thread.

Proper b-screw adjustment is INTEGRAL to shift performance. Both theory and practice back this up. Go read barnetts and educate yourselves.
Since most of us don't have Barnetts readily available, would you be so kind as to post the correct procedure?

My experience tends to support the above posters that the b-screw adjustment on Shimano rds doesn't seem to matter much unless the large cog interfers with the jockey pulley. Please point out the specific error of my ways.
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Old 06-08-11, 03:59 AM   #10
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Seriously ******** information on this thread.

Proper b-screw adjustment is INTEGRAL to shift performance. Both theory and practice back this up. Go read barnetts and educate yourselves.
Bad day at work ?

I don't have Barnett's manual or the big blue book here either.

A properly adjusted b screw has great effect on the performance of a bicycle's transmission and one will realize this when they have worked on derailleurs that lack this feature.

Will agree on the b screw setting on Suntour derailleurs as having more effect while Shimano derailleurs are more forgiving... in either case the derailleur guide pulley needs to clear the largest cog and maintain chain tension throughout the transmission's operating range.

This is fairly easy to do with closely spaced blocks and chain rings but gets a little harder when you are working with wider ranges that push the derailleur to it's maximum cog capacity... working with vintage half step gearing and wide blocks can be a bit of a challenge as B screw adjustment is critical and everything needs to be fairly exact.
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Old 06-08-11, 06:26 AM   #11
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update: I removed my FD and outer chainring to make a 1x7 drivetrain (38 X 13-32t). I set the chain length by Sheldon's advice (big - big no through RD) and the RD is fairly stretched out in the 38 X 32t low gear. When I turn the b-screw all the way out (loose) it still doesn't hit the 32t cog. Quite a bit of room to spare actually.

Should I add a link to make the RD track more closely to the big cog? Is there better shifting with this?
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Old 06-08-11, 07:04 AM   #12
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Bad day at work ?
But....that is a typical response, you seem surprised. It's just a power trip.

*****

The second posting by FastJake is the best why to approach this, to learn the B tension screws use.

My commuter has a 32T rear using 1995(ish) SLX derailler, other than jockey wheel rub when the B tension screw is too far out, I find no difference in shifting.

On my road bike with 2008 Dura Ace components, adjustment of the B tension is critical for smooth(er) shifting.

It seems that on the lower quality Shimano components, the B tension is not as critical for shifting.
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Old 06-08-11, 07:50 AM   #13
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Some of the later Suntour rear derailleurs had little notches on the derailleur that could be used as reference points for proper B tension adjustment. The notches would be aligned with each other when the angle was correct.
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Old 06-08-11, 10:33 AM   #14
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Originally Posted by TurbineBlade View Post
update: I removed my FD and outer chainring to make a 1x7 drivetrain (38 X 13-32t). I set the chain length by Sheldon's advice (big - big no through RD) and the RD is fairly stretched out in the 38 X 32t low gear. When I turn the b-screw all the way out (loose) it still doesn't hit the 32t cog. Quite a bit of room to spare actually.

Should I add a link to make the RD track more closely to the big cog? Is there better shifting with this?
It is big and big plus 2... snap a picture of the drive when it is in it's lowest gear so we can see the derailleur position. It should not be strained to it's limit and should also be providing adequate tension in the high gear position.
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Old 06-08-11, 03:57 PM   #15
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This is the current drive train, hopefully it gives a good idea. The b-screw is backed all the way out and the upper pulley still clears the big cog easily....so it seems to do nothing in this instance, is that correct?

The chain length is big - big plus one full link.

Thanks,
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Old 06-08-11, 04:58 PM   #16
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I'd add at least a link to that chain. No sense straining your RD more than you have to. As long as it doesn't go slack in the smallest cog you're fine. That looks too tight for my taste.

I have no idea what operator was so PO'd about. Maybe he'll explain it, let us all in on what we're missing.
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Old 06-08-11, 05:16 PM   #17
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Note: the chain is on the biggest possible gear combo -- no FD and the outside chainring is just "hanging out" there.
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Old 06-08-11, 08:14 PM   #18
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Note: the chain is on the biggest possible gear combo -- no FD and the outside chainring is just "hanging out" there.
Looks to me as if it's in the lowest gear combination, inside ring - biggest cog. What would happen if you put it in the big to big combination?
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Old 06-08-11, 09:17 PM   #19
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What would happen if you put it in the big to big combination?
Not possible with no FD.

Still, looks tight to me. The long cage MTB derailer should be able to take up all the slack with a longer chain. How does it look in the smallest cog?
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Old 06-08-11, 09:36 PM   #20
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In the case of my Sram 7, the "B" screw works just like the books says it should. The result is shifting is very quiet and flawless.
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Old 06-09-11, 02:06 AM   #21
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Whooops, yeah you're right. This is the lowest possible gear. I should have said the gear which stretches out the RD as much as possible.
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Old 06-09-11, 02:59 AM   #22
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This is the high gear (chain most slack) for reference.

Thanks,
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Old 06-09-11, 01:47 PM   #23
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Not possible with no FD.
That's a good point.
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Old 06-10-11, 06:56 AM   #24
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Needs another link or two.

And I think maybe it's just conceivable that the chain could hook onto the big ring by itself, under the right freakish circumstances... that could be ugly.
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