Bicycle Mechanics - B Screw Maxed - What to do?

I've always had a slight "grinding" noise when in the largest sprocket in back. Did some investigation on-line and it seems my "B" screw needs to be tighted. The jockey wheel seems too close when in the biggest gear. Trouble is, the "B" screw is already in as far as it can go. Ultegra RD.

Almost looks like I need to loosen the RD and "rotate" it so that the "B" screw actually has some room to be useful, but not sure.

What the suggested "next step"?

Buy a longer bolt to replace the strock B-screw or reverse the current bolt in the threaded hole.

tntyz: +1 on the longer screw. Turning it around might work but adjusting it may be difficult.

I have flipped them over, screwed them in,
and had the broader head catch the dropout's stop better .

then you go shopping for a long cage RD , made for the ratio range you want to use..

I have flipped them over, screwed them in,
and had the broader head catch the dropout's stop better .

then you go shopping for a long cage RD , made for the ratio range you want to use..

That's what I was going to suggest.

The second part is a bonus but I agree with that part too.

A new derailer probably is in order. My Sram 7 for instance says the jockey wheel should be adjusted for a 6mm clearance between itsself and the large freewheel cogs.

How many teeth on the largest cog?

+1 on flipping the screw.

I went from a 27T to a 28T and found the maxed out B-screw wasn't enough for my setup. I had the room to remove an inch of chain, which may have pulled the upper pulley a little further from the cog, but I decided to keep the extra inch in there. If you need it even longer, try using a longer screw also backwards.

I tried a longer screw I had lying around, but found that once it got in too far, the screw would miss the hanger. I put the original screw in upside down and screwed it in until it fully engaged the threads on the RD and it works perfectly.

...then you go shopping for a long cage RD , made for the ratio range you want to use..

NO!

It is not the length of the cage that matters... why do you people keep making the same mistake?

A GS (long cage) Ultegra or 105 or DA (any road) derailleur will have the same problem. If you are using larger than 28 teeth you should go with a mountain derailleur like XT (Non-dynasys) to make it work.

part of the benefit is the wide difference of cog diameters/tooth counts,
leaves a lot of slack to be taken up.
that is what the length of the separation of the pulleys does .

now if you dump all but one chainring then the chain wrap amount requirement
is even less ..
the need to flip the B screw is a Symptom .. preload and the pulley wont closely follow the small cogs, and slows the shifting that y'all paid to get.


I have a short cage Chain-tensioner on My Rohloff. :innocent:

you can go way over 28 teeth with an ultegra gs. I run 32 teeth in the back without issue. part of it depends on the length of your deralieur hanger. i agree with the the others, you should get a longer screw or try flipping it around. i prefer the longer screw.

OP: how many teeth on your cassette? All the suggestions given are good, but until we know that we're all just guessing. If you went to a bigger cassette it's possible your chain is too short.

Use a cassette with a smaller large cog...

NO!

It is not the length of the cage that matters... why do you people keep making the same mistake?

"made for the ratio range you want to use." That sounds like spec for the biggest cog to me. I guess maybe we see whatever it is that we're looking to find.

Thanks for all the replies. 27 teeth on the large sprocket. I don't really want to go the route of a RD replacement if it can be avoided.

27 teeth on the large sprocket.

You don't need a new RD. Either your B-tension screw is too short, or your limit screw is adjusted wrong causing the dragging (less likely.)

If you're OK with some open-derailleur surgery, you could also open up the pivot and drill a new hole for the tip of the spring to rest in, so it has higher tension at any given B-screw setting.

When in big/big, how much slack is the RD picking up? The chain could be longer than necessary.

"made for the ratio range you want to use." That sounds like spec for the biggest cog to me. I guess maybe we see whatever it is that we're looking to find.

If it were just the "made for the ratio" comment then it would have been correct. But the length of the cage has no bearing on it whatsoever... a short or med cage made for the correct cog sizes would work fine too.

However it seems we were both wrong - the OP's cassette should work with his current deraileur.

I am surprised this is even a problem. This is the standard setup for this bike (2008 Trek Madone 4.5 w/triple). It's been there since day 1; I just never took the time to figure it out before. Is it possible there is something wrong with the RD installation?

As long as the derailleur is properly screwed into the hanger and the B tension screw is hitting the little tab then it is very unlikely that is is a derailleur install issue.

Actually, maybe it is just a setup issue - perhaps the derailleur is just not properly lined up with the large cog and this is creating the grinding. My experieice with derailleurs hitting the large cogs has been more of a 'rumble' than a grind - the jockey wheel on the derailleur rides along on the cog (actually touching) and bumps from tooth to tooth.

As long as the derailleur is properly screwed into the hanger and the B tension screw is hitting the little tab then it is very unlikely that is is a derailleur install issue.

Actually, maybe it is just a setup issue - perhaps the derailleur is just not properly lined up with the large cog and this is creating the grinding. My experieice with derailleurs hitting the large cogs has been more of a 'rumble' than a grind - the jockey wheel on the derailleur rides along on the cog (actually touching) and bumps from tooth to tooth.

That is a better description of the sound and the way it feels.

That is a better description of the sound and the way it feels.

So the derailleur is close enough to the cog that it is likely touching? THis is unusual as that derailleur should work with that cog... many people report good results mating Shimano road derailleurs with 30 tooth cogs - sometimes even larger -, even though they are rated to a max of 27 or 28 teeth. If this were a new problem, and the bike used to work properly, I would suggest that it might be a problem with the derailleur... but since it has been happening since you got the bike I don't know what to say.

The chain could be longer than necessary.

+1, especially if that RD is standard on the OP's model bike.

I was running a standard DA double, 53-39 front, with a 11-28 rear and DA 7800 SS (short cage) derailleur with no problems. I recently put on a compact, 50-34 crankset, and found that the top pulley in rear derailleur rumbled (hit) on both the 25 rear cog and 28 rear cog when in 34 front. First I tried a longer B screw to no avail: the angle of the B screw changes as I screw in the B screw and it screws right off the edge of the derailleur hanger. Then I tried screwing the B screw in upside down. This solved the rumbling in the 25 tooth cog, but not in the 28 tooth cog. Then I finally solved the problem by getting a B screw with a slightly larger head at a nuts and bolts shop, screwing it in upside down again, and that solved the rumbling problem when in the 28T cog by angling out the 7800 short cage derailleur a little more. So thanks for the tip about flipping the B screw upside down!!!

Having said that, is there any theoretical, shifting downside to this solution? For example, assuming that I set distance (using the B screw) between the top jockey wheel and 28t cog at the same distance on 7800 short cage derailleur as I would set the distance on a higher capacity (33T), higher limit (28T) DA 7900 derailleur, when I get down to using the smaller cogs, will the distance from the top jockey wheel to the cogs be further away when I use the short cage 7800 versus a 7900 (or GS derailleur), thus causing slower shifting? Thanks for your answer!

I was running a standard DA double, 53-39 front, with a 11-28 rear and DA 7800 SS (short cage) derailleur with no problems. I recently put on a compact, 50-34 crankset, and found that the top pulley in rear derailleur rumbled (hit) on both the 25 rear cog and 28 rear cog when in 34 front.

Did you shorten the chain when you switched to smaller chainrings?

If you're OK with some open-derailleur surgery, you could also open up the pivot and drill a new hole for the tip of the spring to rest in, so it has higher tension at any given B-screw setting.

Sometimes you'll even find an extra hole in there for adjustment. If increasing tension on the upper pivot doesn't do it, try reducing tension on the lower pivot.


If it were just the "made for the ratio" comment then it would have been correct. But the length of the cage has no bearing on it whatsoever... a short or med cage made for the correct cog sizes would work fine too.

Except that longer cage lengths pretty much always go along with a larger cog spec.

NO!

It is not the length of the cage that matters... why do you people keep making the same mistake?

A GS (long cage) Ultegra or 105 or DA (any road) derailleur will have the same problem. If you are using larger than 28 teeth you should go with a mountain derailleur like XT (Non-dynasys) to make it work.


you can go way over 28 teeth with an ultegra gs. I run 32 teeth in the back without issue. part of it depends on the length of your deralieur hanger. i agree with the the others, you should get a longer screw or try flipping it around. i prefer the longer screw.

This is right. I have an Ultegra GS RD for my triple. There's no way it's taking a 30 cog.

Did you shorten the chain when you switched to smaller chainrings?

Yes. I shortened the chain based on putting chain in 50 tooth and 28 tooth and checking angle of derailleur against pictures at Park. And a bike mechanic told me taking any more links out might be unsafe to shift in to 50/28. It is borderline. Of course I try not to cross shift in to 50/28, but I have been known to shift in to big/big on some occasions. With regard to the 34/11 position, the chain now bumps on that little tab on the derailleur when in 34/11, but that is not unsafe as far as I know, and I am unlikely to knowingly use that combination for longer than a few seconds. There is a barely visible amount of sag in the chain when I am in the 34/11 (you really have to look hard to discern it) and the chain looks pretty horizontal all the way back to the end of the derailleur cage.

I am still wondering whether there is some downside that I don't know about in forcing my short cage 7800 derailleur to work by using an upside down B screw with larger head up against dropout versus using a derailleur with higher chain wrap capacity and higher rated maximum cog capacity.

Sounds like you might be able to lose another pair of links, which would eliminate the occasional few seconds of your chain dragging over the cage tab... if you shift to 50/28, is it possible to pull a couple of links together to take out an inch? If so, you should do it, since it'll help keep the upper jockey wheel off your big cogs.


I am still wondering whether there is some downside that I don't know about in forcing my short cage 7800 derailleur to work by using an upside down B screw with larger head versus using a derailleur with higher chain wrap capacity and higher rated maximum cog capacity.

Don't sweat it - the downsides are what's being mitigated in this thread.

Except that longer cage lengths pretty much always go along with a larger cog spec.

Sorry, Kimmo. You are wrong.

According to the specs on the Shimano website, the max cog size of both the SS and GS (short cage and mid cage) versions of both the 6700 Ultgrea and 105 are 28 teeth. The max cog size of the SS and GS versions of Tiagra and Sora are both 27 teeth. The max cog sizes for the GS and SGS (mid cage and long cage) versions of SLX rear derailleurs are both 36 teeth. What is different between the short and mid and long cages of any one derailleur is the capacity, which is the amount of chain take-up possible - a longer cage can generally handle a bigger difference in chainring size + difference in cog size.

Sounds like you might be able to lose another pair of links, which would eliminate the occasional few seconds of your chain dragging over the cage tab... if you shift to 50/28, is it possible to pull a couple of links together to take out an inch? If so, you should do it, since it'll help keep the upper jockey wheel off your big cogs.

Don't sweat it - the downsides are what's being mitigated in this thread.


I looked at the chain again and I don't think I want to try to take out more links. It was time to change the chain though and I just put a new chain on with same number of links. This solved the problem of chain dragging over cage tab when in 34/11.

the need to flip the B screw is a Symptom .. preload and the pulley wont closely follow the small cogs, and slows the shifting that y'all paid to get.

This^ is the comment that caught my attention and prompted me to initially ask about theoretical downside of the flipped B screw solution.

Sorry, Kimmo. You are wrong.

Oops, that's a surprise. I stand corrected.


I looked at the chain again and I don't think I want to try to take out more links. It was time to change the chain though and I just put a new chain on with same number of links. This solved the problem of chain dragging over cage tab when in 34/11.

Hmm, must have been down to the extra length of a worn chain. Sounds like you're golden now.

As for the downside, clearing the largest cogs on a wide cassette will always mean a bigger gap to the smaller cogs, unless I'm missing a layer of subtlety...

Hmm, must have been down to the extra length of a worn chain. Sounds like you're golden now.

Ok. Thanks!!


As for the downside, clearing the largest cogs on a wide cassette will always mean a bigger gap to the smaller cogs, unless I'm missing a layer of subtlety...

It is probably a dumb, nitpicking question, but is it possible that a derailleur that is designed to handle a 11T min, 28T max cassette from the get go can stay closer to the cassette cogs across the entire range of shifting on a 11/28 cassette (and thus shift a tiny bit faster) than a derailleur that is designed for a 11T min, 27T max cassette, that is used on a 11/28 cassette only by virtue of flipping the B screw? That seems to be what Fietsbob was implying.

There's no minimum; the maximum sized cog is determined by the distance between the upper pivot and the parallelogram.

And it's totally a nitpicky question - I was going to spend fifteen minutes formulating a thoughtful reply, but since B-screw adjustment has little effect on shifting, I CBF.

Short answer: I doubt it.