I wanted to try and get a bit more large cog clearance on my road bike; I went from the 11-23 cassette that came with the bike to a 12-27. It's a bit tough to get the RD to climb onto the 27t sometimes, and when it does, the jockey wheel is definitely rubbing.

I tried tightening the B screw in all the way, but that had no effect.

So I found a slightly longer screw and replaced the original but even screwed all the way in, there was no change...then I noticed that the screw is not making contact against my frame's dropout - in other words, if you look at these images (from Park Tool's site):



You see that the screw is pointing roughly towards 8:00 or 7:00, and hence, butting up against the little tab of the frame's dropout.

On my bike, the screw seems angled too low (closer to 6:00) and it ends up being screwed in against nothing - no wonder it's not changing the jockey-cassette clearance...


Is there a way to adjust that angle, so the screw is making full contact against the dropout? Would it involve loosening the main RD hanger bolt?

Thanks!

I'll add, for clarity - It's a Dura Ace 7700 (short cage) RD, but the 12-27 cassette is a Dura Ace 7700 9 speed, so I was assured at the time it was installed that it should work (I'm running a 53-39 double up front, fwtw).

Is the RD installed properly? Can it be rotated counter-clockwise so the screw lines up with the tab if you loosen the attachment bolt?

Sometimes running the screw in the opposite direction works better.

I'll have to check that at home tonight. It did seem when I fiddled with it this morning that I could move the RD around but when I tried to tighten the screw it seemed to push the screw out of proper angle alignment. Could it be the RD is too loose in the hanger?

I see that works well for many folks, but I was kind of trying to avoid removing the RD if possible...I didn't want to muck about too much with the adjustments since - aside from issues with the largest 27t cog - it's been shifting pretty well. If this recent development means I need to mess with the hanger bolt anyway, maybe I should try reversing it (for better chance of the screw making proper contact against the drop out?)?

Because the B-screw is always straight it runs on a tangent to the arc of the hanger. So at maximum extension it can miss the stop entirely. You have a few options.

Before doing anything rash, back the screw out (pull the RD back so it doesn't drag against the point of the stop) then find the maximum position before it slops past the stop. That might be enough, and you'll be good to go, with a small lesson learned.

Otherwise, the best thing to do is to use one of those epoxy/metal products to build up the stop extending it back and pushing the RD back. To get an idea how far you'll need to do make an aluminum foil pea and trap it between the B-screw and stop. You can leave it there as a solution, but odds are you'll lose it when removing the wheel. So once you decide how far you need to go, you can do a permanent job with the epoxy. If it's more than 1/8" or so, you'll want to reinforce it. I do this by drilling a 2mm hole and setting a small piece of spoke in.

Another way to do a better repair is to use a welder to build it up backward and dress it to shape with a file. This works for both steel and aluminum hangers, and a skilled welder can do it with a minimum of paint burn.

If I am understanding your post correctly, this is essentially what people are talking about when they say a bike's ability to accept a large cog of a certain size has as much to do with the drop outs, as with the current RD's capacity? Because on this particular frame, the little nub on the drop out is very very small indeed - so I think you are 100% on target - it seems to engage the screw when it's in a few turns, but once you get past 1/3 or certainly half way, it slips off the edge.

I do have a NOS Ultegra long cage RD I could try installing, but am I correct in saying, with that tiny nub on my drop outs (short of the building up of more material you suggested) it likely won't make a difference? I.e., this frame was just designed to to use a small straight-block cassette with a racing RD?

I don't know that it's intentional design, or simply variance among hangers. Most frame builders use stock hangers without modification. If i's a replaceable aluminum hanger, chalk it to design.

Variance in hanger stop tabs is one reason that many (most) modern derailleurs use a stop cam between the Rd and hanger, The cam will engage on just about any hanger, and puts the RD maker in control of B-screw engagement.

Try the foil pea and see of you like the results, then decide if you want to live with the pea (carry spares) or do something more permanent.

Changing RDs may not solve anything, unless you switch to one with a stop cam.

I think you missed how flipping the B screw around is actually a very simple solution and will solve your problem. Since the head of the screw is wider, it will better engage the drop-out. Normally people do this because they need more extension than is possible with the existing screw (or they buy a longer screw).

I have the same problem you're having on my bike, except it happens at about 70% before the screw basically pushes by the drop-out. Just flip it around and you'll be all set. To adjust, rotate the derailleur back, do a couple turns of the screw, and let the derailleur rotate back in place. Check your setting, and repeat until you have it right.

+1, this is simple and may solve the problem very well. You can also get a nut and use a punch to crush the last threads on one side, then attach it to the business end of the B-screw.

The problem with these if the screw is extended all the way is that it can bend the screw over time. I still favor modifying the hanger as the right permanent solution.

I will try the pea, thanks again - and I am curious whether either of my derailleurs actually have the stop cam? (I have the SS version of the 7700 on the bike now); this is the Ultegra 6500 GS RD I have as a spare.

Thank you - if I try this, am I understanding right - no need to remove the RD completely? Just flip it out of the way enough to install the screw backwards? And would it be worth using the slightly longer screw to avoid or at least lessen the risk of eventual bending FBinNY mentioned?

Other than being long enough for thread engagement in the RD body, a longer screw won't make a difference, since the bending forces are based on the angle of contact and amount of screw extended on the business end.

By the way, no offense meant, but your cat video is horrifically distracting and annoying. Maybe once would be tolerable, but at the bottom of every one of your posts is way over the top. Say you don't think it is cute...please.

Did you happen to use a 1" longer chain?

do you mean when switching from a 23t to a 27t? I don't believe so - after installing the new cassette I had my local shop basically do a little RD tweak when installing new shifter cables ...since then, I think I switched the chain out with a new one and basically matched the length to the original. I realize that could be creating a bit of stubbornness when shifting onto the 27t, but would that have a bearing on the jockey pulley clearance/B screw adjustment?

It might have a bearing on breaking some expensive parts!

***********The "B" screw is NOT a chain tension screw.**********

B screws are adjusted for a specific distance between the jockey wheel and the largest cog!!! On my Sram 7 the instructions are to set it at 6mm. The easiest way to measure that clearance is with the end of an allen wrench.

I used this method on one frame and built up this platform using an Arc welder in order for it to provide a footing for the B screw. Best to use a heat shunt though in order to save the paint job.

It's sometimes possible however, to exchange the B screw for a Allen-headed bolt and reversing this, using the larger head to find contact with the platform.

I tried reversing the screw, and the foil pea - It looks like I gained a wee bit of clearance, but am still getting the rubbing.

It actually looks like my dropout may already be worn down a bit - there is clearly paint gone where the B screw makes contact, so I am wondering whether there used to be more surface there.

The bike is definitely usable as is - like I said, a little stubborn sometimes (not always) when shifting to the 27t. I just wanted to get a bit of clearance there if possible...it looks like building up the dropout may be the only choice at this point.

Try the reversed screw combined with the pea. Otherwise it's epoxy/metal time. Remember you don't want to build the stop higher, you want to build it back around the arc.

Yep - I tried both together (reversed screw + pea). I'm going to try another pea or two, just to make sure I've got the shape right...

If you end up needing to build it back over 1/8" around, you'll do better drilling and setting a pin to anchor it properly. Or consider welding if you have a friend who'll do it for a few beers.

BTW- One of the nicest jobs I saw done was by a dentist, who set 2 pins at right angles, and built it up with silver amalgam. So if a dentist owes you a favor.....

How long is your chain? Are you sure too little clearance is the issue here? IME sluggish shifting comes from too much gap between the cog and RD. I try and maintain as little gap between the cog and RD pulley- just enough that the pulley and the cog don't share a link of the chain- when setting up a RD. Also a chain that's too short (depending on RD cage design) can keep the top pulley from tucking up under the cog.

LOL - Idk any Dentists...I'll try to hook up with one.

Definitely not too much clearance - the jockey pulley is grinding right up against the 27t cog.

If the spring tension in the cage pivot is adjustable, then lowering that tension will pull the jockey pulley away from the large cog.

Park tool covers this well in their repair help article on derailleur overhaul.

On a particular suntour rd I was running, I had to adjust this tension when changing from a corncob to a wide range freewheel, and vice versa.

Park Tool Co. » ParkTool Blog » Rear Derailleur Overhaul

rd disassembly would be required.

Chain length also becomes more critical the closer you get to maximum cog size capacity.

If you are going to stay at a max cog of 27 teeth, it may be worthwhile to size the chain using the bib/big method. My cheap way of playing with chain size is to use multiple quicklinks on the same chain to easily add or remove links in 1-inch increments. When the ideal length is determined, I install a good chain with one quicklink.

So, there's the two things I'd check. Please make sure that, if you alter your chain length, you always have at least enough to run the big/big combo smoothly, ie. shift in and out of it, front and rear at the same time.

Park Tool Co. » ParkTool Blog » Rear Derailleur Overhaul