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  1. #1
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    Dura-Ace rear derailleur skipping can't be tuned out!

    I am helping a friend with his Ritchey Break Away steel bike equipped with full Dura-Ace 7800 group (except for FSA compact crank, 50/34) and a 28 tooth largest cog. After returning from a trip with the bike (frame) broken down into its two parts in its travel bag, he reassembled it and attempted to ride it. With the chain on the big ring up front, the drailleur and chain were skipping ONLY ON THE FOUR LARGEST REAR COGS. There was "perfect" adjustment and no skipping on any rear cog when using the small front ring. This defect cannot be adjusted out. Upon close examination what I see is that when the chain is crossing at a sharp angle from the big front ring to one of the large rear cogs, the chain gradually slides to the right on the front-most (upper) jockey wheel until it runs into the outside (right most) plate on the derailleur arm. When it touches the plate, it jumps back to center itself on the jockey wheel, and the process starts all over again. This does not happen when back-pedalling when all is smooth. The larger the cog (further left the derailleur), the more frequent and more severe the skip. The drive train functions hardly at all on the two largest cogs with the large front ring the chain is so bound up. I checked the jockey wheel teeth, and they appear to be in good conditon with minimal wear. Besides this issue with the large front ring and largest four rear cogs there are no other problems whatsoever.

    Not to skew anyone's response, but let me just say that the travel case is semi-soft sided and some crushing could have occurred. I can't see any misalignment of the derailleur hanger, which by the way, is very substantial and made from steel. The rear triangle was always protected with a dropout separator to keep the stays from being crushed. Has anyone encountered such behavior before? Any known causes? Cures? It would be easy enough to replace the derailleur in hopes of a quick fix, but we would like to avoid that if possible.

    Any help would be much appreciated.

    Thanks. Robert

  2. #2
    Insane Bicycle Mechanic Jeff Wills's Avatar
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    From your description, I would check the derailleur hanger alignment with a proper gauge. Since it worked fine "before" and didn't "after", I would suspect it's been bent (possibly twisted) slightly out of spec. It might be OK by eye, but you have to check with a proper tool.

    A friend of mine makes the ne-plus-ultra of derailleur hanger tools: http://efficientvelo.com/tools/hange...ent.html#ultra
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  3. #3
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    Hanger alignment may be a factor, but it could also be a "B" screw adjustment. Possibly the RD is riding a bit high allowing interaction between the Jockey wheel and larger sprockets (through the chain). Take a look, there should be at least 3/4" of free chain along the tangent from the jockey wheel to the sprocket. Put another way, both shouldn't ever touch the same link at the same time.

    It could also be a minor trim adjustment, which is more sensitive when the RD is closer.

    Since the B adjustment is only an eyeball check, start there. Then check trim, but don't be surprised that you have to check the hanger.

    The last and most expensive possibility is that the RD cage is slightly bent or twisted, that's also fairly easy to eyeball, but you may not like what you see.
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  4. #4
    bike whisperer Kimmo's Avatar
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    Quote Originally Posted by Jeff Wills View Post
    A friend of mine makes the ne-plus-ultra of derailleur hanger tools: http://efficientvelo.com/tools/hange...ent.html#ultra
    Wow. Just spent half an hour looking at all his tools. Nice.

    Quote Originally Posted by FBinNY View Post
    there should be at least 3/4" of free chain along the tangent from the jockey wheel to the sprocket. Put another way, both shouldn't ever touch the same link at the same time.
    Hm, haven't come across this idea before; I thought it was enough for the teeth to miss each other. Could you say a bit more about the thinking behind it?

  5. #5
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    Quote Originally Posted by Kimmo View Post
    Hm, haven't come across this idea before; I thought it was enough for the teeth to miss each other. Could you say a bit more about the thinking behind it?
    You want to avoid any direct interaction between the pulley and sprocket (through the chain). The pulley has to lead the chain onto the sprocket, and off when shifting. If the pulley is too close the chain's stiffness resists the motion of the pulley and increases the force needed to shift.

    If you have a clothesline you can experiment derailing the rope. If you try from too far you have to push the rope sideways to get any action, but if you're too close you can't move the rope over the flange. The key is to be as close as possible, without being too close. On a derailleur, you'll find that happy medium to be with about 2 links of chain between them. BTW- it rou struggle through the Shimano adjustment data you'll see a similar guideline for "B" screw adjustment.

    Direct contact though the chain must be avoided because it prevents the cage from rotating fully, and can push the RD off the B stop. That risks the entire RD being pushed back, and can cause a breakaway hanger to let go.
    FB
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  6. #6
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    A little clarification

    Quote Originally Posted by FBinNY View Post
    You want to avoid any direct interaction between the pulley and sprocket (through the chain). The pulley has to lead the chain onto the sprocket, and off when shifting. If the pulley is too close the chain's stiffness resists the motion of the pulley and increases the force needed to shift.

    If you have a clothesline you can experiment derailing the rope. If you try from too far you have to push the rope sideways to get any action, but if you're too close you can't move the rope over the flange. The key is to be as close as possible, without being too close. On a derailleur, you'll find that happy medium to be with about 2 links of chain between them. BTW- it rou struggle through the Shimano adjustment data you'll see a similar guideline for "B" screw adjustment.

    Direct contact though the chain must be avoided because it prevents the cage from rotating fully, and can push the RD off the B stop. That risks the entire RD being pushed back, and can cause a breakaway hanger to let go.
    I am familiar with the B-stop adjustment and the need for the right clearance between the cassette and the derailleur at extreme extension of the derailleur. I will certainly examine this aspect of the setup to see if it is okay. However, the malfunction APPEARS to be initiated at the other end of the derailleur cage, the other wheel, the one closest to the crank, not the one closest to the cassette. The chain slides over toward the outer plate until it hits it and snaps back to the center. The direction of motion is of course in the direction that the crossed-over chain is pulling from the large front ring slanted to the large rear cogs. But why this just start to happen? I have checked that the chain is the right length. It appears to be: the two wheels line up straight up and down on the small-large and large-small ring-cog combinations. Puzzled.

  7. #7
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    Quote Originally Posted by rpenmanparker View Post
    I am familiar with the B-stop adjustment and the need for the right clearance between the cassette and the derailleur at extreme extension of the derailleur. I will certainly examine this aspect of the setup to see if it is okay. However, the malfunction APPEARS to be initiated at the other end of the derailleur cage, the other wheel, the one closest to the crank, not the one closest to the cassette. The chain slides over toward the outer plate until it hits it and snaps back to the center. The direction of motion is of course in the direction that the crossed-over chain is pulling from the large front ring slanted to the large rear cogs. But why this just start to happen? I have checked that the chain is the right length. It appears to be: the two wheels line up straight up and down on the small-large and large-small ring-cog combinations. Puzzled.
    OK, that clears it up. Most likely the hanger or cage got bent in transit. A bent hanger would be confirmed, if you had to adjust the trim when you put the bike back together. Double check the limits while you're at it.

    A bent cage (or hanger) puts the lower pulley too far out of line for the chain. If the chain wants to move outward, the cage is inboard, if the chain wants to move in, the cage is outboard. A twisted hanger or cage has the same effect, except here the pulley's in the right place, but it's line of action is off.
    FB
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  8. #8
    Ride, Wrench, Swap, Race dddd's Avatar
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    The first thing that led me to believe that the hanger was definitely bent was that the indexing at the rear derailer changed when the chain was moved between the different-sized chainrings.

    With the hanger bent, the cage pivot of the rear derailer will no longer be parallel to the axle, and the cage will thus rotate off-axis with the axle now.
    And, since the upper guide pulley of the derailer is offset from the cage pivot axis, any rotation of the cage (as from changing chainrings) will cause the upper pulley to move inward and outward in response to the cage rotation, which is a big no-no for any derailer bike with more than one chainring (even for a friction-shifted bike, as the rear derailer suffers unwanted movement in response to changing chainrings).

    Although the unwanted in-and-out movement would seem to be slight, this is such a classic case of typical bent-hanger symptoms that I can think of no other cause of this indexing malady.

    It should not be too hard to see that the derailer cage is now somewhat less than parallel to the cassette cogs, simply by sighting along the side of one of the (cleaned) larger cogs and noting that both pulleys then do not fall in the same line.
    It helps to shift to big and small chainrings so as to sight down the derailer cage and pullies as the cage rotates to different angles, with one angle usually showing much more skew than another.

    And, if you do try to straighten the hanger at home with houshold tools, several important hints:

    1) Always have a wheel clamped into the dropouts when bending the hanger, even if you have to mount the wheel backwards in order to get wrench clearance clamping on the hanger.
    2) Clamp a very large (12" min) adjustable wrench tightly over the entire threaded hole of the hanger, so as not to distort or crack it.
    3) By carefully positioning the wrench angle before bending, it is possible to correct both bend-in and twist at the same time, as often both may be afflicting the hanger's precise alignment.
    4) It helps to have a "known-straight" derailer at hand to test-mount for sighting purposes, without the cable attached, but with the bike's rear wheel properly installed.
    5) If the cage angle looks good from different angles when carefully sighted along CLEAN cogs, it is good enough for any modern indexing system to work accurately.

    Lastly, I don't think it's likely that the derailer got bent from the inward impact to the derailer's knuckle.
    This is very different from the type of trauma that occurs when a chain-jam bends the hanger while riding, which is the one thing that so often does bend the derailer.
    Last edited by dddd; 09-14-12 at 04:16 PM.

  9. #9
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    Thanks to everyone who contributed some help for my question. I just wanted to get back to all of you and let you know problem is fixed. New derailleur unfortunately, but fixed is fixed, so no complaints. The hanger was absolutely straight so that left only one possibility. Doing the repair at home with new part bought at a good price made this experience altogether satisfactory. Thanks again for the advice.

  10. #10
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    This experience is a prime example of one of the simplest diagnostic tools. If a problem occurs with something that was operating properly just before an interceding event then the first thing to look at is the potential impact of that event. Although it's nice to know all about the b-tension screw there's nothing about carrying the bike dismantled that would change its adjustment, but doing so could certainly could bend either the derailleur or hanger. It does not matter how likely one thinks the connection, and there's no expert knowledge needed.
    There's no such thing as a routine repair.

    Don't tell me what "should" be - either it is, it isn't, or do something about it.

    If you think I'm being blunt take it as a compliment - if I thought you were too weak to handle the truth or a strong opinion I would not bother.

    Please take the time to post clearly so we can answer quickly. All lowercase and multiple typos makes for a hard read. Thanks!

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