Chain Rub with Dura Ace Compact Crankset

Discussion in 'Cycling Equipment' started by oldultrarunner, Nov 4, 2011.

  1. alfeng

    alfeng Well-Known Member

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    Quote: Originally Posted by KLabs .

    ... please post a url (link) of where you can get a compatible 36T 7900 inner chain ring. That would be terrific ... thanks


    Quote:
    Originally Posted by oldbobcat .


    You got me looking and I could not find a 7950 36t ring anywhere, even in Shimano.com. For 130 mm you have several options. but with 110 mm 50-34t is all you get.


    Once again, what we have here is a failure to communicate ...

    To again quote myself:

    • The inner chainring on-or-for a Double crankset is not ramped. So, as far as replacing the inner chainring, the only thing(s) a person truly needs to be concerned with is the BCD + the cosmetics, with the latter being a trivial matter.

    • Consequently, almost any 3/32" 110BCD chainring (except Campagnolo's!) will fit on the Compact 7900 crankset.
    This is from Shimano's tech doc for their Ultegra 6750 crankset (it shows the chainrings & spider more clearly than the images in the 7900/7950 doc):

    [​IMG]
    As you can hopefully see, other than being nickel plated, it is just a nothing-special 110BCD inner chainring, so ANY 3/32" 110BCD chainring can be used.

    Several years ago, I bought a 37t 110BCD chainring to use on one of my so-called "Compact" cranks ... it was a 3/32" BMX chainring.

    Personally, I would just get any 36t 110BCD 3/32" chainring from eBay ... and, if necessary, paint it.

    • FYI. WalMart's store-brand FLAT BLACK spray paint doesn't require primer (99.9% of the time) and should be durable enough ... buff it if a semi-gloss appearance is preferred.


    There is no reason to buy a set of 110BCD chainrings from PRAXIS WORKS when only someone only needs an inner chainring.

    In fact, you would not want to substitute the 7950 outer chainring with anything other than a 6750 chainring unless it was for dire reasons because any other substitute outer ring would not be "hollow" and would probably be cosmetically unattractive where the chainring meets the spider ...

    FWIW. The "chain bounce" suggests to me that your rear derailleur cage was possibly too short ... or, of course, the chain bounce might have been due to something else OR simply an unusually rough patch of pavement the last time you recall making the shift between chainrings with the particular crankset.
     


  2. vspa

    vspa Active Member

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    im very happy with my Ultegra Compact but i would like to know why the chainrings are not interchangeable with my normal Ultegra crank;

    is it for mechanical reasons ? (for example some kind of rigidity or strength reasons)
    is it for commercial reasons ?
     
  3. alfeng

    alfeng Well-Known Member

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    Well, as far as 'I' am concerned, the only reason that you cannot use normal chainrings on the latest Ultegra & DA cranks is mostly cosmetic (or, as you said, "commercial") ...

    However, the sibling 105 crank has comparatively normal profile spider arms and are designed to be used with normal, two-dimensional-by-comparison chainrings ...

    • to repeat ad nauseum, I have found that Campagnolo shifters w/ Shimano derailleurs work BETTER with vintage, unramped-and-unpinned (i.e., flat & relatively thin) chainrings THAN admittedly old but supposedly-mechanically-the-same-as-current-shifters 9-speed 6500 shifters were able to work with ramped-and-pinned chainrings when shifting under a modest load (e.g., cresting a hill) ...
    • based on the description of the Shimano Di2 shifting, Campagnolo mechanical shifters with Shimano derailleurs are on a par with how efficiently the chain is moved between cogs on a Shimano cassette when under load ...
    • based on the description of the shifting capabilities of the new Campagnolo EPS groups, the only advantage of Campagnolo's electronic shifters & derailleurs beyond some bling-effect over Campagnolo's mechanical shifters which are tandemed with Shimano DA-or-Ultegra chainrings is the modest physical effort -- or, lack of when using the electronic shifters -- needed to effect a shift.

    • N.B. perhaps the reviewers who are paid to gush over the wonders of the latest products have failed to communicate properly how much more wonderful the electronic shifting is than what I experience with MY unauthorized pairing of Campagnolo shifters on an otherwise Shimano drivetrain ... as it stands, their description merely echoes my experience of fast & efficient shifting when the drivetrain is under a load which I was not able to achieve with an all-Shimano drivetrain.

    FYI. The newest Ultegra & DA chainrings are supposdly lighter and (¿more importantly?) STIFFER to, IMO, further compensate for the in-built deficiencies caused by the STI shifter's eccentric take-up spools (front & rear / left & right) so there will be less chainring deflection when the chain migrates to the outer chainring than when the crank has an "older" style chainring ...

    Shimano can, IMO, remedy the problem(s) caused by their shifters's incorporated "dwell" by simply making the take-up spools in their shifters concentric ...

    By my reckoning, the eccentric take-up spool in STI shifters is a legacy of the same thinking which brought BioPace chainrings to the World ...

    By my reckoning, the floating upper-pulley wheel which Shimano rear derailleurs have had since ~1983 allows the STI mechanism to be "tuned" to momentarily over-shift to help the chain make the transition, and then the float allows the pulley wheel to center with the cog it is theoretically aligned with ...

    In the end, I reckon that Shimano-only users will either have to wait until everyone from THAT engineering team retires OR when Shimano's STI patents expire OR Shimano-only users can buy Di2 shifters-and-derailleurs/etc. if they don't want to be concerned with which chainrings are being used.

    • The slight modification which has resulted in what I presume is a slight over-shift means that there will be another 4-or-5 years before the current patent runs out ... just in time for the next generation of Shimano's mechanical shifters.

    --​
    BTW. SRAM? Does anyone really care? I presume all the misshifts in CX competition have been with SRAM's Double-Tap shifters ... least we forget, there was a signficant SRAM misshift during the TdF a year (?) ago.
     
  4. oldbobcat

    oldbobcat Well-Known Member

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    Whatever. I stopped cross-chaining when I started using 42-tooth chainrings and 126mm rear axles and Nuovo Record derailleurs. Old habits die hard.
     
  5. ambal

    ambal Active Member

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    Great thread, I've never run more than a 39x25 but i've been thinking of moving to a 27 or 28 rear.
     
  6. alfeng

    alfeng Well-Known Member

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    Hey, gotta love a bike with a NR rear derailleur!

    • I still think that the Campagnolo Nuovo Record may be one of the best looking rear derailleurs ever made
    • it's too bad that the NR (and, other Campagnolo ...) rear derailleur doesn't seem to have the ability to move across a more than a 7-speed Freewheel/Cassette
    • nonetheless, I'm working on indexing a NR rear derailleur to some Ergo shifters just because ... but, I may revert to using a Gran Sport or even a Valentino rear derailleur (which I presume has the same geometry) if I can get it to work because the frame I'm planning to set up probably isn't worthy of a NR rear derailleur!?!

    However, I'm not particularly keen on the NR front derailleur ...

    • Way back when, I had a HOLDSWORTH set up with a full NR group while I rebuilt my low-end GITANE with a Shimano Crane rear derailleur + eventually a second generation Dura Ace front derailleur & a Sugino crankset + NISI tubulars, etc. ...
    • Because I found that the particular Shimano front derailleur worked BETTER than the NR front derailleur, I replaced the NR front derailleur on the Holdsworth with another DA front derailleur ...

    And, even now, my preference is for Shimano front derailleurs because I find that they work better ... yes, I know saying THAT is a heresy to most Campagnolo users.

    ---​

    FWIW. IMO, the only time to cross-chain is when the bike is being set up to ensure that the rear derailleur cage is long enough to take up any potential slack in the chain to preclude the "chain bouncing" which you experienced on a bike which apparently wasn't set up properly ...
    Regardless (this is for the OP & others), cross-chaining should ideally never be used even if it can be achieved without grinding the chain on the outer chainring OR the outer plate of the cage because there is more wear on the smaller chainring(s) + smaller cogs than when the equivalent ratio is used with the outer chainring ...

    Racers, however, may not be able to avoid the small-small OR the large-large options as they are working through their gears.
     
  7. oldbobcat

    oldbobcat Well-Known Member

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    I doubt that a Nuovo Record derailleur would index well because the jockey wheel doesn't follow the contour of the cassette/freewheel at all and some overshifting is always necessary.

    Regarding chain bounce, forget I ever mentioned it. I prefer a loose chain because I don't crosschain to small-small because of bounce, friction, and rub against the big ring (even with 53-39), I like to use big-big on occasion, my chain line is spot-on, thank you, and yes, 50-36x11x25 or 53-39x12-27 is a lot of spread for a short-cage rear derailleur to cover, but I just don't see the point in buying a GS version.

    With short chainstays every system on a rear axle that's wider than 120 mm will involve "sacrificial gears," combinations that are at least unpleasant to use because of friction or chain slap. On older 5-, 6-, and 7-speed systems the main limiting factor was the width and lack of flexibility of the chain. On the newer systems it's the combination of the width of the cassette, the shortness of the chainstays, and the combination of close proximity and wide range of the chainrings. And the spindle length of triples has always been an issue, but less so with narrower spacing.

    If my Dura-Ace 7800 system had half-step gearing I wouldn't mind using small-small, but with a 10-speed block why bother? The chainrings aren't available and I doubt the front derailleur would be happy with a 3-tooth gap.

    The triple users I know spend most of their time in the middle ring, giving them pretty good access to an entire 9- or 10-speed cassette. They don't tell me why they never use small-small, but I surmise that it's because of any combination of cross-chain friction, small-cog friction, chain-slap, chain rub on the front derailleur (regardless of trim), and the availability of a similar ratio on the middle ring that's just plain less troublesome. Similarly, it's likely they don't use big-big because of cross-chain friction, friction from too much tension on the derailleur springs, and often some chain rub on the front derailleur.

    Now I know you can fine-tune out some of these conditions, but usually not without exacerbating something somewhere else. Shorten the chain to get rid of slap on the small ring and you get too much tension on the big one. Put a spacer behind the drive-side cup to get rid of rub on the big ring and you mess up the chain line from the big ring. Back off the cable tension on the front derailleur to eliminate rub in big-big, and there isn't enough to jump the chain up to the middle ring consistently (especially so with older Shimano 3x9 systems).

    Now I know that you will keep experimenting because that's what you do and your curiosity and perseverance are admirable. You might find an ideal kludge that involves Campagnolo 10-speed levers, SRAM Apex derailleurs, and Shimano cranks and cogs, and looping the cable kinda funny before it hits the pinch bolt, but most of us don't have that box of spare parts in our garages and most of us are quite tolerant one or two small compromises if it means more time to ride, work, be with our families, or kill time on cyclingforums.com (joke's on me), and a generally less troublesome system.

    I'm going out for a ride before it gets dark.
     
  8. Eichers

    Eichers New Member

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    Hi oldbobcat, this may or may not be relevant but my friend is using a DA 7800 SS RD successfully (without any issue) with an 11-28T 10spd cassette and a 50-34T FSA compact crankset :)
     
  9. oldbobcat

    oldbobcat Well-Known Member

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    I have no doubt. ;-)

    It was a nice ride. I may go for a longer one tomorrow. The weather will be nice.
     
  10. danfoz

    danfoz Well-Known Member

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    Whan I had a compact, except for the cassette size (12-25) this was my identical setup - DA 7800 RD and FSA compact... same rubbing as the OP. Unless running a frame w/super long stays, the chain is gonna rub against the large chainring in the smallest 2 gears while on the small chainring regardless. Personally I didn't see it as "not" successful, just wasn't my preference to have the rub.
     
  11. swampy1970

    swampy1970 Well-Known Member

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    Chains are more efficient when they're tight. Plus, things work better when it's still on the bike and hasn't bounced off either the chainring or the cassette.


    http://www.ihpva.org/HParchive/PDF/hp50-2000.pdf

    For the 36 tooth 110bcd, FSA make some very nice rings in that size.

    As for the original poster - make sure the chain is on the correct way. The 7900 chain needs to have the links with the writing on facing out. If you see no writing as you look at the bike from the right hand side of the bike then you need to put the chain on the other way. This is described in the tech notes on shimano.com AND in the install sheet that came with the product. If it's on the wrong way it will be noisy.

    I noticed that shifting wasn't as smooth with the 7900 as it was with the 7800... until I started taking things apart and noticed very shoddy cuts at the end of the outer casing which was rubbing against the inner gear wire. Shifting was greatly improved after a quick clip and file.

    A couple of pages about chainlines. It boggles the mind really. In an age when very fast gear changes are possible, people still feel hard done by when they have to spend 1/4 second flicking the left wrist to change between the large and small chainrings. It's not like you have to take your hands off the bars and reach down
     
  12. Eichers

    Eichers New Member

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    Hi danfoz, that's interesting but my friend isn't experiencing any rubbing with small/small. Although, I have an FSA 50/34 compact crankset on another bike and yes it does rub a minor amount. Mine is a cheaper Vero FSA version and his is the top of the range CF FSA version. At this stage, the minor amount of rubbing does not concern me either :)
     
  13. oldbobcat

    oldbobcat Well-Known Member

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    You are absolutely correct. Any inner ring with a 110 mm bcd should work fine.
     
  14. oldbobcat

    oldbobcat Well-Known Member

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    This study also concluded that chain drives are more efficient with larger cogs, another reason for not using small-small or other combinations in that vicinity, and that efficiency lost through cross-chaining is pretty insignificant.

    Cross-chain police, arrest me, I'm going to continue to use big-big, maybe with a slightly shorter chain.
     
  15. davereo

    davereo Well-Known Member

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    They mine as well arrest the entire chain gang seeing as I use big-big quite often. I would lay down heavy wages on most cyclist using this combo and never admitting it.

    Small-small is a different story. I usually only go small as a bail out option so I am shifting back on my large once the hill is behind me.
     
  16. alfeng

    alfeng Well-Known Member

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    Oh ye of little faith!?!

    I actually made what I considered to be a successful preliminary test using a Gran Sport rear derailleur a couple of months ago, so my comment was more of a tease (not that anyone other than myself probably cares since most people who are inclined toward using vintage components prefer restoration rather than extension of the utlization beyond the original friction mode) ...

    • I presume the Nuovo Record & Valentino rear derailleurs will respond the same as the Gran Sport rear derailleur ... so, some of the delay is due to deciding which of the three derailleurs to use ...

    Using a vintage Campagnolo rear derailleur is both for aesthetic reasons & to a much lesser extent the novelty (... because I apparently can!).

    Unfortunately, indexing a vintage Campagnolo rear derailleur may have an even lower priority than proving that Shimano STI shifting would benefit from using a Rapid Rise rear derailleur ...

    • Here's the XT rear derailleur that I modified with a Road derailleur pulley cage for the fore mentioned project (but, depending on the cassette, I may eventually use a "stock" Rapid Rise rear derailleur which has an SGS cage):

    [​IMG]

    ... Maybe I'll finally get around to assembling BOTH project bikes this Winter!

    BTW. It only takes 5-to-30 minutes for most bench "tests" ... committing to a moderately serious road test is another issue ...

    And, I can't stress it enough, unless a Shimano user is using a Di2 drivetrain, they (meaning, even YOU!) would benefit from installing a set of Campagnolo shifters on an/(your) otherwise DA-equipped bike UNLESS the rider is a Flatlander.
     
  17. alfeng

    alfeng Well-Known Member

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    Glad to hear you may use a more appropriate length chain for your bike's rear derailleur's cage in lieu of changing derailleurs OR simply changing the cage (it can be done!) to eliminate some of the chain-and-rear-derailleur issues you've mentioned ...

    BTW. What is it with you guys in Boulder?

    • Chisolm apparently spent 12+ years being clueless as to how to set up a front derailleur (i.e., thinking it was difficult)
    • and, you prefer using the wrong length rear derailleur cage and thereby open-the-door for unnecessary on-the-road problems
     
  18. danfoz

    danfoz Well-Known Member

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    @KLabs,
    my chain stay length was likely the culprit - typical tight racing geometry. Your friend is a lucky man, I've ridden a few different compact geared bikes all with the same result. Ultimately, my biggest gripe wasn't the rub, it was the big "throw" between chainrings. These days I've found my preference, especially in races is with a non-compact 50/39. The tight ratio allows me to make a quick two gear up-shift on the rear, and a quick downshift on the front. A combination that allows for a change of front chain rings that essentially feels like a single downshift on the same chain ring. Great in a make it or break it scenario as the road steepens running at redline while slowly running out of gears in the big ring. But not so important for smelling the roses.
     
  19. alfeng

    alfeng Well-Known Member

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    FWIW. Cross-chaining may be prevalent & it may not be particularly inefficient (I disagree on this point, but ...), but it is a bad habit which you both should try to break because it makes you/whomever look like more of a newbie than either of you are ...

    Regardless, cross-chaining introduces unnecessary wear on the SIDES of the cogs & upper pulley wheel wheel --

    • wear is friction ... friction is wear ... unnecessary friction is unnecessary wear ... you may not notice if from ride to ride, but ...
    • worse, friction is lost watts no matter how small an amount.
     
  20. oldultrarunner

    oldultrarunner New Member

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    I ended up taking the bike on Monday to a new bike shop that does only service; the owners are ProTour mechanics and know their stuff. The problem is in the short chain stay which by design has an angled Ti which is affecting chainline. Since the rub only occurs when I'm in the 34 tooth chainring and 11 or 12 cog in the back, I just use the big chain ring for that gearing. But I did learn that the DA 7900 groupset is not nearly as good as what the 7800 was in their opinion (and mine as well, as it's terribly noisy and not as precise in shifting). My 7700 DA triple on my old Trek 5500 is a much better drivetrain than the 7900 at over $2K.
     
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