11-28 cassette, chain length and b-tension screw adjustments

Discussion in 'Road Cycling' started by jdewberr, Apr 3, 2012.

  1. jdewberr

    jdewberr New Member

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    Hi All,

    Please steer me to the proper forum if I am in the wrong place.

    I recently installed an 11-28 cassette on my road bike. In doing so I'm learning that my b-tension screw is probably wrong and I might need to install a new chain. I made the chain a bit short to keep the top pulley from touching the 28-tooth cog when in 39 & 28. In doing so I pretty much don't go into

    53x28 (10th)
    53x24 (9th)
    53x21 (8th)

    I'm reading Sheldon Brown's stuff on setting this up, but I am wondering if my mix of cassette and derailleur (cage length) won't work.

    I'd greatly appreciate any war stories from those out there who do their own wrenching.

    Thanks!!!

    Joe
     
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  2. kdelong

    kdelong Well-Known Member

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    A bike shop can tell you if your RD and cassette are compatible. Either get the model number off the RD or better yet, take it with you to your LBS so that they can identify it. OR you can give us the make and model number of the RD and I am sure that one of our members could tell you the maximum tooth cog that the RD is designed to be used on. Shimano used to stamp it on the cage but I don't think that they do that anymore. Can't say for sure for Sram and Campy RDs.

    Here are a couple of DIY links from Park Tools that can help you if you find that your RD has enough capacity for a 28 tooth cog. As for the chain, if you have the section of chain that you removed, it is a pretty simple procedure to re-attach as much as you need with a Power Link or equivalent. If you threw it away (don't ever do that again) some bike shops carry bulk chain and may be able to provide several links rather than a whole new chain.

    http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

    http://www.parktool.com/blog/repair-help/chain-length-sizing
     
  3. oldbobcat

    oldbobcat Well-Known Member

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    I would promise nothing outside of the derailleur manufacturer's data sheet, that a particular cassette size will work. If Shimano says an Ultegra 6700 derailleur will handle a maximum of 27 teeth, and you insist on 28, I'll do my best to make it work, but no promises. That said, I've heard of guys pushing a rear derailleur by two or three teeth, but it's one of those things you won't know until you try it. So much depends on the length and shape of the derailleur hanger, and to a smaller extent the chain length.

    If I can offer one unsolicited tip, though, it's to make sure the chain is long enough to use big-big, even if you never intend to. The time will come when you will use it, unthinkingly, and lock the whole system up with consequences best left to the imagination.

    Now there's a business opportunity--casting long derailleur hangers for guys who want to use MTB cassettes with road derailleurs. The shifting would lose a little in the translation, but, hey, it would be cheaper than a new Deore derailleur.
     
  4. swampy1970

    swampy1970 Well-Known Member

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  5. jpr95

    jpr95 Active Member

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    I have an Ultegra 6700 rear (medium cage) with an 11-28 cassette. Smooth as silk shifting. It definitely has to be set up properly and dialed in, but once you do, it's great.
     
  6. alfeng

    alfeng Well-Known Member

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    +1.

    BTW. 'I' prefer to adjust the B-screw when the derailleur is OFF the bike ... it makes the process a little more tedious BUT it is easier (IMO) & you don't subsequently grind the tip of the screw into the derailleur hanger.
     
  7. swampy1970

    swampy1970 Well-Known Member

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    Normally, I just release the rear mech to frame bolt by a couple of turns and pull the unit backwards - it moves the screw off the frame.
     
  8. oldbobcat

    oldbobcat Well-Known Member

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    Oops. Must have been thinking about 6600. Thanks for bringing it to my attention politely.

    I adjust too many b-screws in a week to keep removing and re-installing derailleurs, but I do swing the cage so I'm not grinding the end of the screw into the hanger. This works for adjusting limit screws, too.
     
  9. An old Guy

    An old Guy Member

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    I think people make too much out of "proper" adjustment.

    I run anything from a 11-23 to a 16-30 on my bikes with no adjustments. Same chain. Never touched the "b" screw.

    I don't know the cage lengths. I expect they are whatever is most common. One is a Shimano. One a Campy.
     
  10. jpr95

    jpr95 Active Member

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    11-23 is a 12-tooth difference, and 16-30 is a 14-tooth difference, so it wouldn't be too hard to split the difference with the b-screw adjustment and never have to touch it. Neither should be a problem for a short cage, which is probably what you have. Throw a 11-28 or 11-32 on there, and I guarantee you'd have problems both in the parallelogram (b-screw adjustment) of your derailleur and in the cage length.
     
  11. An old Guy

    An old Guy Member

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    The "b" screw sets the distance between the upper derailleur pulley and the largest cog on the cassette. The proper difference to be concerned with is between the 23 and the 30.

    ---

    I can and have used the following cassettes without any derailleur adjustments or chain length changes:

    11-23, 11-25, 11-27, 11-30
    12-25, 12-27, 12-30
    13-25, 13-27, 13-30
    14-25, 14-27, 14-30
    15-27, 15-30
    16-30

    As I said "people make too much out of 'proper' adjustment."
     
  12. jpr95

    jpr95 Active Member

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    The b-screw rotates the derailleur in relation to the sprockets, effectively changing the angle at which the parallelogram of the derailleur moves the chain across the gears. The first part in setting it is so that it doesn't interfere with the largest, inner sprocket, so yes, that's part of it, but far from the whole story.

    I suppose you could also run 20" wheels and not adjust your brakes... /img/vbsmilies/smilies/rolleyes.gif
     
  13. AlanG

    AlanG Member

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    Well I'm no expert mechanic, but assuming that when using the 30, the pulley is at the recommended distance... You don't see a difference in shifting crispness when that distance increases when the 23 tooth cassette is installed? If that is the case, then why is the b screw needed? They could just give a lot of clearance.

    A friend of mine bought a used bike from an amateur racer and the b screw was set wrong so the pulley contacted the 25 tooth cog and made noise. I don't think this could be so good as far as wear is concerned. So it did need to be adjusted in that scenario..
     
  14. An old Guy

    An old Guy Member

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    Chrispness in shifting: On modern cassettes each cog has a small set of teeth that are formed to assist in shifting. The rear wheel turns 1 revolution between times when those teeth are aligned properly to assist in shifting - about 6 feet. Shifting is never chrisp. (I could go on about how the torque on the pedals varies during a pedal stroke and how the torque is seldom optimal for the shifting when those teeth are in the proper position but you should get the idea.)

    Yes, if the derailleur pully hits the cog, one needs to adjust the "b" screw.
     
  15. An old Guy

    An old Guy Member

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    Perhaps you could tell us the whole story. According to Sheldon Brown's pages the "b" screw sets the clearance between the cog and derailleur, and also balances the tension in the 2 derailleur springs.

    I have doubts about giving spring tension balance as an intended goal. It seems to be that the spring tensions are changed, but the change has no real effect. Changing chain length will also affect the spring tension balance, but no one suggests changing the chain length to balance the spring tension.

    I think the angle change is simply due to the clearance changing and not a sign of an important performance issue.
     
  16. alfeng

    alfeng Well-Known Member

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    FWIW & IMO. Well, for the umpteenth time ...

    I have found that shifting is ALWAYS "crisp" with Campagnolo Ergo Shifters + Shimano's 9-speed Cassettes & Shimano rear derailleurs AND/OR any chainring (modern or vintage) & Shimano front derailleur (I prefer Shimano's 6500/6503 front derailleurs, BTW) ...
     
  17. jpr95

    jpr95 Active Member

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    I think the angle of that parallelogram is incredibly important. Think about it--the farther away from the cassette that the derailleur pulleys get, the more flex there will be in the chain between those pulleys and the cassette. Get it far enough away, and you won't shift, though I imagine it would have to be pretty far for that to be the case. Keep them close together, and you'll get nice, crisp shifts. Further away, and the shifts get mushy.

    I'm sure I've probably read Sheldon Brown's page on derailleurs, but I don't recall anything about "balancing spring tension". AFAIK, rear derailleurs have springs such that the default position (no cable tension) is to go to the smallest/outermost cog. That's why brifters have either a button or a smaller lever to upshift (to a smaller cog), because all you're doing is a controlled release of cable tension. The large lever is for shifting to a larger cog (downshifting), which is working against the springs by pulling on the shift cable.
     
  18. An old Guy

    An old Guy Member

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    "Mushy" is a word without much substance.

    I did go out and look at my derailleur and cassettes both the 11-23 and 16-30. Both function as they should with the same "b" screw setting. It appears the longest segment of chain that is free between a cog and the pulley is on the order of 2 links (1"). The shortest segment is on the order of 1 link (.5").

    And the biggest gap is at the big cog - 23 on the 11-23.

    The gaps on the small cogs 11 and 16 as well as the large 30 are all about 1 link (.5").

    In a blind test using an 11-23 I don't think you could tell the difference between "b" screw settings - all the way out and all the way in.
     
  19. alienator

    alienator Well-Known Member

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    The B-Screw is important and to say otherwise is to demonstrate ignorance of its importance and function. Without it there is no way to insure the proper gap between the upper pulley and the largest cog. It's that simple. In fact, if I didn't adjust the B-screw when I mounted an 11-27 cassette on my Campy hub, my Record RD's upper pulley would have kept hitting the chain along the 27T cog and would have ruined either the upper pulley, the chain, or the 27T cog.....or maybe all three. As for "balancing spring tension", you only need to balance spring force with the cable tension necessary to get proper up and down shifting of the RD without or with minimal noise. If the RD isn't moving, the spring force and the cable tension are balanced. It's that whole "if inertia is constant, forces are in equilibrium" thing, i.e. Newton's first law.
     
  20. swampy1970

    swampy1970 Well-Known Member

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    It's a farkIng screw - and the least important of the three back there. Install rear mech, set limit screws, set cable adjuster for correct indexing and take a quick looksie at the gap between the upper pulley and cassette in all gears. See which gear puts the pulley closest to the cassette and spend a few seconds to adjust then go ride...
     
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