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Thread: Rear Cassett

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    Rear Cassett

    Is anyone running a 12x23. I currently am running a 11x32 on our c'dale road tandem . I am unhappy with the gear spacing . I am going to a 12x26 on my road single and thinking about putting the 12x23 that is on it now on our tandem .
    I feel like we should still have deep enough gears because we have very rarely used the small ring on the front.
    We live in Kansas so there is not alot of hills , however we do have our wind.
    Gal. 2:20

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    hors category TandemGeek's Avatar
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    We have run 12-25's in the past and have some friends who run 12-23t and 12-25t; our current default is 12-27t, noting that North Georgia is not flat.

    For a 12-25t you need to shorten your chain will also need to run your B-screw all the way out on your XT rear derailleur. If you run a 12-23t or find that you still have a lot of gear chatter in the smallest cogs with a 12-25t you may want to switch over from the XT to an Ultegra or 105 RD which will have a shorter cage.

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    SDS
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    We run 11-21 or 11-23. If it is steep enough going up so that we will want the 28 X 23, then we will want the 53 X 11 going down. This is with what appears to be an XTR long cage rear derailleur. I don't know anything about MTB parts, so that is as far as I can go.

    I live in North Texas, where hills are scarce.

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    An 11-23 or 12-23 is a fine rear cassette for tandem use. Even if you encounter hills, most tandems are equipped with front triples so assuming you have a 32 tooth small ring you'll have a 37 inch gear. That's plenty until you hit more than a 10% grade for more than a couple of miles. I'm also assuming that you're not carrying a touring load. A long cage rear derailleur like an XTR or Dura Ace triple will handle that cogset but you might need to take a few links out of the chain. In flat to rolling country you'll be fine.

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    Cat 6 Steve Katzman's Avatar
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    For a 12-25t you need to shorten your chain...
    Why would you need to shorten your chain? The chain is at its loosest when it is on the smallest cog. So going from an 11 tooth smallest to a 12 would leave less slack. Shortening the chain would only preclude the possibility of replacing the 11-32t if they ever needed it for hilly terrain.

    I'd say you might want to shorten your chain to save weight and minimize the possibilty of chain suck. Not that chain suck would be any more prone to happen than it is with the current 11-34t cassette.

    I also do not follow your logic for the need to adjust the derailleur B-screw. While it may make shifting crisper for the lowest gears by bringing the jockey pulley closer, if left alone, the shifting shouldn't be any worse than it is now. Same goes for changing to a shorter derailleur. If they are keeping the triple crankset on the front, they will still want to get the long (triple) version of the Ultegra or 105 RD, which wouldn't have that much of an effect.

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    hors category TandemGeek's Avatar
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    Quote Originally Posted by Steve Katzman
    Why would you need to shorten your chain? I also do not follow your logic for the need to adjust the derailleur B-screw.
    I tend to be a bit anal about setting up the shifting and my suggestions tend to follow suite. So, you're absolutely right; you don't need to do any of these things unless you experience shifting problems or discover that you have chain slop issues. However, that said, I'll share my logic for "suggesting" that chain length and derailleur angle are worthy of attention when changing cassettes, particularly ones that have vastly different low-gear sprockets.

    Chain Length: As for why I (not necessarily you or anyone else) would change the length of my chain, it's because a chain that is long enough for a 32t rear sprocket will be 4.5" longer than it needs to be for 23t sprocket (32 - 23 = 9 x .5" chain pitch = 4.5"). In my book, 4.5" is a lot of excess chain. While it's better to have a chain that's a bit too long vs. a bit too short, the best shifting performance will come from a chain that is as short as possible without being too short. A chain that is the proper length vs. being too long will also tend to minimize chain suck and chain slapping on the stay if you happen to enounter rugged roads every now and again.

    Derailleur Angle: Rear derailleurs are designed to work with gearing that falls within certain parameters, i.e., a given capacity and maximum sprocket size. A long cage (SGS) XT derailleur is designed to work best with a 34t max sprocket (off-road gearing) & triple crank set-up whereas the Ultegra/DuraAce/105 long cage rear derailleurs (GS) are designed to work best with a 27t max sprocket (road gearing) & triple cranks. In addition to the length of the cage -- which helps to pick-up the extra chain required for the larger capacity of a triple + 34t sprocket -- the derailleur itself is a bit different; optimized for use with the max size sprocket. You can more easily exceed the capacity of a GS-like RD (short/medium/road triple) by using a rear sprocket that is too big vs. running a SGS-like RD (long/extra-long/off-road) by using a sprocket that is too small; however, shifting is not optimal with either.

    The angle adjustment screw, or B-screw as it's often called (Shimano's contribution to RD lingo), is used to position the jockey pulley and, as you would expect, screwing it in moves it away from the cassette and screwing it out moves it closer. You adjust the B-screw to set RD to be as close to the largest sprocket as possible. If the B-screw is too far out, the jockey pulley can end up dragging against the largest sprocket when the bike is in its lowest gears which can create shifting problems. Conversely, if the B-screw is too far in you reduce the angle between the jockey pulley and the sprockets: ideally, you want that angle to be as great as possible for crisp shifting, noting that the derailleur works by pulling the chain up and down the cassette by dragging on or off sprockets. Therefore, any time you change a rear cassette where the largest (low-gear) sprocket size is changed the B-screw "should" be adjusted and, as already noted, the chain "should" also be checked for proper sizing to ensure it doesn't end up being too short or too long.

    In short, the rear derailleur / cassette / chain are all part of an integrated system and changes to any one part in the "system" usually require an adjustment to one or both other parts get optimal performance. If you don't mind sub-optimal performance, then adjusting the chain length and/or B-screw adjustment is not necessarily "needed". For me, given the scant amounts of time we have for riding, I place a high value on problem-free performance and do what I can to keep it that way. Clearly, it's not essential for everyone...
    Last edited by TandemGeek; 03-18-05 at 08:27 PM.

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    LeMond Lives! Dusk's Avatar
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    Quote Originally Posted by Lonnie Seachris
    I feel like we should still have deep enough gears because we have very rarely used the small ring on the front.
    I was looking at my triple and what it would do if move to a double. I solved it by looking at the two gear tables.

    If you know what gears you use now, just put together a gear table of them and one of what the new gears would give you. Then you will know. What I say or use works for me and what do you care what works for me. Do the numbers and go with what you know!

    Why have strangers tell you what you already know better than them?

    Cheers,
    Dusk

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