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  1. #1
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    Shimano M980 Double clearance issues.

    I have a customer that im building a frame for who is planning on using for his build. He is currently using them on a bike he is riding. So using something else is out.

    The crank in question is the 26-38 ring combo(same as the triple, but sans big ring). I was planning on using the Deda 29er stays. Chainstay length is 440mm. Hoping to fit a 2.2ish tire.

    I drew everything out based on the published Shimano spec, and I am not happy with the lack of clearance associated with anything on the drive side. To clear the rings it looks like I need to bring the stay way in on the BB and if I keep everything symmetrical the stays start running into each other, plus tire clearance kinda sucks.

    Has any one run into this? If so how did you deal with it? As Im kinda hoping im missing something and Ill realize it tomorrow.

    Thanks.

  2. #2
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    Shimano M980 Double clearance issues.

    With 440 CS length, a triple crank should fit no problem with regular chainstays (the older Deda's or TT).
    I ran into a similar issue with the new Deda stays where the big bend made them hit the seat tube (34.9) when mitered. I could've made it work but didn't seem right so I opted for the regular true temper stays with less bend. I crimped the heck out of them but this frame has CS length of 420. I was able to put them inboard from the bb faces 7mm and get a 2.4 tire with XX1 and the 32t ring. You may need to go in a bit more to clear his crankset tho.

  3. #3
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    At 440mm, you should have zero problems with a variety of chainstays. The (29er) Dedas may actually have *too much* bend in your case. Otherwise I'm not sure what to say - I regularly build bikes with 42 or 42.5cm chainstays that would fit that crank, with ~70mm of clearance for the tire at the knobs. So keep fiddling, it's plenty doable. Are you dimpling or manipulating the stays at all?

    As far as I remember (I rarely build mountain frames for anything but single ring now) the chainring positioning is actually not the same for the Shimano double cranks - it's still ~50mm chainline but that's measured to between the 2 rings (like road double chainline) - ie the smaller ring runs between where the granny/middle would be on a triple, while the larger ring runs between the middle/outer. So you may have more room than you think.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  4. #4
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    Quote Originally Posted by Walt View Post
    At 440mm, you should have zero problems with a variety of chainstays. The (29er) Dedas may actually have *too much* bend in your case. Otherwise I'm not sure what to say - I regularly build bikes with 42 or 42.5cm chainstays that would fit that crank, with ~70mm of clearance for the tire at the knobs. So keep fiddling, it's plenty doable. Are you dimpling or manipulating the stays at all?

    As far as I remember (I rarely build mountain frames for anything but single ring now) the chainring positioning is actually not the same for the Shimano double cranks - it's still ~50mm chainline but that's measured to between the 2 rings (like road double chainline) - ie the smaller ring runs between where the granny/middle would be on a triple, while the larger ring runs between the middle/outer. So you may have more room than you think.

    -Walt
    I agree it should be no problem at all, which is why I asked. I too dont build many mountain frames that use multiple chainrings (cant wait until front derailleurs finally go extinct).

    The 980 crank is a triple crank just without a ring in the outer position, Shimano calls out a 46.8 chainline. So it would be more like a triple crank with a 38t middle ring. The 985 is the dedicated double crank, which I agree would not be causing any issue.

    I think your right, the 29er stays might have too much bend, I do have an old deda stay ill play around with. And I was doing a little massaging of the stay.

    Ill play around a bit more, just hopeing someone had run into this before.

    Thanks

  5. #5
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    Ah, so I was wrong about the chainline. That will definitely make things tighter. You will probably want a shallower bend chainstay. The Deda 29er stays have a TON of bend in both directions, which really isn't ideal in most cases for multiple rings (at least IMO). Let us know how it ends up working out! And tell your customer to get with the times and ditch the front mech!

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  6. #6
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    Quote Originally Posted by Walt View Post
    Ah, so I was wrong about the chainline. That will definitely make things tighter. You will probably want a shallower bend chainstay. The Deda 29er stays have a TON of bend in both directions, which really isn't ideal in most cases for multiple rings (at least IMO). Let us know how it ends up working out! And tell your customer to get with the times and ditch the front mech!

    -Walt
    Its really kind of a stupid set up, not sure whey the didnt/couldnt offer those rings on the dedicated double crank.

    Ill head back to the drawing with the old deda stay tomorrow and see what I can come up with and relay my findings.

    From my playing around i agree the 29er stays dont seem to be ideal for multi ring setups.

    Think its going to be a tough sale to ditch the front mech for this cat unfortunately.

    Thanks for your help.

  7. #7
    pvd
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    I don't use the published numbers. I belive that they are making engineering assumptions for the builder. Not good.
    Always measure actual crank ends and rings for clearance. Model in 3D if possible. That will make life much better for you.

  8. #8
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    That is a good point. I have a couple of "model" cranksets (ie too trashed to use) here that I use to double check, and I have a notebook full of crank and chainring offsets I've scribbled down over the years as well. I don't usually look at the Shimano spec sheets but if they're building in some kind of assumed clearance, that's annoying.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  9. #9
    pvd
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    I just go to a bike shop and measure off a bike when I need too. Simple enough.

  10. #10
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    That is some really good food for thought. Your right any assumptions on that has the potential for lots of headaches.

    Im still building my mach up parts stock, Id much prefer to measure it with my own hands.

    Learn something new on every frame.

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