Strong Chainstays for a Fat- Mtbr.com
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  1. #1
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    Strong Chainstays for a Fat

    Hi !
    I'm working on a frame project for a steel, fillet-brazed, Fat that will be used in 1x11 configuration.
    The issue I have is that the customer is the kind of guy who breaks *a lot* of frames, usually the chainstays just behind the BB. He recently broke his Fat (a big-name production steel frame) after 1500 km. I replaced the chainstay and replaced it by a Fat chainstay from Ceeway (they look like Deda...), removed the CS bridge to avoid having a stress riser, but it broke again after 5500km... (hey ! that's already much better than 1500km !)
    Back to the custom frame I'm working on: do you have any idea where I could get stronger fat chainstays ? Or should I use a kind of yoke ? Or some sort of gusset ? Any ideas or examples are welcome !!!
    I plan to use stronger 3/4" seatstays to increase the rear-end stiffness, but I'm really worried by the CS !!!
    Thanks for your help !
    Francois
    Edelbikes / France

  2. #2
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    Do you have a sense for why it broke?

    Was it a short-cycle fatigue at the weld leg? ...fillet brazing and a tube with more flexibility would solve this.

    Is he a heavy coaster, and the downward force of his weight 'pushed' the BB down breaking the CS as a cantilever?

    Or is he strong on the pedals and torqued the CS to failure?

    ...Or does he bash logs with his chain ring?

  3. #3
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    If you're that worried about strength, I would just use some 3/4x.049 or something similar and bend/shape as needed yourself. If chainstay length/clearances are an issue, sure, do a yoke.

    Good luck!

    -Walt

  4. #4
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    Quote Originally Posted by tgwms View Post
    Do you have a sense for why it broke?

    Was it a short-cycle fatigue at the weld leg? ...fillet brazing and a tube with more flexibility would solve this.
    Yep, that's what I thought when I repaired his frame: try to leave as much flexibility as possible with no CS bridge. Looks like it wasn't enough... ;-)

    Quote Originally Posted by tgwms View Post
    Is he a heavy coaster, and the downward force of his weight 'pushed' the BB down breaking the CS as a cantilever?

    Or is he strong on the pedals and torqued the CS to failure?

    ...Or does he bash logs with his chain ring?
    The guy is a bit large/strong (approx. 230lbs and 1m90), and seems to generously hammer on the pedals... ;-)
    In both cases it's the inside of the drive side chainstay which started to break, first time just behind the bridge, and on my repair it broke just at the limit of the haz of my fillet, less than 20mm from the BB.

    Thanks for the suggestion Walt, I have some tooling to bend 3/4" tubing, so I might try something like this.

  5. #5
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    Brute-force reinforcement might simply push the failure mode somewhere else. Perhaps try distributing the stress instead?

    Could you adapt a lug-style 'flair' overlap to the chain stay bridge to spread the stress over a larger area of tube? ...similar to these brake bridges:
    Name:  olmo-ssn-brakebridge3.jpg
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    Name:  olmo-ssn-brakebridge2.jpg
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    A full bridge might be too rigid and abrupt . I'd be concerned about failure at the end of the bridge:
    Strong Chainstays for a Fat-26286362370_aea5bbe908_b.jpg

    Or here's a two-birds-one-stone approach: braze on a chain protector which acts as more of a tube reinforcement than a protector. Similar to carbon frames. Taper the ends to feather the stress on the tube.
    Strong Chainstays for a Fat-737814285_634.jpg

  6. #6
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    The Nova NOV_COCS_SBND_29W chainstays are pretty burly. They're a 25.4 oval instead of 24, So the bigger contact patch can't hurt. All the ones I've bought have a 1mm wall, not .9mm. Also, with the 31 degree bend, they're pretty appropriate for fat bikes.

    Nova calls them "NOVA MTB 29W S-BEND 25.4mm OVAL x 450mm" on their site, they also have a thinner wall one that is much less burly.

    Good luck! He sounds like a crusher.
    Myth Cycles handbuilt bike frames
    Durango, CO
    http://www.mythcycles.com

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