Chainstay/Seat stay braces- Mtbr.com
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  1. #1
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    Chainstay/Seat stay braces

    I've broken 2 steel hardtail frames in the past 2 years by cracking the seat stays. Both had fairly obvious weaknesses in retrospect, but since seat stays aren't particularly stressed they took me by surprise.

    Framebuilders, do you have any seat stay guidelines? What are your feelings about braces? How about wall thickness? Does SS OD mean as little to you as it does me? I'd love to hear your opinions and anecdotes.


    My next personal frame is going to have 14mm .8mm wall seat stays, with a small bridge.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  2. #2
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    Where did you crack the seatstays? That is indeed an odd thing to have happen.

    I use either 16mm or 19mm x .8 or .7, depending on the size of the rider and the likelihood of lots of crashing/abuse. I don't think I can recall a seatstay ever having problems, though. So maybe I could geek out and go smaller/thinner. Not worth it to me to try, but guys like Rob English use like 3/8" tubing successfully.

    The seatstay bridge is pretty meaningless but I do them just because people freak out otherwise. I don't do chainstay bridges unless I need to mount a fender or a kickstand, waste of effort otherwise.

    -Walt

  3. #3
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    Quote Originally Posted by Walt View Post
    Where did you crack the seatstays? That is indeed an odd thing to have happen.

    I use either 16mm or 19mm x .8 or .7, depending on the size of the rider and the likelihood of lots of crashing/abuse. I don't think I can recall a seatstay ever having problems, though. So maybe I could geek out and go smaller/thinner. Not worth it to me to try, but guys like Rob English use like 3/8" tubing successfully.

    The seatstay bridge is pretty meaningless but I do them just because people freak out otherwise. I don't do chainstay bridges unless I need to mount a fender or a kickstand, waste of effort otherwise.

    -Walt
    First was an on-one 456 evo 2.
    Name:  1145950d1499727736t-456-evo-ii-456crack.jpg
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    The 2nd was on a frame i built, .6mm (whoops) 16mm (edit- 19mm) stays from nova, right at the edge of a fillet braze. No brace.


    Really, the failures just got me thinking about stays and braces- I'm a 1%'er when it comes to loads a frame is exposed to, unfortunately.
    Last edited by scottzg; 09-28-2017 at 09:04 AM.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  4. #4
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    Can we have a full picture of this frame?
    From what I see, a bridge is not needed as the SS's co-join for the upper section to the seat tube. A full picture might reveal a better understanding of what is going on here.

    Thanks
    Eric
    If I don't make an attempt, how will I know if it will work?

  5. #5
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    Quote Originally Posted by Eric Malcolm View Post
    Can we have a full picture of this frame?
    From what I see, a bridge is not needed as the SS's co-join for the upper section to the seat tube. A full picture might reveal a better understanding of what is going on here.

    Thanks
    Eric
    I no longer have the frame, but here's a picture of one off the internet. The upper part of the wishbone is 2 stays that have been welded together. It cracked between the tip of that weld and the brazeon.

    Chainstay/Seat stay braces-s1600_dsc4340.jpg
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  6. #6
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    I really like hearing about frame failures, in any circumstance. We simply don't know how stressed something is until it breaks. I wish it was something framebuilders talked about more. Thanks for posting up your tubing choices. I've used a lot of 16mm x .8 and .9 depending on the load. No failures, but they're all young frames.
    Myth Cycles handbuilt bike frames
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  7. #7
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    My only thought on the On-One is that it's *really* hard to make a strong TIG joint on something like that - and then you're putting a water bottle boss right across from the end of the crotch/weld area? Not the best way to design things, though I guess it's a neat look.

    Then again, these are mountain bikes and there is a certain *hit happens aspect to anything you do on a mountain bike. Stuff that is built well can still get broken, because mountain bikes get beat to hell.

    -Walt

  8. #8
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    Weld it back together, and then weld a bar on top of both tubes.

  9. #9
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    I've used a lot of 1/2" seatstays, either straight (.028" on a road bike, with a standard caliper brake bridge), or bent (.035", mainly to limit crimping/ripples on the inside of the bend) for a wishbone stay, brazed to a 3/4" stub.

    That on-one failure looks to have initiated at the end of the braze. Possibly the inside of the tube was grazed by the drill bit when they drilled the hole for the threaded fitting. Either way, that's two stress risers at the same place on the tube.

  10. #10
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    Thanks for the picture

    Looking at the structure of this design, the co-joined welded upper end is very strong and will not flex. There is no V-brake to introduce a spreading force into the area or bending moment from ST to Drop-out. The location of the brazed fittings is not wise as there is double heating involved and a thining of material right at that point. But I am more inclined to look at the metalurgy and wonder if a stress from weld zone to unwelded material has been created. Adding a bridge in the original design/build may have reduced risk but more likely a heat/steel issue that I would call a 'hard/soft' grain break, and invisible until an event puts a force through the frame to expose it.

    Eric
    If I don't make an attempt, how will I know if it will work?

  11. #11
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    Hey Walt, (Anyone?) I have a2007, 26", specialized MTB, that I'd like to convert to a 'gravel bike' type thing. The biggest tire the frame can accommodate with a 700c rear wheel is 32mm. I'd PREFER to use a bigger volume tire however, the seat stay brace is in the way. How much would removing the brace, or (filing it in half,) affect lateral stiffness of the bike? Thanks

  12. #12
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    Quote Originally Posted by eebowler View Post
    Hey Walt, (Anyone?) I have a2007, 26", specialized MTB, that I'd like to convert to a 'gravel bike' type thing. The biggest tire the frame can accommodate with a 700c rear wheel is 32mm. I'd PREFER to use a bigger volume tire however, the seat stay brace is in the way. How much would removing the brace, or (filing it in half,) affect lateral stiffness of the bike? Thanks
    Not perceptibly.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  13. #13
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    Unless it's a rim brake bike, removing the brace shouldn't matter. Go for it.

    -Walt

  14. #14
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    Thanks, It's a disc brake frame.

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