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  1. #1
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    DIY Disc Mount For Retro Kona

    A while ago I posted a thread about my 1995 Kona Explosif. I have had the bike for a few years and tried a few different builds and finally settled for a "Retro-Mod" build. The one issue that bugs me with such a build is the frames inability to mount disc brakes. This meant that I had a V-Brake at the back and a disc at the front. Not exactly elegant. Some might call it a "Mullet."

    I looked at getting a mount welded on but decided against it as the cost would be rather high in comparison to the frames value. Also, I might want to restore the bike to a period correct build in the future. I looked at bolt on adapters but they are few and far between and the ones that are available dont look very good. They look flimsy or are suited to chunkier aluminium frames. Eventually I decided to try make my own. As Jeremy Clarkson always says "How hard can be?" It also helps that I work in engineering and have access to some pretty cool design tools.

    Step one was getting a 3D model of my frame. I stripped off a few components and "scanned" the frame using a very high resolution GOM 3d metrology scanner. This gave me a very accurate 3D model to design the disc adapter around.

    DIY Disc Mount For Retro Kona-frame-scan.jpg

    Next I used Inventor CAD software to design a bracket that is simple to manufacture, relatively light yet strong. Its very similar to an idea Specialized had to retrofit discs to some of their old bikes called a "Sharkfin" Its not as elegant looking but I wanted it to be simple and cheap to manufacture.

    DIY Disc Mount For Retro Kona-disc.jpg

    I did a bit of FEA on the bracket to check its strength. To be honest I have no idea what sort of forces a typical disc brake generates but can tell you that if bike designers used my assumed forces bikes would have a lot more material around the drop out.

    DIY Disc Mount For Retro Kona-fea-crop.jpg

    Finally I had it manufactured and fitted it. I did modify it a little by drilling a hole to cable tie the brake hose. Otherwise it fitted perfectly.

    DIY Disc Mount For Retro Kona-bike-2-.jpgDIY Disc Mount For Retro Kona-pc026035.jpgDIY Disc Mount For Retro Kona-pc026036.jpgDIY Disc Mount For Retro Kona-pc026038.jpg

    Only issue is I discovered the caliper was leaking mineral oil all over the disc and pads so I haven't had a chance to test it. Its at the LBS getting sorted and hope to take it for a proper test this weekend.

  2. #2
    artistic...
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    Good job. What´s the weight of that contraption?
    WTB: Bomber Z2 1 1/8 steerer, in good to excellent shape OR bomber rebuild kit.

  3. #3
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    Quote Originally Posted by colker1 View Post
    Good job. What´s the weight of that contraption?
    Weight is around 200g.

    Edit: Just checked in the CAD software. Weight for the arm and bracket to the boss is 130g, so not a lot of additional weight.
    Last edited by Stuartfleming; 1 Week Ago at 11:06 PM.

  4. #4
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    Nice mullet (the thing hanging off the back).

  5. #5
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    Quote Originally Posted by Stuartfleming View Post
    Weight for the arm and bracket to the boss is 130g, so not a lot of additional weight.
    You could recoup some of that by removing the redundant canti bolt on the drive side, as well as the dog collar cable guide on the seat post.

    Good on you for not getting a disc mount brazed on.

    The elegance might not be in the part itself, but the elegance is in the fact that you kept the frame original and still managed to update it.

    Grumps

  6. #6
    Sneaker man
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    Since you're CAD'ing it, any way to do a stress anaylysis of brake forces through 1 seatstay vs 2? I like the idea, but then part of me gets worried that you're doubling the force on the single stay. Maybe an additional brace down to the chainstay.
    All the gear and no idea.

  7. #7
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    Quote Originally Posted by mik_git View Post
    Since you're CAD'ing it, any way to do a stress anaylysis of brake forces through 1 seatstay vs 2? I like the idea, but then part of me gets worried that you're doubling the force on the single stay. Maybe an additional brace down to the chainstay.
    With the distance from the hub to the brake mount, the force is reduced considerably by the fact that it's acting as a lever. Wouldn't it reduce the force to something similar to a rim brake since it's the same distance from the hub? If OP's worried he could always add a brake arch to equalize it across both seatstays.

  8. #8
    artistic...
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    Quote Originally Posted by noapathy View Post
    With the distance from the hub to the brake mount, the force is reduced considerably by the fact that it's acting as a lever. Wouldn't it reduce the force to something similar to a rim brake since it's the same distance from the hub? If OP's worried he could always add a brake arch to equalize it across both seatstays.
    Good call
    WTB: Bomber Z2 1 1/8 steerer, in good to excellent shape OR bomber rebuild kit.

  9. #9
    Ride Fast Take Chances :)
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    Nice work. Look more than strong enough for what ever riding your going to do. The long lever will put very little load on the brake boss and tying the rear stay from axle to boss will add even more strength.
    The only load I would be worried about would be a twisting motion from the rear offset. You will already be braking so the small amount of binding shouldn't be noticeable.
    Making shit harder than it needs to be isn't awesome, it's just...harder.

  10. #10
    Anytime. Anywhere.
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    I had something similar on my 98 Explosif, but I can't remember the brand. I don't remember the deets, but it contributed to throwing my rear triangle out of alignment. I got it straightened and went back to a V on the rear. It's still my very cool townie bike. I commend your ingenuity, and wish you luck.
    I got some bad ideas in my head.

  11. #11
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    Quote Originally Posted by Travis Bickle View Post
    I had something similar on my 98 Explosif, but I can't remember the brand.
    Brake Therapy?

    They were the ones like a tie rod end, where the length was adjustable.

    Grumps

  12. #12
    DFA
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    3D modelling? Nope.
    FEA? Nope.
    Does it work? Yup.


  13. #13
    DFA
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    Quote Originally Posted by Uncle Grumpy View Post
    Brake Therapy?

    They were the ones like a tie rod end, where the length was adjustable.

    Grumps
    Brian is on the site. Use to post in the DH forum a bit. We had an interesting discussion on full floating brake theory and design via PM a few years ago.

  14. #14
    Sneaker man
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    Quote Originally Posted by noapathy View Post
    With the distance from the hub to the brake mount, the force is reduced considerably by the fact that it's acting as a lever. Wouldn't it reduce the force to something similar to a rim brake since it's the same distance from the hub? If OP's worried he could always add a brake arch to equalize it across both seatstays.
    yeah probably...but it's still putting that force through 1 side as opposed to 2... Not so worried about the OP, more if I put soemthing like this on my ti frame

    I guess i just had some issues in the past, not exactly this... my Yeti arc when I put discs on it would bend the seat stay and cause terribe vibrations. No issue with v-brakes, no issue with the exact same brakes on another frame. So i'm just a bit wary these days.
    All the gear and no idea.

  15. #15
    Sneaker man
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    double post
    All the gear and no idea.

  16. #16
    Phobia of petting zoos.
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    Quote Originally Posted by DFA View Post
    I'm a sucker for bikes with those big mofo swing arms, like that Iron Horse, the San Andreas and the Foes LTS/Weasel.

    Grumps

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