Carbon Fiber Full Suspension Front Triangle- Mtbr.com
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  1. #1
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    Carbon Fiber Full Suspension Front Triangle

    Sup guys and gals, long time lurker coming out of the shadows.
    I'm a mechanical engineer and MTB rider/framebuilder since the late '90s.
    Finally signed up today to share a project I've been working on.

    I had come here several times for ideas, answers, and inspiration.
    So hopefully the "work" I've done can be helpful to someone.
    The project blog is at Carbon Fiber Mountain Bike Frame Construction
    A few photos from the project are posted below.

    I set out to build a carbon fiber composite front triangle to replace the aluminum front triangle in my full suspension Rocky Mountain Element. Basically, the construction was:
    1. Measure the existing frame to duplicate suspension pivots and component clearances.
    2. Design the replacement frame and fixtures in SolidWorks.
    3. 3D print clamshell plugs segmented to my printer size
    4. Vacuum bag wet carbon over the plugs
    5. Remove the clamshells and assemble into the alignment fixture
    6. Bond and reinforce the clamshell construction
    7. Wet layup [multiple] over the hollow clamshell construction using an elastic band compression.
    8. Finish (working on it now)


    The blog is more of a photo bank and stream of consciousness dump.
    Not a complete step by step instruction manual.
    Any input on clarity or readability is welcomed and appreciated.
    I need to make chronological reading more obvious (like episode numbers?).

    Anyways, I put a couple images below.
    Check out the project HERE, and lemme know what you think.


    Thanks and cheers!

    Carbon Fiber Full Suspension Front Triangle-frame-design.jpgCarbon Fiber Full Suspension Front Triangle-20171010_192831.jpgCarbon Fiber Full Suspension Front Triangle-20171105_143540.jpgCarbon Fiber Full Suspension Front Triangle-20180107_143756.jpgCarbon Fiber Full Suspension Front Triangle-20171227_183601.jpg

  2. #2
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    Have you tried a propane torch instead of a heat gun to remove the bubbles? I've done a lot of epoxy coats on wood, and apparently there is some chemical effect from CO2 that helps bring the bubbles to the surface and burst them. When I first read that, I thought it was BS, but I've done experiments both ways. One of those tiny butane torches or a small propane torch work way better than a heat gun or hair dryer.

  3. #3
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    Very interesting. I will try that. Thank you!
    ...any idea where the bubbles are coming from? Or how could I diagnose their origin?

  4. #4
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    When I make countertops out of wood, the slightest little sliver will hold some air that tries to escape.
    I think that roughness/holes also creates nucleation points for outgassing, but that's just a theory. Definitely smoother surfaces create fewer bubbles, but I've never had _no_ bubbles.
    One last thing that might help is a vacuum chamber and degassing the epoxy. Pretty sure that a little air does dissolve in the epoxy during mixing. I use a vacuum chamber to take bubbles out of smaller pieces and it definitely helps.

  5. #5
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    Thanks for posting this. This is a clever way to use 3d printing to construct a carbon fiber frame. I'm learning about carbon fiber construction so this is very informative. I've got a few questions if you don't mind:

    Is there a reason you decided to use plain weave rather than unidirectional?
    Does the plain weave count as 1 layer of fiber or 2?
    How many layers do you add in a session?
    Is there a reason you used elastic wrap rather than vacuum bag for the later steps?

    Also, between yesterday and today, a few of the posts on your blog seem to have gone missing. Best of luck on this project!

  6. #6
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    Hi Cluh,

    Uni was terribly hard to use with wet layup. I purchased a large quantity of it when starting the project because... well, bikes are made with uni, right? But the nature of uni made it incredibly challenging to cut/plan, wet out, and get it to sit flat when laying up on a bike frame.

    Also, and this is a big one, unidirectional does not "pass" resin through itself as readily as any weave. So if you wet-layup 5 layers and the top layer is a patch of uni, then all the excess resin under it has to squeeze up through it. Weave has holes and the resin passes through, but uni holds the fibers tight next to each other and the resin requires a lot more work to push through.

    Working with uni pre-preg obviously would eliminate many of these problems.

    Weave was far easier for me with wet layup.

    Twill loves to conform to curves, and this is why I bought twill instead of plain at first. But when cut into smaller pieces the twill weave will come apart easily. It seemed like it almost wanted to flick apart. I also found it hard to track "straight" when cutting. By the end of my roll, I was 20deg off perpendicular to 0-90 of the weave.

    Plain weave was easy to lay flat on the frame, easy to work with in small pieces, passed resin, and easy to cut straight.

    The best thing I tried for this frame was braided tube. It was expensive, but it was so very, very convenient to cut 8" of tube, rip it down the middle, and lay it flat. Perfect rectangular piece every time. It conformed to anything (better than twill) and it wouldn't come apart. And I could pull the orientation from about 30deg separation all the way to 90deg+ separation. Very versatile, but you pay for it.

    So I only used Uni for very specific reinforcements where I could get it to reliably sit flat and stay put.

    Does weave count as one or 2? One layer, but what is more important is the orientation of the fibers. So, a weave will apply a matrix of 3k bundles in the 2 directions. When tallying my fiber plys I counted the 2 orientations as separate to understand the final mass of fiber in any one orientation. But this made it tricky to calculate a final wall thickness because I multiplied the ply tally by .005", which is the thickness of the weave itself ...not exact. I am still working on a more accurate method. This is a tool I'm sure seasoned composite fabricators have perfected, but developing it myself might take a few iterations to get good data.

    How much per layup was driven by resin "pumps". I would only layup as much fiber as I could wet with 2-3 pumps. If the layup required more than that I was putting down too much. It took a while to get a feel for how much carbon 2-3 pumps would wet out, but you can't really calculate this when using a bunch of small pieces. I'd imagine it is different when you are wetting out large sheets; far more predictable.

    Elastic versus vacuum... go check out the blog for a more thorough understanding. I simply couldn't get vacuum bagging to work for the complex frame structure. It was uneven, unpredictable, caused the plys to shift or bunch-up, and was simply frustrating/laborious. Basically, the film can't contort to apply good compaction around each tube AND the radii transition between them. The film sticks to itself and doesn't naturally apply even circumferential force.

    Comparatively, the elastic was very easy to apply applied purely circumferential compression around all the tubes and transitions. Only on the flat sections did I need to apply supplemental compression with the pads and clamps. Your shape may be different.

    Keep in mind I tested layups extensively before designing the frame, so I was able to modify the design of my frame shape to match the compression method (reduce flat sections, eliminate negative curvatures).

    Missing posts? I hope not. I see 79 posts in the dashboard. Which ones disappeared? Try clicking on the MORE POSTS or ARCHIVE buttons on the blog.

  7. #7
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    Sounds like you were getting "bridging" with the vacuum bag, which you need to "pleat" in order to fight that effect. It's super f'n annoying, I'll give ya that, and is a common problem myself and many others have encountered at some point, s'why I try to avoid vac bags when possible. (Not always possible.)
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  8. #8
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    Thanks a bunch for your insight. I hadn't noticed the archive tab on your website. I've got plenty of reading to do. Thanks for documenting all of this. It should help steer me in the right direction.

  9. #9
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    Vacuum Bag Problems

    Hi Drew,

    You are right, I did experience bridging.
    I then used a pleated bag, but this didn't solve the problem.
    Bridging between film contact points or the inability for the 2D film to drape against the 3D surface was not the problem.
    Using vacuum on a bag constructed of film is not an optimal method to compress completely around a male core evenly. Here's why...

    Atmospheric pressure is only able to compress the layup perpendicular to the film surface. When you lay 2 pieces of film on either side of a male core, they form a seam where the film must transition away from the core surface. At that seam, the atmosphere is not able to apply force radially to the section... or perpendicular to the surface. See the drawing, the film can't compress "down" or "up" on the core.
    Carbon Fiber Full Suspension Front Triangle-vacuum-bag-round-section.jpg
    If the core were placed in a seamless bag, like inside a balloon, then yes it could compress evenly around the section. The seam is the problem.

    Elastic banding offers an advantage because it is able to apply perpendicular compression all the way around the layup and radially against the core surface.
    Carbon Fiber Full Suspension Front Triangle-elastic-band-round-section.jpg

    The caveat with the banding is that the compression comes from the tension, so it doesn't apply much pressure to a flat section and it won't apply any to a concave section.
    Carbon Fiber Full Suspension Front Triangle-elastic-band-problems.jpg

  10. #10
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    Gotta love a solid explanation. Kudos
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  11. #11
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    Quote Originally Posted by Drew Diller View Post
    Gotta love a solid explanation. Kudos
    \m/ thanks

  12. #12
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    !Another Clear Coating Fail!

    I tried again to clear coat the frame, and my epoxy did this:
    Name:  20180327_153028.jpg
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    I've not seen that particular effect before.
    I had washed the frame and let dry an hour before applying the resin.
    Was that not long enough?
    Or has my resin gone bad?
    Post HERE
    My carbon fiber full suspension frame build blog:
    https://carbonmtb.blogspot.com/

  13. #13
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    ...Hmm.

    ...

    Spitballing: Could it be static electricity? Do you need to rotate the frame, like on a gimbal or at least a rotisserie type device?

    I'm asking questions rather than giving answers because my finishing experience is likewise limited.
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  14. #14
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    Interesting spit ball Drew. That certainly is what it looks like in the photo. It was being held/grounded/handled no different than all the other times I put resin on the frame.

    Current hypotheses:
    • I washed the frame, but didn't sand AFTER the wash. Could I have left soap residue or ruined the physical surface finish by not sanding directly before the resin application?
    • I used a new Scotch-Brite pad. Perhaps it was covered with a release agent and that was deposited on the frame?
    • I hadn't covered/sealed my resin pump nozzles while they sat in open air for a month since the last layup. Did the resin in the pump nozzle absorb moisture from the air and ruin the resin? It did not come out cloudy or crystallized; it looked normal.


    Inspecting the frame this morning (24hrs+), the resin I 'wiped' on and let cure on the frame doesn't feel completely hard in some places. Some of it feels slightly soft. Sanding it produces little balls of rolled up dust... not dry clean dust. But this is only noticeable on parts of the frame, not the whole surface. And the cup of leftover epoxy hardened up completely. It feels normal.

    So, maybe there was something on the frame surface, and that something prevented the resin from wetting out properly and also prevented proper cure?
    My carbon fiber full suspension frame build blog:
    https://carbonmtb.blogspot.com/

  15. #15
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    Another hypothesis: I used a new sanding block, and it might be stearated.
    My carbon fiber full suspension frame build blog:
    https://carbonmtb.blogspot.com/

  16. #16
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    Soapy water . . . .

  17. #17
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    FINALLY a decent clear top coat

    After all those troubles, finally the top coat on my composite frame is smooth, UV resistant, and clear[ish].

    I did not figure out why the epoxy did not cure. But I scrubbed the frame with acetone until all the tacky was gone. Then scraped and sanded. Then more acetone. Then prep for paint.

    I used this 2-component urethane gloss clear in a rattle can:
    Name:  20180413_155900.jpg
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    And it worked extremely well:
    Name:  20180413_155906.jpg
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    Getting close to building this baby up.
    \m/ woot.
    Very excited.

    More pics HERE.
    My carbon fiber full suspension frame build blog:
    https://carbonmtb.blogspot.com/

  18. #18
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    Rolling Frame

    Assembled and rolling.
    Small details to button up before hitting a trail.
    Rolling
    Carbon Fiber Full Suspension Front Triangle-20180528_144228.jpg

    I also posted my CAD files in case anyone is interested:
    https://grabcad.com/tgwms-1/projects
    Carbon Fiber Full Suspension Front Triangle-frame.png
    My carbon fiber full suspension frame build blog:
    https://carbonmtb.blogspot.com/

  19. #19
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    Ace!
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

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