Carbon Tube Alum Lug Questions- Mtbr.com
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  1. #1
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    Carbon Tube Alum Lug Questions

    Hi and Happy New Year everyone!

    I'm starting a long process of building my first frame. Currently reading through as much as I can find here on the forum. (Lots of good stuff posted by Walt. Thanks Walt!) Though, I don't see much about building with carbon tubes and alum lugs. I know the downsides about building a frame like this but my mind is made up. Can some more experienced frame builders point me in the right direction about building a frame like this? Preferably some documented mountain bike builds. Recommended adhesives, fits, designs. Any general techniques previously used with pros and cons. Pretty much any advice would be appreciated. When I do searches I generally only find regular brazed lug or full carbon builds.

    The Bike Plan:
    I would like to build a 29+ full suspension bike but most likely it will become a normal 29er hardtail to keep things a little less complicated for the first build. Geometry will not be too aggressive, think XC bike. It will also be a smaller frame size for my wife. I will machine the lugs myself and will be making tooling/fixtures as needed. Going with a carbon/alum lug frame because my wife and I think they look cool.

    Info about me:
    I'm a manufacturing engineering and a competent machinist. Never built a bike frame or worked with carbon before. I'm also not a good welder. I am good with CAD software, Fusion 360, and I can fix just about anything mechanical. All this means is I've got the general skills but just lack the specific knowledge for frame building.

  2. #2
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    The essence of what you want to do is no different to doing a lug frame. Instead of using a filler in the form of silver or brass, you use an epoxy. Jig and a good design. If using Carbon tubes, these will be straight off the shelf tubes? Makes Chainstays difficult.

    My 2 cents worth. I am watching with interest as I have a steel/carbon jointing that I am considering at the moment but all I need to know is what type of Bonding agent to use. Everything else is as per doing a conventional steel build.

    Don't over think this, correct 'glue' and standard methods of measure, cut, mitre then join together in the jig.

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

  3. #3
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    Thanks for the reply Eric. At this point I plan to use off the shelf carbon tubes. I think I can just cut them to length like steel tubes. The chainstays will be a pain especially if I go for a 29+ frame. However, I've got a crazy idea of making my own BB and integrating the chain stays to it. Doing some eyeballing with my Jones Plus shows it may work out but I haven't modeled it yet. I've seen similar solutions on gearbox full suspension bikes such as the new Resistance Insolent. It may be a heavy solution but I don't really care at this point.

    Let me know what glue you end up using on your project and how it works out. It will be several months before I start gluing things together. I'm still in research and design phase.

    As a side note. Ive been reading Bamboo/Carbon build writeups for reference. Its similar joinery but of course bamboo and alum most likely require different glues.

  4. #4
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    I'm going to give the only easy answer first and then move on to other parts.

    What is probably the most popular adhesive is 3M DP420, which is going for a good price here . You can just squeeze it out and mix by hand, but getting a dispenser gun is very convenient and pretty cheap if you get an off-brand like this. Just make sure it fits whatever size epoxy you get. Finally these mixing nozzles work incredibly well. You just squeeze and the epoxy comes out ready mixed. Unfortunately, they're for a single use only as the epoxy will cure in them and clog them up. At around a buck a piece, this seems incredibly wasteful, but considering how much DP420 costs and how these will allow precise placement, it's not so wasteful after all. Certainly much less bad than CO2 cartridges. (Wow, the English in there was probably more bad)

    On to other stuff...

    I'm aware of no online tutorial/blog/record of making bikes with aluminum lugs. Like you already found, there's highly unsophisticated bamboo stuff all over the place. I say unsophisticated not because of the bamboo itself, but because the techniques use to join the tubes are rather crude. The best thing I've seen on a documented DIY carbon build is the legendary Damon Rinard page which is saved here. Whether this will help you at all with your aluminum lugs is questionable, but it's a fun read if you're into that kinda thing. Incidentally, Damon uses West Systems epoxy, IIRC. Probably this stuff would work for you, too. But he's got a completely different application.

    As for designing lugs, you'll need to have much greater overlap than with steel lugs because the strength of the epoxy is much less than brass or even silver. So I guess you could measure/estimate the area of a steel lug and then look up the strength of silver and compare to epoxy and up-size your joins accordingly. Or you could look at some old frames and just base it on that. If you've run into Robot Bike Co (carbon frames with Ti lugs), I'd caution you that their lugs are inside/outside so can be much shorter

    Carbon Tube Alum Lug Questions-3d-printing-robot-bike-co.jpg

    (Unsolicited side track -- these highly complex lugs actually take advantage of 3D printing in a way that most 3D printed parts don't. Thus, are pretty cool, if ridiculously extravagant, IMO -- now back to your regularly scheduled program)

    Tubes:commercial roll-wrapped carbon tubes are controlled for ID and not OD, so you may consider making internal lugs rather than external lugs. Maybe get some tube samples first and see if they meet your tolerances for bond clearance (hey, something else to consider!)

    I think you're on the right track with the chainstays. If you make an aluminum yoke, you may be able to get away with straight tubing. Same goes for the seatstays.

    This is a cool sounding project. Keep at it and listen to the collective advice of this forum to keep it as simple as you can for your first build. Sounds like you realize this by keeping it a hardtail. Going with plus isn't out of the question, but it's certainly not making things easier on you. In any case, get through it and learn stuff. You're going to screw up more than one thing. That's okay. Learn from it and build #2!

  5. #5
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    Wow! Thanks for all the links! And I will for sure read through Damon Rinard's build. His carbon bike looks awesome.

    I've got another question. Where is a good place to buy carbon tubes? Walt posted a link to Composite Resources but are there any other good suppliers? I've found some through google searches but are of unknown quality.

  6. #6
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    Carbon tubing specifically for bikes:

    rock west composites

    Note, I'm pretty sure this stuff was designed with road bikes in mind. So, you'll have to decide if you want to make a mountain bike out of it. In general, the round stuff they make in house and the crazy shapes come from Dedaccai.

    This will be your other source of bicycle specific carbon stuff:

    thecno

    They're in Italy. I've never ordered from them before and don't know how easy they are to deal with nor how long it takes for stuff to arrive.

  7. #7
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    If you can anodize the aluminum before hand, you'll get a better-than-good bond, and prevent cathodic corrosion for a long time. Forgive me if you already knew this.
    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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    I had heard of anodizing helping bond strength but thanks for the reminder.
    At some point I should probably do some tests with gluing scrap bits together. At least before I glue together the real frame. Maybe I'll compare some anodized vs regular aluminum parts too just for fun.

    Drew: Where are you based at? If you're near Southern California I wouldn't mind testing a few of your carbon bars.

    As a side note:
    I plan on Type II anodizing all my machined aluminum parts. Maybe even a slightly thicker hard anodizing (Type III) for slightly better wear resistance. We do it to our machine tools at work and it holds up to some serious abuse. It's a bit more expensive so maybe I will just hard anodize the dropouts, yoke and bottom bracket to save a bit of money.

  9. #9
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    Drew, as I'm guess you're aware, some/many(?) builders sand off anodization, asserting a better bond without it. Do you understand the reasoning for this? Is it maybe macroscopic scratches while sanding with a not-so-fine grit?

    I've also heard the reasoning that anodization (or fiberglass, etc) isn't needed for insulation because an even bond line should take care of that. What do you think of that?

    Since this is the internet, and possibly worse, a biking site, I'll be clear that neither of those is intended to be a loaded question.

  10. #10
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    I've been curious about this because I want to do something similar in the future (though more in the alu-alu bonding context) and read up a bunch. A few things I took note of:

    - Galvanic corrosion with carbon/alu (deal with using beads or veil).
    - Bond line geometry (width preferable to length because of deformation, similar to bolts).
    - Pretreatment: roughening vs CAA vs PAA (SAA is not recommended, sounds like it is possible to get away with just roughening but less reliable).
    - Failure mode is usually gradual weakening of the bond to ~zero over a long time period. Caused by hydration of bonded oxide surfaces forming a weak layer among other things.

    I don't really know anything about it, but there are tons of papers out there on this stuff. If you are OK to spend a bit then just get 'Handbook of Aluminum Bonding Technology and Data' by J. D. Minford.

  11. #11
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    When I first heard about gluing anodized aluminum the reasoning was the complete removal of the natural oxidation layer that typically inhibits adhesion. Though... I can think of two problems with this: 1) Anodizing, to the best of my knowledge, is a type of oxidation. 2) An anodized surface is fairly smooth and a rough surface is usually preferable for adhesion. (Brazing, gluing etc.)

    Essentially I've heard it both ways and haven't seen any concrete evidence either way. Hopefully someone with some first hand knowledge chimes in. Regardless, I'll do some of my own testing before I glue up my frame.

  12. #12
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    from Bonding of Aluminium by SAPA. Data is pretty variable between publications though. Not sure how much it matters for a bike.

  13. #13
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    Thanks for the book reference cerebroside.
    I did some googling and found a book preview containing most of the pages on google books. Here's the link: https://books.google.com/books?id=zt...atment&f=false

    I guess I know what I'm reading these next few days.

  14. #14
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    Aluminum indeed forms Al2O3 aka alumina in air. This reaction is crazy fast to initiate, though I'm not sure how long it takes to finish growing. And it self limits to a very shallow depth. Anodizing forms the same stuff, but much deeper. Given how fast alumina forms, I don't quite understand how sanding it could affect bonding chemically. However, as mentioned above, I could see the physical/mechanical properties of the surface changing. If you find anything about this in your reading, please report back.

  15. #15
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    I think the data pages on bond strengths aren't in the preview IIRC. From other stuff PAA seems to be the best overall, and is what they use for aircraft, but might be more effort than it's worth for this sort of thing. Seems to vary a lot by adhesive.

    I still have no idea what I'd do for pretreatment if I started my project. Probably just roughen it up and cross my fingers? Post up if you find out anything interesting.

  16. #16
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    Quote Originally Posted by Feldybikes View Post
    Aluminum indeed forms Al2O3 aka alumina in air. This reaction is crazy fast to initiate, though I'm not sure how long it takes to finish growing. And it self limits to a very shallow depth. Anodizing forms the same stuff, but much deeper. Given how fast alumina forms, I don't quite understand how sanding it could affect bonding chemically. However, as mentioned above, I could see the physical/mechanical properties of the surface changing. If you find anything about this in your reading, please report back.
    Off the top of my head it was explained as partly surface roughening, partly surface pore size of the oxide layer which is controlled by anodization, partly removing weaker hydroxide layers. Didn't read the whole book though.

  17. #17
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    Can you save me some time and describe what PAA is and looks like?

    I'll post back if I find anything conclusive. Though, it may take a while, this type of reading is fairly dull and usually puts me to sleep. hahaha

    Slightly off topic: Be careful what abrasive you use for preparing your tubes. For example, don't use Scotchbrite or similar products. Scotchbrite is aluminum oxide and tends to mechanically imbed itself into the metal you are preparing. Which in turn causes voids and other problems when brazing. I speak from first hand experience on this one. We had huge problems with this at my last job. Most people probably know this already but figured I'd post it anyway.

  18. #18
    would rather be ruined
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    https://www.techstreet.com/standards...uct_id=1744145

    if your talking about phosphoric acid anodizing thats also covered under BAC5555 but mainly its down to hairs and pores in a similar way to carbon forging

    HOWEVER THERE ARE OTHER METHODS

    The Pi Joint is not new but has been made easier for robot to make due to 3d printing We used this type of construction in composites (and also in F1 for bonding titanium flexures into the aero wishbones long before bikes) We also surface convert aluninium titanium (why its white when we glue it together not titanium coloured) and steel with again phosphoric acid on steel giving some of the highest yield strengths

    Also the anodizing method can vary Boric sulphuric acid also is another (wont leave corrosive residue or potential) we have used as well as some who still prefer chromic film even though its the weaker substrate

    there used to be an adhesive sadly no longer made which far outperformed dp420 and even the huntsman formulations we use do so now but its horses for courses you will get by with virtually any epoxy if your using good surface prep

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