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I'm planning to make a diy carbon-alloy bike, Bastion Cycles style. In which I bond carbon tubes to 3d printed aluminum lugs. I live in China and can't find T700 or above tubes online for sale, there's only T300 ones. Would they be enough? I'm 50kgs and output low power. If I remembered correctly, the last gen Trek Domane (half internal routing version, and not the flagship one) is made of t300 carbon? Will 2mm thick tubes be enough?

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    Shouldn’t you just look at the strength and weight of the tubes? And maybe elasticity.
    – Michael
    Dec 12, 2022 at 12:01
  • No, because carbon fiber is a anisotropic material and it couldn't be defined with one strength value (which is what manufacturers give you), and the software I'm using doesn't have simulations for anisotropic materials.
    – Zyleyus
    Dec 12, 2022 at 13:26
  • But the tubes you can buy are symmetric and uniform in all directions, are they not? Surely there are specifications for their bending and torsion properties.
    – Michael
    Dec 12, 2022 at 13:28
  • @Michael Carbon fiber tubes are anisotropic not because the shape is asymmetric, but because the fiber sheets don't have equal properties in all directions, thus resulting in any final products being anisotropic.
    – Zyleyus
    Dec 13, 2022 at 2:37
  • Carbon tubes are normally designed to be loaded in the longitudinal direction only. Transverse strength is normally "good enough" for common handling loads.
    – Therac
    Dec 13, 2022 at 4:44

2 Answers 2

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It might be difficult to get an exact answer to this question on this forum. There may be some things we can say about carbon fiber bicycle construction.

enter image description here

The figure above is from this document by Toray Composites America, a manufacturer of carbon fiber (I believe their parent company is Japanese). Carbon fiber is typically offered in grades. T300 and T700 are described as standard modulus.

There are actually several ways to build carbon bikes. Most of us probably think of molded bikes. Those are built from many sheets of carbon fiber that are cut, laid up onto a mandrel (like a mould, but the mandrel is inside the structure), and then put into a mould and baked in an oven to cure the resin. Bikes using this construction method may often mix fiber types. When you hear a bike being described as "high-modulus carbon fiber", I believe that means that some of the fibers used are from grades like T800, which Toray labels as intermediate modulus. These are lighter but more brittle, if I recall correctly. You wouldn't make the whole structure out of T800. The actual high-modulus carbon fiber is, I believe, too expensive to be of practical use in bikes. Anyway, more premium bikes may have a higher proportion of intermediate-modulus fibers in the layup, plus they may use a more complex layup (i.e. more sheets that are cut in more complex shapes) that saves overall weight.

I'm not familiar with the issue of T300 vs T700. If a bike was described as being made of T300, it's possible that some of the layup is T300 and some is T700.

The OP is actually discussing buying carbon tubes and making a bike. This is similar to how many metal frames are constructed. From the diagram, we can see that T300 has significantly lower tensile strength than T700. I believe this means that a pure T300 tube would break at a significantly lower strain than a pure T700 tube. A side question is: when you say T300 tube, is that in fact pure T300 fiber, or are the tubes made of mixes of fiber?

Absent input from someone who has actually worked with these fibers, I would probably not want to build a bike from tubes that are pure T300 fiber, unless I was able to create a test scenario and a test rig to mimic the strain of realistic riding conditions.

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  • Actually I've learned a lot about carbon bikes in the past years, I know everything that you just said. But they are of little use because my budget only allows Bastion style (seems that you haven't heard of them, plz have a look). Their tubes are weaved by a machine, and as far as I can tell from limitted videos online, they do not have different thicknesses and layups here and there like those traditional carbon frames. I have a T800-labelled bike and the walls seem to be somewhat 1mm, So I guess thickening the wall and lowering the "carbon level" is viable.
    – Zyleyus
    Dec 12, 2022 at 14:59
  • I've heard of Bastion. From their description, it sounds like they may filament wind their own tubes. That's an alternative construction method which I admittedly didn't cover; it's currently more rare. Because you asked about buying tubes, it doesn't matter how they're constructed. You're still looking to get tubes of a certain material (or mix of fiber grades) and a certain spec (e.g. thickness, maybe the layup, other relevant parameters may exist). The bottom line stands: we don't know if T300 tubes suffice, there's reason to think they're weaker, so don't proceed unless you know different.
    – Weiwen Ng
    Dec 12, 2022 at 21:56
  • If a bike is described as T300, you can be sure T300 is the best grade they use. Don't expect underselling. As to whether cycling "high modulus" is actually intermediate modulus, good question... I've always assumed it to be HM CF, and thus useless for a non-athlete, but using IM CF would make way more sense, since a bike is strength critical and not just stiffness critical.
    – Therac
    Dec 13, 2022 at 4:47
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T300 tubes are 4 times strong as steel ones. Even the old, junk steel frames do not have 2mm tubes. To be safe, get tubes in higher diameter rather then thicker walls. Say, 30mm tube with 1mm wall thickness is better than 20mm tube with 1.2mm wall thickness.

Just check the orientation: these tubes have 3K carbon on sides and UD (unidirectional) in the middle. Make sure UD side faces up/down.

Since you are using aluminum joints, you have to wrap them with glass fiber first...

This is not scientific, but from my experience, it will be %100 safe.

One small detail: I've seen T800 DIY bike frames which cracked, because T800 too brittle. If you can get T700, but not T800.T300 is ok. Getting the wall thickness right is tricky though...

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    The term "carbon fiber is xx times stronger than xx metal" is kinda non sense for me from a engineering degree. If strong means tensile strength than accroding to the picture the other guy had put in his answer, 500 kpsi is only slighly over that of Aluminum, even less than Cromoly, not to mention the bike specific steels such as XCr which is aroung 1300. And those steel tubes are of less than a mil of thickness, which is hard to compare to a 2 mil carbon tube.
    – Zyleyus
    Dec 12, 2022 at 15:10
  • I guess you don't know Bastion Cycles either, they're great and you should have a look. They have cylinders extruding from the lugs, which means they apply epoxy on the inside of the tubes rather than let silver flow on the outside like a traditional steel lugged frame. So GF is not needed.
    – Zyleyus
    Dec 12, 2022 at 15:11
  • @Zyleyus, from the viewpoint of someone who broke a whole lot of metal in a previous job, I also find it nonsensical. Steel comes in a whole lot of grades, spanning an order of magnitude in breaking strength, and nearly as much for yield strength.
    – Mark
    Dec 13, 2022 at 1:17
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    @Barış Atasoy My numbers came from Columbus's catalogue, which should be the most common steels on a steel frame. Nonetheless increasing the diameter does inspire me.
    – Zyleyus
    Dec 13, 2022 at 2:35
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    These strengths are for the FIBER, not for the resulting composite. Unidirectional composites are typically up to 60% as strong as the fiber. For bi-directional, best-case you get 30% (30% warp, 30% weave, and 40% epoxy). So a T300 tube is at best a match for high-strength aluminum.
    – Therac
    Dec 13, 2022 at 4:30

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