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  1. #1
    mtbr member
    Reputation: iwantalitebike's Avatar
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    Jun 2009

    Making custom tubes for bike project

    Well, with my winter time her in northern ill I plan to build a carbon and bamboo frames. I plan to start with the carbon. I was wondering if anyone had some info on make custom top tube, down tube, ect. I'm not sure when layering, if the number if wraps can be done in I process or should I do a single layer at a time. I do plan to vacuum bag these tubes. Any info is greatly appreciated. Thx
    I ride a bike, therefore I am!

  2. #2
    will rant for food
    Join Date
    Jan 2010
    I've made a handful of bamboo/composite frames. I have spent all my free time of 2013 on developing my tools for doing that cleaner and more efficiently - haven't made a frame since literally a year ago.

    Your description is confusing. You want to make carbon tubes? K. If that's the case, what parts are meant to be bamboo?

    If you're anything like me, you want a carbon seat tube because finding a perfect bamboo pole to ream the innards out of for seat post compatibility is a f*cking CHORE.

    Number of layers of carbon that can be done in one process depends on the epoxy you're using, and whether or not you're using a heat source for curing the epoxy, or using ambient room temperature.

    For example, West System epoxies are room temp cure, and the pot life is so small that you can only do a few layers per, and you have to move fast and things can go wrong quickly.

    Heads up, if you don't know what pot life is... don't take this the wrong way but you need to go back to Google for now.

    For that reason I use epoxies that cure at around 200 F with the use of an oven or high watt infra red lamp.

    Easiest way I've found of making carbon tubes is using a polypropylene mandrel, wrapping that with carbon/epoxy, wrapping *that* with Dunstone tape (secure the ends with Kapton tape for good anchor so the shrink tape can do its job), and sticking the affair in an oven. When the cured assembly is removed from heat and allowed to cool, the super high thermal expansion rate of the PP mandrel means it will almost slip out on its own after it shrinks.

    The caveat here is that polypro doesn't stick to epoxy. You can't use just anything with a high expansion rate.

    Word of warning there, though: some PP rod stock arrives slightly bent along its length. If that happens, it'll need to be turned down on a lathe.

    Try making something other than a tube first. Suffering a mandrel that is irrevocably stuck inside a carbon tube... f*cking sucks.

    Here's a tube I made with the above method. Search for "polypropylene rod stock".

    Whatever you do, don't use a metal mandrel. You're going to have a very bad time. The big boys can do it because they use super high polished and chromed mandrels, and they remove cured tubes with hydraulic tools.

    And for pete's sake since you are starting out please don't make anything hyper light weight. You want to keep your teeth.

    Take all of above with grain of salt, I am not an engineer.
    Disclaimer: I run Regular Cycles (as of 2016). As a profiteer of the bicycle industry, I am not to be taken very seriously.

  3. #3
    mtbr member
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    Jun 2009
    Thanks for the reply. I should have elaborated more. I plan to make a carbon frame and also a bamboo/carbon frame.

    The info is very helpful. With the higher temp curing, do you need to cure at a particular temp i.e. 200*f or somewhere close? Have you ever cured in a oven?

    Another question i have, when making a tube, and say its determined to be 7 wraps of 2x2 3K cloth. Can I wrap at one time, or does it need to be 1-2 wraps at a time?

    Do you have pros/cons of vacuum bagging or do you wrap the carbon with tape?

    I plan to make all the tubes individually then epoxy them together, then wrap the joints.
    I ride a bike, therefore I am!

  4. #4
    will rant for food
    Join Date
    Jan 2010
    If you think you can roll a mandrel onto cloth to the tune of 7 circumferential wraps from one cut of cloth, yeah go for it. You need a fairly large flat surface to do this. Other shapes are more difficult in terms of many plies at once. But you need to consolidate (lay the ply as flat as possible) as you go if you're wrapping in one swoop, and each wrinkle will translate to the next layer.

    I prefer to wrap with tape as described with the expanding mandrel.

    I should have noted that the mandrel diameter at room temp is not the same as the cured tube inner diameter. For example I had a mandrel that was 31.75mm diameter at room temp climb to 33.25mm diameter at 240 F (I believe those were my numbers, if not exact, very close), and the latter was approximately the finished tube's ID.

    I don't like vacuum bagging because the compaction is kinda weak, just atmospheric pressure. If you had an autoclave it would be a different story. Also vac bag materials are just kinda irritating to work with in my opinion. There are some cool alternate bag materials instead of using tacky tape, like the C-channel method with lay-flat vac tubing:

    Quick Lock Seals

    The main cons of the shrink tape / expanding mandrel method:

    - Squishy dimensions depending on temperatures as noted above.
    - Shallow epoxy pockets will form under the spiral of compression tape, which needs some finishing work to look nice.

    You will need to vacuum bag for the frame joints, though. So you'll need to learn to vac bag one way or the other. I just really don't like vac bags and I've spent the last year going to elaborate lengths to avoid using them.

    Tubes are pretty much the only composite part shape where compaction is relatively simple.

    You're aware of areal weight, yeah? Saying something is a 7 layer layup is somewhat meaningless because a ply of one kind of carbon can be three times thicker than another kind. I can't provide you any recommendation here, I haven't made an all-carbon frame yet.

    Be safe.
    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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