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  1. #1
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    4130 rack building

    First let me say that I am a fair mig and arc welder but know nothing about frame building. Many years ago I also took a class in oxyacetlyene welding. Here is the question: I have decided to try my hand at some custom rack building for a touring bike I am building up. I am not doing this to save money I am doing this because I want something custom and that I made myself. I have some 4130 3/8"tubing and a way to bend it. Problem is a lbs I know of offers to build them and has for years and they brass braze them. Every internet search I do says that this is not the way to join this kind of tubing. It says tig welding with a complicated annealing process is the way to do this. Of course they are talking about severe aircraft applications. Can I braze or any ideas? (have not learned to tig yet)

  2. #2
    weirdo
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    Go for it! I`m a machinist with welding experience and recently started practicing brazing mostly for bicycle purposes. If you already know how to weld, it won`t take long to become at least "good enough" at brazing. Even more usefull than your actual welding experience is the experience that you probably have in fitting, squaring up, etc. I spent about four hours doing practice brazes over the course of a few weeks (had brazed a little before, but never tiny stuff) then brazed low rider bosses into my fork and they came out pretty good. Not quite sure if I understand what you`re asking about whether to braze or not- consider that frames are brazed all the time. TIG or bronze brazing will be perfectly acceptable, either one.

    EDIT: If you`re interrested, here`s a thread where the experts gave me an online crash course in brazing and walked me through the problems I ran into.
    http://forums.mtbr.com/showthread.ph...ight=tig+braze
    Last edited by rodar y rodar; 03-08-08 at 06:54 AM.

  3. #3
    Senior Member limeylew's Avatar
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    Quote Originally Posted by fixiehorse View Post
    First let me say that I am a fair mig and arc welder but know nothing about frame building. Many years ago I also took a class in oxyacetlyene welding. Here is the question: I have decided to try my hand at some custom rack building for a touring bike I am building up. I am not doing this to save money I am doing this because I want something custom and that I made myself. I have some 4130 3/8"tubing and a way to bend it. Problem is a lbs I know of offers to build them and has for years and they brass braze them. Every internet search I do says that this is not the way to join this kind of tubing. It says tig welding with a complicated annealing process is the way to do this. Of course they are talking about severe aircraft applications. Can I braze or any ideas? (have not learned to tig yet)
    Brazing has always worked well for me on 4130 tubing and I've never had a failure.

    Its probably best if you do a good job of mitering the joints.

    Good luck with your project.
    Lewis.
    A cyclist is a cyclist's worst enemy.
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  4. #4
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    The tubing used in making racks is anealed 4130, welding it will not cause it to harden significantly since there tubing is never quenched as part of any of these processess. Normally if people say anealing is needed they don't know what they are talking about.

    It's interesting how the various craft groups hold different prejudices. On the motorcycle groups where people were mostly from mild steel backgrounds, all sorts of misconceptions about 4130 existed. Normally aircraft guys are very knowlegeable assuming you are talking to the guys who really do it. Interestingly I read a text by one aircraft expert welder that stated categorically that 4130 should never be brazed, because it propagates cracks. I guess I might take that seriously if I was building an aircraft, but good or bad it works on bikes.

    Brazing does work on racks, and part of the reason probably has to do with the rather limited loads involved. I use silver for racks.

    You can weld racks also. It is hard to run a perfect bead around the tubing. Some racks are held with spots (yuck) and others are held with straight runs along flatened sections. Surly does that and it is a widely practiced approach in industry. Where the tubes won't shift in one direction due to a 3 dimensional structure, but they are heavily loaded in the other direction, in line with the plane. Triangular TV towers pre-cable, and some shoping carts do this. Check out some stuff and copy what you like.

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    Thanks for your replies. I realize that when I did my internt searsh I just used 4130 and brazing. This is what gave me numerous articles on how it should not be done. Once you mention bicyles and 4130 brazing is talked about all over. I will try to take a picture when I am finished. Thans again.

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    Oxy-Acetylene welding is an oft employed joining method in aircraft applications. NON copper plated filler is preferred...apparently, at welding heat, the copper gets into the grain structure and can lead to eventual cracking.

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    also for the most part the 4130 wire you find these days isn't copper coated while the mild is. The copper is also unheathy to breath in and it is vapourizing into the air at welding temps and somehow or other particles may end up being breathed in.

  8. #8
    Goathead Magnet aley's Avatar
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    I was once told by a former framebuilder for NHRA Top Fuel dragsters that 4130 with an ER70S-6 filler is approved for frames that will not be postweld heat treated - apparently it's hard to find a heat treat oven that will take an entire Top Fuel frame, and even if you did it would be likely to warp. :-)

    Another option for welding would be to use a 300-series stainless filler (E308 or E309). The austenitic grain structure give the filler metal good ductility, which prevents it from cracking at the weld root.

    Either of these is going to be more easily accomplished with a TIG welder, where you have very tight control over the heat input and amount of filler metal. Oxyacetylene welding has traditionally been used in aircraft construction, although I don't have enough experience with oxyacetylene to know how it compares with TIG for joint quality.

    If your technique is good and your joints fit nicely, brazing should also work fine for a rack.

  9. #9
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    This is my rack. Mostly comprised of scraps of .313x.035 4130 I scrounged up at work. I brazed it and it is sturdy enough to hold my wife (110 lbs or so), though I'm sure if I were to actually ride with her sitting on it, I wouldn't be able to steer at all... Go for it.


  10. #10
    ong
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    I've also been building racks out of 4130 lately (working up to making a frame). Using "brass" (low-fuming bronze) to fillet braze the tubes together works very well, and people have been doing it for a long time. You can also do "slip fits" to form closed loops, by using a plug of smaller tubing to join the open ends of the loop and filling the gap with silver or brass (capillary action sucks the filler metal into the joint).

    Here is a good introduction to the process -- I found it very helpful.

  11. #11
    Senior Member Nessism's Avatar
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    One caution to brazing anything is to make sure you get all the flux off afterward, including on the inside. The flux is acidic and will attack the joint so it must be removed. Soaking in water typically takes care of the job.
    Becareful buying/selling bike parts on-line. I learned the hard way. :(

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  12. #12
    weirdo
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    Quote Originally Posted by Nessism View Post
    One caution to brazing anything is to make sure you get all the flux off afterward, including on the inside. The flux is acidic and will attack the joint so it must be removed. Soaking in water typically takes care of the job.
    Does that go for brass/bronze flux too? I`d heard it wasn`t nearly as bad as flux for silver.

  13. #13
    Senior Member Nessism's Avatar
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    Quote Originally Posted by rodar y rodar View Post
    Does that go for brass/bronze flux too? I`d heard it wasn`t nearly as bad as flux for silver.
    Not sure about the relative scale between the two in terms of how acidic it is. I do know that brass flux is a LOT more difficult to remove - gets glassy and very hard. Hot water works much better than cold. Most framebuilders have some sort of soak tank with a heater they can drop the frame into and leave it for a few hours.
    Becareful buying/selling bike parts on-line. I learned the hard way. :(

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