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  1. #1
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    Social Tandem/Velo Frames: 4130 vs Carbon Fiber & Al; MIG vs TIG Welding

    Years ago, I built bikes (but not frames) from old parts. It was huge fun, but then life got in the way (school, job, etc.) 30 years later, I am starting to build frames/bikes again for fun. Now though, my goal is to build side by side tandems and velomobiles that my wife and kids will actually ride in with me.

    So here is my first newbie/old man (the times have changed) questions:

    - I can do the math and calculate bottom tube sizes, but does anyone have any "real world" experience or rules of thumb for sizing 4130 bottom tubes for LWB recumbent bikes? Stiffness will obviously not be an issue for a side by side tandem going 5-10 mph around the neighborhood or down a flat bike path; I just don't want to add any more weight than absolutely required, but don't want to have a "bent bent" every time I hit a small bump in the road...

    - It appears the entire industry is moving toward Carbon Fiber and TIG Welded Aluminum frames. However, being a newbie welder and planning on my first few prototypes to most likely be "flawed experiments", I am thinking it would make more sense to start out practicing with 4130 and a cheap arc welder (just to get my design right and welding skills up to snuff). Does that make sense?

    I was originally going to go with lugged construction and brazing, but I can't get the flexibility of design I want using pre-manufactured lugs. Then I saw a cheap Flux Core Wire Welder on line that all the reviews say is great for all sorts of light duty welding. However, old threads on the welding forums said using MIG/Flux Core Wire on 4130 Bicycle Tubing was a really bad idea. So, now I am looking at a cheap arc welder (from Harbor Freight). Here are the issues and related questions:
    - The way the local community college does classes, I need to finish the brazing course before the MIG course and the MIG course before the TIG Course, meaning it will be months before I can start TIG welding with any sense of knowing what I am actually doing). So, would using the Flux Core Wire Welder really be that bad just for low speed, low impact, prototype building?
    - The way the forums describe TIG welding (i.e. incredibly difficult to do well, get cancer from breathing the gas, etc.), would I be better off just using bamboo or something for prototypes and then laying up carbon fiber once I get the design correct?

    Thanks in advance for any help/advice,
    Joe P.

  2. #2
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    I say stick with 4130. You can modify it and fix it easily. The technique that you would use is called fillet brazing (or brass welding by the motorcycle guys). The industry is going with aluminum for no particular reason. There are lots of aluminum bikes that are heavier than steel. For your purposes, oxy acetylene brazing may be the best bet. You can also gas weld, mig or tig. Anything else is probably a bad idea. If you go onto welding forums, they will probably tell you that the practices that have been used in framebuilding for the last many decades will never work, so they are of limited use unless you tig.

    Carbon is out for your applications. Too expensive and there is no reason to go that way.

  3. #3
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    Unterhausen,

    Thanks. I appreciate the quick feedback. When you say MIG should be okay, would that include using a basic Flux Core Wire Welder or a simple 120V Arc Welder like the ones they sell at Harbor Freight:

    http://www.harborfreight.com/interes...der-98870.html
    or
    http://www.harborfreight.com/interes...der-98871.html

    I am not trying to be cheap per se, but after 20+ years of marriage, I think it would be easier to ask for more expensive tools if I first was able to prove I can do this reasonably well with less expensive ones....

    Thanks again,
    Joe P.

  4. #4
    Randomhead
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    I know nothing about mig, but I do know that home-built recumbents are often built using mig. I doubt that an inexpensive machine is what you want, but I just don't know.

  5. #5
    Senior Member PaPa's Avatar
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    Quote Originally Posted by joepaiva View Post
    I can do the math and calculate bottom tube sizes, but does anyone have any "real world" experience or rules of thumb for sizing 4130 bottom tubes for LWB recumbent bikes?
    I assume this is to be a 'clean sheet' design. If so, we are missing a few crucial numbers to accurately suggest tubing sizes. Are the frame(s) mono or multi-tube? What is the anticipated laden weight? Also need the wheel base. And side-view print would also be helpful.

    Since most available 4130 tubes above 1" (25.4mm) are .049" wall minimum, MIG would probably be the cheapest and easiest method. When I say 'MIG', I mean a wire welder with/shielding gas - the gas used is usually a C75 mix (argon & CO2). Be advised, that many cheapy flux-core wire welders cannot be converted, so if you decide to buy a 'flux core' welder, just make sure it can be easily converted into a 'MIG', which requires little more than a bolt-on kit and a bottle. For wire, ER70S-2 or ER70S-6 will suffice (Note: wire for shielding gas welders 'MIG', is different than wire for flux-core welders, so make sure you get the correct wire for the process used (flux or MIG).

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