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Old 05-30-09, 07:43 PM   #1
redfrequency
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Is MIG welding safe for chopped bikes?

I'm starting to get into chopping bike frames and making some wacky frankensteined monstrosities. I'm specifically working on a tall bike right now, which is essentially a frame stuck on top another frame (welded at the seatpost and forks on both frames).

My question is, is MIG welding safe enough for this type of joining? Looks are definitely not important!

Thanks!
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Old 05-31-09, 03:25 AM   #2
just4tehhalibut
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Well the Greenspeed recumbent trikes were MIG-welded and possibly a few bike makers out there still MIG so really the machine is okay, it just depends on the user. And frame components, And frame design...

Not sure what you mean about attaching via the seatpost, you mean seat tube, I'd assume. Anyway, saw this on Youtube (via the Bike Rumour blog, thanks) about some cyclists in Africa, they make a tallbike
http://www.youtube.com/watch?v=7Npub...layer_embedded


Only glimpses but interesting slashup.
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Old 05-31-09, 09:58 AM   #3
sstorkel
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Originally Posted by redfrequency View Post
My question is, is MIG welding safe enough for this type of joining? Looks are definitely not important!
Are you a good MIG welder? I find that the thin tubing used for bicycles is pretty challenging to MIG weld properly. TIG welding gives much more control and is generally more successful... for me.
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Old 05-31-09, 11:54 AM   #4
NoReg
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Look at some of the existing MIG threads here. lots of info. Your project is less challenging. You can add to the strength of such frames with gussets.
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Old 05-31-09, 08:34 PM   #5
redfrequency
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Thanks so much for the replies! Yes, I do mean the seat tube, not seat post. For every ones information- I'm welding the bottom bracket shell of the top frame to the seat tube of the bottom frame. I got a 1'' piece of pipe and stuck it down the fork tube through the head tube of the bottom frame and welded it at the bottom and at the threads. I then recut the threads to get the head set cups, spacers and nuts on. I cut off each arm of the forks on the top frame, leaving just the stump. I'm now cutting the pipe the the correct length and welding the stump to the pipe. Using 2x4's with c-clamps on each bottom bracket shell to keep frames straight and centered. The welds so far seem pretty strong and solid. didn't know if they were prone to crack anymore than any other welds or tubing in general. Definitely will be putting gusstes in front of and behind the bottom bracket shell on th top frame and seat tube/seat stays on bottom frame. Thanks again for the replies!
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Old 05-31-09, 10:15 PM   #6
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Assuming one can produce proper welds with adequate penetration, sufficient accuracy, and no undercutting, etc... The usual stuff, then the main problem with MIG is cold starts. The wire that initially feeds and is consumed is colder than further down the joint, and while the general weld may be fine the starts may not be. There can be similar problems when incorporating spot welds. Since tubing in general and small tubing in particular requires quite a few stages to weld, these can be specific problems.

Also, there is a lot of different MIG. A gas shielded MIG process is way better than a cheapo box feeding fluxed wire.
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