Hi,

I am fortunate enough to be in a evening metal working class and hope to start working on bicycles soon. During the first term, we did forge work and oxy-acetylene work. It was great to start with the fundamentals.

One of my projects was a fun bike parking rack welded from scraps of mild steel pipe using oxy-acetylene.



Next term we will start doing stick welding (SMAW) and MIG welding (GMAW). I hope to work on a cargo bike by chopping and re-welding a few old frames that I bought off Craigslist. In preparation, I have read the Atomic Zombie's book and was surprised that the author doesn't mention much about watching out for what metals the old donor frames are made from.

It seems that many old donor bikes are made of chromoly and other non "mild steel" materials. So, my question is "how much should I worry about trying to weld thin chromoly with a MIG welder?"

My metal working instructor is an artist and does a great job with teaching, but doesn't emphasize structural welding.

I will be trying the techniques recommended by Brad Graham in his book, but am also interested to know if other folks have recommended reading or other suggestions about welding together old frames without a TIG welder.

Thanks for any information.

It's very hard to MIG weld bicycle frame tubing -- it's just too thin, and you don't have enough heat control. Even if you could make a successful weld, MIG is messy with lots of splatter that you'd have to tediously grind/file off. It's just the wrong process for bike frames, kind of like trying to give yourself a haircut with a lawn mower.

Since you've developed some proficiency with gas welding, why not try fillet brazing?

Yeah, stick with what you are practiced in -gas welding. Its how we built WW2 fighter jets afterall. Small torch for thin steel. Not easy, but doable. Try to fillet baraze as mentioned, or get some lugs and braze something together...
Better yet, stay late after class and get someone to show you the tig. If you can gas weld, you'll love the tig. Then you're right in bike frame territory. Start saving your pennies, you'll want to get one!!

~Steve

Lots of bikes are mig welded, it may even be the industrially prefered process. Lots of sheet metal is MIG welded. Just for starters you want to separate out the flux core MIG welding process that like stick leaves a lot of nasty glaze on everything and MIG using a gas shield. Flux core really doesn't belong here. That said if you can find some other way of doing it do. Welding is an art and guys can do a lot of different stuff with a lot of different stuff. Bikes have been welded as long as they have been around, and TIG has only been around for a small part of the time.

Brad is mostly working with mild steel, and that steel is at least about 2x the thickness of chromo in most frames. He is also a very tallented welder. With some formal training at which he excelled. He welds just about everything in his project books with a 3/32" electrode and a DC arc welder. Just so people can know that the projects can be done with basic gear. I would love to see him weld a "real" bike frame with that gear. My money would be on him, though the beads would not be what people expect in custom work today. Using arc is also tedious because every strike for a tack or a bad start will require that much of the whole joint is cleaned out all over again. Not fun. I know some of his techniques, and they are othodox which is another way of saying few people in the bike biz know anything about them.

yeah, I guess it depends on what he's realistically expecting out of his first mig welded thinwall chromoly frame.

doconabike, what do you want to make? what for?

Good question, and thanks for all the above suggestions.

My main goal is to learn about bike frame building and have fun learning. My secondary goal is to make a decent cargo bike. I realize that I may burn through some tubing and I may have a monsterously heavy bike once I add enough tubing to support my poor beginner welding skills. Another goal is to continue practicing welding and expand my skills. I also hope to learn some brazing this term. I realize there are many goals there, but I am hoping that they will all converge.

If you want to learn bike making skills, then largely MIG is out in left field. You can probably MIG a cargo bike together, try some Chromo tube and see where it gets you. I would imagine most cargo bikes are made out of thinwall dom tubing, about twice the weight of chromo, but not the type that are interesting over here.

MIG welding is easy, GOOD MIG welding is very tough. I had to certify welding pipeline back in the late 60's, underwater 140' deep (dry) with MIG and it was one SOB to do with Xray quality.

On the other hand I've found welding thin metal such as exhaust systems on a car is a snap with a good MIG machine. I would think that getting a decent weld on a bike frame wouldn't t be too difficult but the thing to beware of is even penetration. A MIG weld can look great but really suck for strength if it ain't good.

That is the big issue along with cold starts which one doesn't get with TIG or gas welding. As you know, both stick and mig introduce filler as a condition of sustaining an arc. That can mean the filler and puddle aren't ready for prime time when the weld boots up. Even assuming all the settings are perfect. With welding small tubes in awkward spaces, there is usually a fair amount of starting and stopping. A person doing this might consider identifying the high load areas, and ensuring they don't terminate and restart in that exact space. Like when running the underside of the headtube/downtube, run a beard from 4:30 to 7:30, not starting at 3:00 and stopping at 6:00, and then starting at 6:00 and stopping at 9:00.

Another problem with MIG is that there is a range of sophistication in the users and machines. One person's idea of a good weld, or what a given set is capable of, is variable. That goes for TIG also, though with TIG it probably affects the aesthetics moreso than the structure, in good hands. Gas is about the only system where the simplest torches have the correct process and all the relevant qualities. But then with gas it doesn't look that great usually.

On the positive side, looking at what survives for eons in the bike weld biz, it doesn't seem that weld quality is as important as the NAHBs lot might suggest. In fact some of the welding looks overworked on the really nice bikes, and may be benefiting from the same breathing room the really disgusting welds are benefiting from. Keeping in mind there are no standards or testing procedures in general use.

I've built a few recumbents by chopping up donor bikes and have used both MIG and brazing. My mig is a cheap Sears Craftsman but does have adjustable speed and temp settings. I always use shield gas. It is extremely easy to burn through the thin tubing used on some bikes. Still, it's also very possible to make a strong and attractive weld. I do typically have to do some grinding and file work to make them pretty. I usually do some file work on my brazed joints, as well, although to a lesser extent.

I would say if you're making custom bikes to sell using thin-wall chromoly, then learn to braze. If you're using cheap donor bikes to create something just for yourself, mig does just fine.

Here's a bike I built for my wife using a Fuji and a French Jeunet frame, plus a few odd pieces of tubing. They weren't labeled, but I'm guessing the Gran Tour SE was chromo while the Jeunet was 1020 or something else. Some parts are mig-welded and some are brazed. That fancy lugged headtube is from the Jeunet