Framebuilders - How to tell the frame's material

I am looking to buy a used steel road bike, but I don't know how to tell the difference between Steel, Aluminum, and Chromoly. I heard from a friend that Chromoly welds are nearly invisible, Aluminum welds are thin, and Steel welds are fat. Is this true? Will a magnet stick to Chromoly? Thank you.

Hmmm... How can I answer your question, which is a quite reasonable one, without sounding like a kindergarten teacher?

All steels are alloys containing iron, carbon, and other elements like molybdenum, chromium, manganese, nickel, phosphorus, zirconium, vanadium, and others. The simplest steels are basic carbon alloys, and some very low end bikes are made of carbon steel tubing. Because carbon steel isn't particularly strong, the tubing is usually straight gauge rather than butted (butted means the tubing wall is thicker at the end than it is in the middle), and the tubing wall is usually fairly thick. So, carbon steel frames tend to be relatively heavy. A stronger steel alloy is chromium molybdenum (or, chromoly). Since chromoly is stronger than plain carbon steel, it can be drawn with thinner walls, particularly in the middle of the tube, resulting in weight savings. So, chromoly is steel; it's just stronger than carbon steel, so it can be made lighter. A magnet will stick to chromoly.

I think your friend, when he says steel welds are fat, is probably referring to a tube joining method called fillet brazing. It isn't welding, but uses brass filler to create a smooth relatively large radius joint between tubes. Brazing is done at a lower temperature than welding, and in a brazed joint the tubing itself doesn't approach a temperature which would melt it.

Welding, on the other hand, involves melting the tubes at the joint so the tubes essentially become one piece. Most welded bike frames today, whether steel, titanium, or aluminum, use TIG (Tungsten Inert Gas) welding. In TIG welding, a tungsten electrode heats the metal you are welding and gas (most commonly Argon) protects the weld puddle from airborne contaminants. TIG welding produces clean, precise welds on any metal.

This is an example of fillet brazed joints (these are by framebuilder Dave Kirk):

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Frame%20Tube%20Joining/DaveKirkFilletBrazedHeadTube.jpg

This seat tube cluster is a beautiful example of TIG welding (by framebuilder Carl Strong):

http://i32.photobucket.com/albums/d7/k4drd/Bicycles/Frame%20Tube%20Joining/CarlStrongTIG.jpg

Hope this helps.

That was the best answer I have ever received to a question. Thank you.

Now for another question: I have a small air cooled welder at my house that can (at the moment) only weld steel. I have practiced welding steel square tubing together, and made a nice big cube. Would I be able to use that welder to weld chromoly? I am wanting to attempt the double reduction (http://www.canosoarus.com/08LSRbicycle/LSR%20Bike02.htm) drivetrain on a single speed road bike. Would the welds from that welder hold up to the stress? I was going to get a bottom bracket from any bike I could find and weld it here:

http://i29.photobucket.com/albums/c292/Arice77/Giant%20Brass/here.jpg


And by bottom bracket, I mean just as pictured. Maybe less of the tubes though:

http://i29.photobucket.com/albums/c292/Arice77/Giant%20Brass/here-1.gif

I am going to switch the front chainring to the left side of the bike, and have a freewheel on the left side of the crank (which is in the newly welded bottom bracket). On the right side of the crank will be a chainring the size of the front one, and it will be chained to the freewheel that is attached to the back wheel. What do you guys think of my plan?

I don't know anything about your welder or your welding skills, so I can't comment on your plan other than to say welding thin walled bicycle tubing without cooking the tubing is a learned skill that takes lots of practice.

As far as your gearing idea is concerned, IMHO it's a worthwhile challenge. This youtube video shows a derailleur equipped bike with an added crank similar to what I think you're considering.

http://www.youtube.com/watch?v=FgIL6eHHgZU

I don't know anything about your welder or your welding skills, so I can't comment on your plan other than to say welding thin walled bicycle tubing without cooking the tubing is a learned skill that takes lots of practice.

My welder (actually my dad's, I am 17) is a Lincoln Electric Weld Pak 3200HD (http://cgi.ebay.com/Lincoln-Electric-Weld-Pak-3200HD-K2190-1-Welder_W0QQcmdZViewItemQQitemZ250406581781QQsalenotsupported). Of course, I would practice more before actually welding anything to the bike.