Lugs vs. Welds

Welding processes & Metalurgy

A piece of unheat treated carbon steel has a tensile strength of about 25,000lb/in sq. A piece of heat treated alloy steel can have tensile strengths greater than 400,000 lb/in sq.

Way back when, low end bikes like Schwinns were welded together out of low carbon unheat treated steel. They had to go with thick heavy tube sections to get the strength they needed. The high end bikes bake then used heat treated alloy steel. One of the problems about welding the metals of that era was once you welded the part you lost the heat treat and strength in the area within a couple of inches of the weld. Also due to the temperatures and atmosphere around the weld the metals would oxidize and some of the alloying elements would percipitate out and cause crystiline bounderies around the welds that were very brittle and could cause sudden failures under impact load.

To solve this the heat treated alloy tubes were silver soldered together using lugs at temperatures low enough to effect the strength and reliability of the tubes heat treat. The lugs also added reinforcement to the high stress transition areas of the frame.

Many new alloys have been created over the last 30 years. Many of which are air hardenable and weldable. Welding processess and materials have improved over the last 30 years so the alloys can be welded without risks. Add to that the new tappered tubes that are thick in the welded high stress areas and thin in the low stress areas.

Now a welded frame can be made lighter than a lugged frame. I am guessing welding takes less manpower than lugging and silver soldering. (certainly when you leave off the great paint details they use to hand paint around the lugs).

I like to flick the top tubes of the high end bikes at the bike store and am always amazed at the tone they make. That hollow high pitch tone you only get from very thin high strength heat treated metal. The old school bikes did not have that tone. The new frames are not as pretty but pound for pound they are a lot stronger than the old school frames.

azmadoc said:

Why one or the other? What's the real difference besides looks and old skool props?

Click to expand...

Generalizations:

Lugs: Add material in a place where it makes a lot of sense. Add weight to a frame. Probably have some benefits in repairability, although this is more related to brazing rather than lugs per se. Look cool, to me. Allow for contrasting head tubes, which look cool to me. Take more time to build well. (the main reason almost all production bikes are welded). Great frames can be built with lugs.

Welds: Save weight, sometimes. Shorten build times, usually. Allow for a wider range of frame designs. Look ordinary and plain, to me. Great frames can be made by welding.

Don't forget fillet brazing, and don't stress on joining methods. If you want a lugged bike, there are some fantastic ones out there. If you want a welded bike, ditto.

For true happiness, choose a builder whose values and opinions mesh well with your own, and trust their judgement.

--Shannon

in my opinion many of the above links are absolutely useless. you're reading about how lugs are more sturdy by people who SELL lugged bikes. can you say advertising? i sure can.

according to sloane's complete book of bicycling, who quotes douglas hayduk's bicyclie metallurgy for the cyclist, there is no difference between a lugged frame or an unlugged frame. both can be made to be as strong as the other. "Any well-made frame has tubing ends accurately mitered so they butt tightly up against the tube they are to be welded, brazed, or lugged to. Inaccurately mitered tubing makes for a weak joint whether it's lugged or not".

A cheaply made frame is weaker than a more expensive one, because less care is taken with those butts. Lugged steel frames tend to cost a lot more than unlugged ones, not because they're lugged, but because they offer a certain "sex appeal" and, dare i say it, snobbiness. Unlugged frames are cheaper to make, sure. But take two $3000 steel frames next to each other, one lugged, one not, and chances are very good they're of equal quality, since equal care is taken with the butts.

lay the lug-debate to rest already. (by the way, am i the only one who thinks lugs do NOT look good?)

sd