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Reading various questions and answers around here, I've heard a lot of people say you can tell the quality of the bicycle by looking at the welds. How do I know what a good weld looks like as opposed to a bad weld. In this answer its stated that good bikes have smoother welds. However, when looking at my bike, the welds don't really appear to be smooth at all, even though I know it is a high quality bike. What should I look for when looking at the welds of a bike? Pictures would be greatly appreciated.

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The question presumes that you can tell the difference by looking at it. I doubt anyone can on a finished bike - amazing what a coat of paint can hide. A great looking weld that is not properly heat treated after the weld is complete is a bad weld, and paint will cover that up pretty easily. –  mattnz Jun 21 '12 at 3:52
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Well, then why all the people telling others to look at the welds on a bike, to determine if the bike is of good quality. –  Kibbee Jun 21 '12 at 16:48
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I have worked with people who care a lot about whether welds fail (it's one way to press the "make wings fall off" button) and they seem to think that some bad welds look awful, and should be avoided. But other welds look fine until you see the x-rays or break the join, at which point you see flaws. But if they look pretty and there are no obvious breaks in the bead or overheating dips that just means they weren't done by a complete amateur. My bike has dodgy welds like that (I am a complete amateur) but hasn't broken in 10,000km because it's not operating anywhere near the material limits. –  Kohi Jun 22 '12 at 0:49
    
@Kibbee: A high quality bike will be finished to a high standard, mean ugly welds won't pass Quality checks. A low quality bike MAY have ugly welds, because it's cheaper to let the ugly welds through production into the market. So a bike with Ugly Welds is likely to be a low quality bike. The corollary does not hold - A nice looking weld does not mean it is a high quality bike. –  mattnz Jun 26 '12 at 2:02

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I found This site about welding to be somewhat helpful. There's an image near the bottom comparing a good weld to a bad weld on aluminum. The good weld is actually the one that isn't smooth. The type of weld used differs on the type of material used (steel vs aluminum). A lot of the information on this site talks about coloring, so you can tell if they went too slow and burned the material, but this would often be covered up by the paint on a bike.

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Keep in mind that bike frame welding is usually done with a multi-flame jig, so that the entire joint is heated uniformly and the weld is done rapidly -- different from the referenced article. This is a technique pioneered by Trek for aluminum frames (where the rapid welding made it practical to use AL in the first place), but it is now SOP even for most steel frames (where the rapid welding prevents the steel tubes from losing their temper). –  Daniel R Hicks Jun 19 '12 at 16:24

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