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Lug Thinning?

Old 12-31-14, 08:38 AM
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Tandem Tom
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Lug Thinning?

I came across this term/idea while reading yesterday about restoring a vintage frame. Am I correct that this can be done to an existing frame to give it a more finished look?Is this something a newbie should/could attempt?
Thoughts??
Thanks!
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Old 12-31-14, 10:40 AM
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unterhausen
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A lot of older bikes are being held together by their lugs. I would limit thinning to major defects
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Old 12-31-14, 12:22 PM
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Actually, thinning lugs near the shorelines not only gives them a more finished appearance, but also reduces stress risers where the thick lug material abruptly stops at the shoreline creating hard points. Thinning the lug near the shoreline at the points mitigates stress risers.



Lugged Steel Frame Construction|Wikipedia

Originally Posted by Wikipedia
In most lugged steel frames, the lugs have a simple pointed shape. The lug's curves maximize the strength of the joint, while minimizing the possibility of stress risers, which would otherwise make the frame prone to cracking at the end of the lug.

At increased expense, frames may have lugs which are cut and filed by hand into fancy shapes, both for slight weight savings and as a display of craftsmanship. In addition many frame builders file the lugs to thin them, in order to reduce stress concentrations. These ornate or fancy lugs may be painted to accentuate their appearance.
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Old 12-31-14, 12:29 PM
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And Typically the Lug is thinned before the frame is assembled and Tubes Fitted..

but if you are doing a repaint anyhow and already have a decent knack with Metalwork ..


Just be careful , thin the lug tips, not notching the Tube itself..
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Old 01-01-15, 09:56 AM
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I've only seen one frame break at the shore line, and it had what I consider excessive lug thinning.
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Old 01-01-15, 11:01 PM
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Assertions that thinning lugs reduces stress risers and increases frame durability are repeated often enough by respected builders that I think there is probably some truth to them. This is from the Spectrum Cycles website (Tom Kellogg and Jeff Duser).

Originally Posted by Spectrum Cycles website
SO HOW DOES FINISHING THE LUGS ADD STRENGTH?

Good question. Most cast lugs have relatively thick edges. This thickness causes a "stress riser" or a point where stresses are concentrated where the structure of the frame suddenly becomes thicker at the lug. This stress riser is exacerbated by the relative thinness of modern tubing. These weak points can be nearly eliminated through lug finishing. For many years, we have taken the time to remove close to 30% of the material from the lugs we use before we even begin the frame assembly process. Thinning the lug wall significantly reduces localized stress and frame weakness and increases durability. As an added bonus, lug work also makes them appear cleaner and more delicate when assembled and painted. And while almost anyone can solder lugs to a crisp edge, the difficulty lies in the finishing of the lug's contours. Because of their shape, the contours of the surface of the lug are much more difficult to perfect. Here is where we excel. Look at our lug work and see the difference Spectrum makes. The cleanness, sharpness, and uniformity of our lugs are a reflection of our art, our experience, and our passion for perfection.
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Old 01-02-15, 08:39 AM
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I almost aways agree with Unterhuasen who gives great advice but not in this case. I think it is okay to thin the lugs. And it is great practice for when you build your own. They are overly thick on most vintage frames. Iím a framebuilder that came from the 70ís where lug thinning was considered to be a mark of excellence. The reason most frames have bulky lugs is because it was too much time and effort to file them down. All my custom frames Iíve built with lugs have been worked over a lot both before and after brazing. I work on the edges before assembly and do almost all the thinning after they are brazed. I do that for several reasons but that is beyond the scope of this subject thread.

My recommendation is to start on the sides to establish your desired lug thickness. Your goal is to have even thickness all the way around. It is much harder to remove metal there because your file strokes are limited. You have to be careful to not let the file slide off of the lug onto the tube which both rounds the edge (a mark of poor craftsmanship) and scars the tube. Holding the file at the right angle helps keep some part of it on the lug so it doesnít entirely slip off. Most pros that thin lugs leave a thickness of 1.2 to 1.0 mm.

Basically lug filing is a 3 step process. 1st remove bulk with a course cut file, 2nd remove file marks with a fine cut file and finally polish out with 80 grit emery cloth. I use my finger as backing while others use a file or some other kind of block. Be careful with going over the lug tips with either a file or emery (particularly if using a shoe shine motion) because they can be taken down too far in the blink of an eye.
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Old 01-02-15, 08:52 AM
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Thinned lugs also suggests that the builder took more time and attention to his build then another frame with lugs not worked on. Like many aspects of building practice gets one far more efficient. So practice on scrap first. Andy.
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Old 01-02-15, 11:08 PM
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I have nothing against thinning lugs, but old bikes may not have properly mitered tubes. In fact, it's more common than not
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Old 01-02-15, 11:20 PM
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What frame do you have, and what's the paint condition? Are you repainting anyway? Are the lugs painted or chromed? Would a simple touch up of the rest of the paint suffice? Are you going for a wall-hanger or a rider? Are you trying to get it all original, or are you going to mix and match with modern parts? How steady are your hands? Have you done any filing before? Have I asked enough questions to drive you nuts yet? Should I ask more?
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Old 01-03-15, 10:36 AM
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I picked up a 1984 Miyata 610. I have been looking for more of a randonneuring type bike and this one seems to fit the bill. Going to consider 27" to 700c conversion. Also I have been wanting to refinish a frame. I know that it is not always cost effective but it is something I have want to try. I am a woodworker and have a spray set up and booth so that part is covered.
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Old 01-03-15, 12:19 PM
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With an '84 Miyata frame I wouldn't be too concerned about the tubes being properly mitered.
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Old 01-03-15, 12:26 PM
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And, it sounds like your woodworking experience will set you up well for the thinning.
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Old 01-05-15, 08:46 AM
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I still wouldn't do it, you aren't going to turn a miyata into an Eisentraut. And I wouldn't count on the construction to be perfect either. On the subject of stress risers, the lug is going to be a stress riser no matter what you do. The material under the lug is constrained, and its sitting right next to material that isn't constrained the same way. It's an obvious stress riser. You could thin to zero right at the edge, but what's the fun in that? Even then there would be a stress riser.

I may post a picture of the frame with very thin lugs that cracked. If any frame was going to be saved by thinning the lugs, that is the one, the lugs are very thin. But logically, even a radically thinned lug is going to more than double the thickness of the tubing, and lugs almost always have a couple of sharp points. This frame cracked on the head tube starting at a point on the lug. I figure there were a multitude of sins in the construction of that frame, but having a relatively flexible lug didn't help. Paint job was awesome though. I respect Tom Kellogg as a framebuilder, but a marketing spiel is not exactly proof of anything. I bet I can go on the wayback machine and find any number of defunct web sites of newbies that say they are the best framebuilder ever -- maybe including the guy that made the frame that I fixed that cracked. I have a Ph.D. and my thesis involved studying fatigue. But I wouldn't write such a thing because I am not going to do the testing required to validate the conclusions made. That's the danger of education, you become more cautious about publishing poorly informed opinion

Someone cut apart a batch of older frames to look at the workmanship, it was uniformly bad even on some fairly well regarded brands. I wish I could find the pictures. The old guys considered the lugs to be the structural element that carried the loads. The idea that there should be an internal fillet that connects the tubes independent of the lug is a fairly recent development.
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