Fillet Brazed Frames

Photo © Pereira Cycles (via Flickr)

Fillet brazing is rarely used in bicycle frame construction today. It’s a beautiful, but labor-intensive construction method that requires considerable skill on the part of the frame builder. The “fillet” itself is a layer of brass melted into the frame joint at relatively low temperature, then filed smooth. Fillet brazed frames are easily recognized by the unique appearance of one tube flowing seamlessly into the other. In my opinion, the aesthetic qualities of fillet brazed frames rival those of lugged-steel. Current builders using at least some fillet brazing in their frame construction include Pereira, Nobilette, and MAP (among others).

17 Responses to “Fillet Brazed Frames”

  • Jason says:

    Besides the beautiful appearance of the joints, does the strength of the joints compare equally to lugged joints?

  • Paul says:

    Ahh, Cannondale, your welds were enough to melt my heart…

  • Alan says:

    @Jason

    This is from Steve Rex, a highly respected frame builder in Northern California:

    “Custom frames are joined either by using fillet brazing, by using lugs or by tig welding. Simply put with a lugged frame, the tubes are joined by being brazed in a sleeve (the lug), while with a fillet brazed frame, the tubes are joined by the brazing alone.

    The big advantage of fillet brazing is versatility. Any tube size or angle, is possible. Lugs confine the maker to the exact diameter and angles of the lugs. The fillet also allows to raised (extended) headtube that enables the rider to get a higher hand position. Strengthwise the fillets produce the smallest stress riser because they taper out to nothing.

    Stress risers are a problem with some lugged frames, and these are more likely to develop cracks near the joint, usually next to the lug. To avoid this problem, we use silver solder with lugged frames, which has a lower temperature liquidus state (1200 degrees F) and puts less stress on the tube. We also taper the lugs.

    TIG joints also create more of a stress riser because of the short transition from the highly heated joint to the tube. Fillet brazing is done with brass, but at a lower temperature than when brass is used with lugged brazing, where the brass has to hit an 1800 degress F to capilate through the lug. Fillet brazing is done closer to the melting point, 150 degrees cooler.”

  • Andrew says:

    I definitely enjoy the look of fillet-brazed frames. The clean lines are actually why I often prefer TIG-welded frames (or carbon, hahaha) to lugs from a purely aesthetic level.

  • Andrew says:

    Thanks for those framebuilders’ links, by the way. I expect I’ll be spending a lot of time ogling Flickr photos of works-in-progress…MAP in particular has some gorgeous shots.

  • Helton says:

    I am seriously interested in building some frame at home just for fun, but with as high-quality as I can get, sometime in the remote future (post-retirement?).

    After a lot of consideration, it seems that fillet-brazing is by far the most handy option to make it, because if you have time to cut, miter and align the tubes together by yourself, fillet brazing is the technique requiring least and cheapest equipment (torch kit, flux, filler metal), compared to welding.

    Besides, one doesn’t need to file and sand the brazing to get that perfectly round fillet shown on the photo, specially if the bike built is not supposed to suffer a lot on racing or cargo-hauling.

    Nice picture, and it’s always good that someone reminds these unusual options.

  • Daniel M says:

    Following on Steve Rex’s comment, I would point out that Rivendell manages to do a sloped top tube and extended head tube, and thus higher hand position, by spec-ing their own lugs.

    If I am mistaken please let me know. And thanks for putting together this wonderful site. I am a new reader and this is my first comment.

  • tdp says:

    “Besides the beautiful appearance of the joints, does the strength of the joints compare equally to lugged joints?”

    Absolutely, if not stronger

  • Doug R. says:

    I may love the fillet brazing, however, the beautiful artistry of my Rivendell’s lugs is incomprable! I love great design work!

  • Alan says:

    @Daniel M

    Welcome and thanks for visiting and sharing your comment!

    You are correct; Rivendell designs their own lugs. I was at their shop recently and one storage area had boxes and boxes of raw investment cast lugs ready to be shipped to their builders. The upsloping top tube does in fact make it easier to set-up a high handlebar position. Their use of a threaded steerer and quill stem help as well.

    Regards,
    Alan

  • Lovely Bicycle! says:

    Daniel M says:
    Following on Steve Rex’s comment, I would point out that Rivendell manages to do a sloped top tube and extended head tube, and thus higher hand position, by spec-ing their own lugs.

    Yes, and I understand that spec-ing one’s own lugs is not the only way to do this; existing lugs can be altered as well. My Royal H. Cycles mixte has an extended head tube and altered lugs.

    Fillet brazing can be nice, and so can TIG welding. It really depends on the framebuilder and how they approach it. There are new builders like Icarus who do amazing things with fillet brazing, so I think it may be experiencing a renaissance.

  • Tony Pereira says:

    Thanks for the mention. When I started building I was drawn to fillet brazing for two reasons: that was how the first mountain bikes that I loved so much were built, and I had an oxyacetylene torch (and kind of knew how to use it). I love the way fillets look and the design flexibility they allow. Steve’s quote is spot-on regarding strength. Let me know if you have any more questions.

  • Robert Anderson says:

    Stephen Bilenky (BIlenky Bikes of Philadelphia, PA) whose links appear down this page, also does filleted frames. As well as other kinds.

  • Finley says:

    I am a bit confused, so please forgive my ignorance. I am far from a metallurgist, but my layman’s intuition would be to assume that a lugged joint would be stronger than a fillet brazed one. Is “strength” in this context pure weight bearing ability and shock resistance, or is it more of a case of “neither will break during heavy usage at one time, but over time the fillet brazed joint will last longer?” If it is the former, I would think that the nature of the lugged joint would lead to a greater amount of strength, due to having ‘tubes within tubes’ as well as having a greater steel to braze area, considering the greater number of points that get brazed. It seems like the brass is being asked to be the lug as well as the glue in a fillet brazed frame. This is by no means meant to bash fillet brazing in any way, as I think it is gorgeous, and manufacturers, especially custom manufacturers wouldn’t use the technique if it was sub par. I just genuinely don’t quite get it.

  • Chris Moore says:

    Mercian in England does beautiful filet (as well as lugged) work. They’ve been doing it for over 60 years.

    Merciancycles.com

  • Bob says:

    @ Finley

    The reason that lugged joints are not inherently stronger is that more heat needs to be applied to the tubes which weakens them.
    As for custom angles with lugs it is possible to build a couple of degrees out of the spec of stock lugs. Enough to have a 3 degree slope on a top tube. It is also possible to make custom lugs by fillet brazing or welding tubing together. That not only allows custom angles but unusual tubing diameters.

  • Hunter says:

    Curt Goodrich is another that uses fillet brazing. In his shop now he has a beautiful singlespeed w/ ecentric and Rolf hub that he has made a down tub shifter for. Check out here http://www.curtgoodrich.com/ and his blog for info on the custom down tube Rolf hub shifter

 
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