My first frame was built during a fillet brazing class. I’m now planning my second.
I’m curious of the advantages/disadvantages/considerations of using a lugged bottom bracket over fillet brazing the bottom bracket?
To achieve the angles I want, the rest of the junctions will likely be fillet brazed. Aesthetics questions aside, are there advantages like reduced heat cycles, better alignment, or less warping and is there really much of a difference?
Thanks in advance
If you want a nice smooth look, I hate filing bottom brackets. But my brazing technique is not very good so there is usually a lot of filing to do. Lugs don’t have that problem.
[quote=“matthoult, post:1, topic:4699”]are there advantages
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Assuming you can get a lugged shell with the sockets pointing in the right direction, I do think lugs are a little easier since the lug holds the tube pretty much in place. Some people add pins to make sure.
Where I think the lugged shell really helps is with the chainstays and making sure the right matches the left so your wheels sits straight. You can tack one side and then insert the other, insert a wheel and adjust the insertion by a tiny bit to make sure it’s all straight before you tack the 2nd side. This really only applies to through axle or vertical dropouts. With the horizontal slotted ones it’s not an issue if one is a tiny bit longer than the other - that’s what the dropout screws are for!
Filing lugless fillets inevitably thins the steel, right at the worst place, in the HAZ where the tube is already weakened.
Best way to make a lugless frame is with small neat fillets that don’t need any post-braze shaping. It’s OK to cheat a bit until you get your fillet game up to snuff, but I recommend leaving a little for the painter to do with putty, rather than trying to make perfect “show” fillets. If powercoating, JBweld works for fairing in fillets, doesn’t melt in the paint curing oven.
Another caveat while your fillets aren’t perfect enough to leave them alone: don’t use the thinnest tubes. Consider going down one size in diameter and up a notch or two on the thickness, for some safety margin in case your shaping thins the tube a bit at the fillet edge.
I encourage all FBs to make sample fillet-brazed joints and break them in a vise with cheaterbars, to see how small a fillet you can get away with. If the fillet breaks, it wasn’t big enough, but I think you’ll find they can be pretty small and the tubing still fails before the fillet.
Smaller fillets add less weight to the frame but more importantly they add less thermal mass and less heat overall, so the joint cools more rapidly. Smaller HAZ with less tempering. Less grain growth/recrystallization.
So the frame ends up stronger with smaller fillets — right down to the point where they’re too small and it’s the fillet that breaks. So ideally you want the size barely above that minimum.
That said, you don’t have to flirt with the absolute smallest. It makes you look bad if a fillet breaks, so err on the side of a bit too big to ensure its’s always the tube that fails before the fillet.
Another option, especially if you have odd angles and there’s no lugged shell that fits, is make your own lugged shell. Then the fillet shaping is done on the lug sockets, no chance of thinning the tubes. It’s a lot of work though! Here’s one I did that way in '84, got that out of my system so I never needed to do another one!
We are vibing here. 
This is very helpful! Thanks. I’m planning on using vertical DO’s, so this is good to know. Appreciate your help!
Thank you, Mark!
I have been on bike forums for years and always appreciate you sharing your experience, expertise, and wisdom.
This info is gold. I learned this the hard way – I’m just finishing up my first fillet-brazed frame, and I tackled the chain stays like I learned in my lugged UBI class: I fully brazed the dropouts to the chainstays, THEN I had to try to make the chainstays exactly the same length in the jig, which I didn’t get right, so ultimately I had to file the (vertical) dropout slot to correct a rear wheel misalignment.
If you fillet braze the BB, first fit the BB, the chainstays, and the dropouts up in your jig – and only then tack things together. Use tabbed dropouts so you have a little wiggle room if your chainstays aren’t exactly the same length.
I decided a while back that I’m only building with lugs unless there is a really good reason not to, like with a mountain bike frame. It’s not because of any technical or ability reason, I only build for fun anyway. You can fairly easily bend the chain stay sockets outwards. If you bend them to get a different angle with the seat tube, it’s best to fully support the major diameter of the shell, or you can bend it. Found this out the hard way.
