c_booth

Specifically oversize down tube to standard headtube?

Obviously there would be no buildup of braze around the sides of the join, added to that the fact it's a relatively high stress area, so instinct tells me it's not the best of ideas - can anyone assure me otherwise?

I guess it's not something one would ordinarily consider since you'd just use an oversize headtube (which I'll probably end up doing) or standard DT, but the friend I'm building for is fairly insitent on having fillet brazed frame with a quill stem, and a 28.6 DT would probably be a bit on the noodly side for him.

I know there are ways 'round this with ahead/quill converters or sleeving the standard dia. headtube etc - or simply persuading him away from a quill stem altogether.

So I suppose i ask out of intrigue as much as anything.

Appreciated as ever.

Andrew R Stewart 

There should be no issues with same size tube fillets if done with the usual care. The sides will have a fillet but it will be internal. Not obvious to the untrained builder but it had better be there. If you can't get this internal fillet with your practice joints (you are going to practice something you haven't done as yet...???) then you shouldn't offer to build this design. Andy.

reddog3

As Andy noted you "should" end up with an internal fillet. If the mitres are really tight, as they should be, one thing I've done is file a couple small notches in each side with a triangular file to insure the filler will flow to the inside. It helped in practice joints, but you never know about the finished product. If it keeps sucking up filler- you're good. One thing I don't care for is the feeling that I need to add a little more heat to make certain the headtube is hot enough inside the joint. You can see the heat spread from inside the headtube, but it's always a real juggling act for me. Just takes practice, and the more you do the more comfortable you get. My problem, as a hobbyist, I don't get to do enough frames.

That said- I don't like the appearance when the downtube and headtube are close to the same diameter. You have a nice radius to the fillet transitioning to a "flat" on the sides. It just don't look right to my eye.

c_booth

Thanks Andrew.

Yeah I'm taking internal fillets as a given (from pov of my question) - I find they tend to form more readily on (inside) the side of the join (due to the tight internal angle at that point which I guess creates a heat pocket) than they do on the top and underside, so should be no issues there. I've built a couple of fillet frames plus a few stems (not to mention a whole heap of practice fillets), so I'm fairly confident in the integrity of my joins (well as confident as one can be without having tested them through yrs of riding). It's more the geaneral knowedge side of things where I'm lacking, and you only pick that up through yrs of experience.............or coming on somewhere like here of course

c_booth

I actually agree with this. He seems bent on having a quill stem though (he wants something vertically adjustable).

fietsbob

Consider a miter cut with 'wings', to wrap around the sides ,
it will add surface to braze .
It's what Brompton does on their main frame tube joints.

Andrew R Stewart 

"Consider a miter cut with 'wings', to wrap around the sides ,
it will add surface to braze ." -- fietsbob

I have done a stem this way. The extension was slightly bigger a diameter then the quill. I left two strips of the extension on either side of the miter, then wrapped them around the top of the quill, kind of like a lug. One additional aspect that was cool is that the top cap has a thicket quill wall to rest on.

Another method is to squish the DT to form an oval as it meets the HT. Then miter as usual. Andy.