Fully welding a seat tube to the BB

[Mike Mcdermid]

Heres something we were discussing on Facebook

and it seems it falls into two camps those that do and those that don't

for those who weld which method do you have proof works for you?

[Eric Estlund]

I fully braze mine.

There are a billion production bikes out there with only partially welded ST's.

I still choose to go all the way around.

[Curt Goodrich]

Back in the olde days, when I was paid to weld, my employers insisted on welding all the way around. When I fillet braze I do the same thing. I see no reason not to other than to save a little time.

[Amaro Bikes]

I guess some people who tack the whole frame together before startint to weld/braze would not go through the complete st/bb junction, but to me it seems much more reliable to do a full st-bb weld. So far I've been always doing so, as first step from the whole process (bb-st), just 4 sequence weld and alignment check before then going back to tack time and adding rest of tubes to this reference spine structure. Seems to make sense and so far has been nice to me, but when I see quite some superproof and hypergood builders doing the all-frame-tack process, I guess it would work fine as well if not fully welding the st-bb.

As Curt said, sure time has a word to say in this question, anyway this is about an extra hour at most so for small production figures shouldn't care too much

Cheers

[VertigoCycles]

Always.

[Rody]

Done it both ways....

Production house making standard frames, partially welded under the DT

Custom building, all the way around

Never saw a single difference in performance or longevity, comes down to personal satisfaction knowing I did the best possible fabrication I'm capable of.


The next question becomes the debate to which is better: fully welding the downtube to the BB and coping the seat tube to fit or fully welding the seat tube to the BB and coping the downtube to fit. Ardent supporters of both exist.

[Jonathan]

I fully weld ST to BB here, mostly because I was taught that way and it made sense. I know folks do it the other way, but I never thought one way or the other about it. I know there have been some failures on small bikes that didnt have a fully welded DT/HT junction where a TT joins the miter. I just weld everything up I can.

[bencooper]

I don't - I mostly fillet braze, so I think it's better to have tight mitres on the DT than fillet the ST first and have to cut away a bit of the DT for the ST fillet. Could do a little fillet, I suppose, but prefer to tack the whole frame first then do the fillets freehand.

One of those "no right answer" questions I think.

[Mark Kelly]

The next question becomes the debate to which is better: fully welding the downtube to the BB and coping the seat tube to fit or fully welding the seat tube to the BB and coping the downtube to fit. Ardent supporters of both exist.

And I'll wager that the split is driven by their vision of how a frame works: those for whom the downtube is the spine of the bike vs those for whom the seattube is the heart of the bike.

[fortyfour]

I originally fully welded the Down Tube to the Bottom Bracket, and then miter the seat tube to the DT/BB junction. But when I started building from my present location, I changed to fully welding the seat tube to the bottom bracket for a number of reasons (I do not have any structural testing to support this method, only reasons from a practical application standpoint):

0. Seat tube to bottom bracket perpendicularity is most likely one of the most important aspects of the frame. (IMO) If that is off, then that starts to throw off head tube/seat tube perpendicularity to the bottom bracket. Head and seat tube in the same plane, who's centerline's are perpendicular to the bottom bracket center line, rear axle center line and front axle center line.

1. Welding the ST to the BB allows for a large vent hole between the Seat Tube and BB (I use .75"). Water tends to be able to get into the seat tube no matter what, and by welding the ST to the BB, you have a clean run down into the BB for the water/moisture to drain through a drain hole in the BB. (I use a M5 x .8 tapped hole in case someone wants to plug it). Using the other method, after long road trips with bike up on top of the car, driving through lots of rain, I would have to pull the seat / post, flip the bike and watch the waterfall come out of the frame as that vent hole is much smaller when I was doing it the other way (most if not all of my mountain bikes have a 1.5" O.D. DT and pretty much all of my Road frames have a 1.5" O.D. DT or a 1.375" O.D. DT)

2. On a mountain bike, the down tube is most vulnerable to dents/dings IMO. If you fully weld the down tube to the bottom bracket, and that tube needs to be replaced in the future, you're now dealing with getting that old down tube out, but also having to extract it from the overlapping seat tube. If you fully weld the seat tube to the bottom bracket, it's much easier to deal with the down tube as it is overlapping the seat tube, or at least that is my thought. The latter may have you replacing both seat and down tube possibly.

I can see both sides of the coin, but in both instances of which tube overlapped the other, I have always fully wrapped all the welds.

[Jake Rosenfeld]

I fully weld the st to bb. after seeing how much it moves around during welding I really don't want to deal with a wonky bb with all the tubes attached.
I copied Ulrich's style of fully piercing the bb for a few bikes, when you insert the st and weld you are welding to the side of the tube and the edge of the bb hole so the thickness differential is somewhat negated and the weld just lays in like buttah. I stopped doing this cause I couldn't make the rest of the welds on the bike look that good and it was out of place.
as far as alignment went I found it to be a far superior joint to control as you were welding

[blasdelf]

I'm particularly curious to hear what people do about this when building aluminum frames

[Jake Rosenfeld]

I'm particularly curious to hear what people do about this when building aluminum frames

I did it the same way.... almost. st to bb but I welded the hidden part first and tacked the back and then stacked the dt on and welded it all up. this way I didn't need to take so much out of the corners due to the size of the bead. so its not quite fully welded I guess.

[John Winston]

I fully weld ST to BB. If memory serves me correct, a certain ex-Merlin employee spoke about this and has some actual data to back up that this was a better/stiffer/stronger method. I believe its somewhere here on the VS.

[zank]

I fully weld the ST to the BB. On the aluminum front triangles I've done so far, I end up having to clip the points on the DT compound cope pretty far back to make room for the fillet. That's how Frank the Weldor did it at the Metal Guru class.

On an overlapping TT/DT, I fully weld the DT to the HT and then cope the TT to fit. There probably isn't much debate on that joint though.

And as a side note, Frank clipped the chainstays quite a bit too to fit around ST fillet.

[Jake Rosenfeld]

I fully weld the ST to the BB. On the aluminum front triangles I've done so far, I end up having to clip the points on the DT compound cope pretty far back to make room for the fillet. That's how Frank the Weldor did it at the Metal Guru class.

On an overlapping TT/DT, I fully weld the DT to the HT and then cope the TT to fit. There probably isn't much debate on that joint though.

And as a side note, Frank clipped the chainstays quite a bit too to fit around ST fillet.

Its a whole nother thing that aluminum.... it's kind of like working with crayons vs a mechanical pencil

I have done a AL bb like you've described but I think I like being able to follow the miter lines with the torch. my guess is that production bikes are just stuffed together and what you see is what you get as far as the bb being welded.

[progetto]

Same process for me, fully weld seat tube. If I'm doing a small frame with short head tube, I also fully weld down tube to head then insert top tube.

I'm particularly curious to hear what people do about this when building aluminum frames

[cschaafsma]

I always fully weld the seat tube to the BB. It's the spine of the bike. I just file back the tips of the down tube miter to make room for the weld bead.