What is the general consensus on procedure for butt joints in tubes, like that of couplers or collars?
I’ve been building more with heavy wall collars recently, and I’m about to weld up some couplers. So I thought now is a good time to ask what people reckon the best bet for welding these kind of joints is.
I’ve done about 10 frames with collars I think, I add a decent chamfer to tube and collar and have a small gap (0.2mm) between the two. Then I purge and lay a slightly proud bead over the joint (with a heatsink). This seems like the most intuitive way to do it for me. I weld with straight current and just dab the normal way.
I know some people leave them square and with a more significant gap, seemingly around 0.5-1mm.
I’ve also seen some people leaving no gap and welding without filler.
What do you guys reckon is the best bet?
Cheers
Honestly, I think the best practice is to use a butted seat tube. Collars add time, are much harder on the reamer, and I have seen them crack at the joint (likely user error).
The only butted seat tube that does not exist is a steel 31.8mm tube for a 27.2 seatpost.
Is there a reason why you prefer to use collars?
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I think I mainly like the heavy wallness of it. It’s true it’s more effort and more to go wrong. For an MTB or other bike I want to be really solid its nice to have a hefty section at the top tube/seatstays intersection.
I haven’t seen any butted tubes that are 1.5+mm wall thickness that are available readily in Europe, although it would be great if they are around.
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a bit of a side note for you here, but it might be interesting;
i’ve turned a few seat tubes from 34.9mm OD, 1.6mm wall tubing ( 31.6 nominal bore) . I’m not sure if this is good practice or not, grain structure etc, but were talking about seriously burly tubes, and they’ve given me no problems…
1.6 at the top/ thorugh the junction, nice gentle taper down to 0.8mm straight section in the middle, nice gentle taper back to 1.2 at the bottom.
I cut the end off square, set the stock tube up as true I can in a three jaw chuck, put a pipe centre in the end and take light cuts. goes fine, the thick wall is enough to resist the clamping load of the chuck without it slipping; I usually finish the tube off about 50-60mm from the chuck to be safe, and I don’t part it off on the lathe.
I first did this when I wanted to save weight on a seat tube I needed for a full suspension build, which wanted a REALLY long butt at the bottom to help support the main pivot housing, but have since done it twice on hardtails with success.
might be an option for you if you want to avoid the join, and have access to the heavy wall tube, and a lathe.
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