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PhilDAS-
I asked in a framebuilding Facebook group and the people replying asked about tube wall thicknesses (.8mm on the seattube, .9mm on the toptube. The tube for the centre of the wishbone thingy is top tube) and recommended I use reinforcement.
I think you see a lot of frames where the seat stays meet the seat tube at the same point the top tube does and they use a single piece of reinforcing sleeve but that wasn’t how I wanted to build so we end up with this.
M_V
Yeah they spread the load from the end of the tube over a larger area. The reinforcement piece may not be much bigger than the diameter of the tube but it’s huge amount more area of contact than the end of the tube would be.
I think if you filleted straight to the seattube you’d be putting a lot of heat into a pretty thin tube and that’d most likely result in distortion of the seattube. This way the hot fillet is done to a heavier piece of metal and then I use lower temp silver brazing to join that to the seattube. If the reinforcement piece gets distorted during the fillet braze it doesn’t matter as much because the filler material can take up the discrepancy.