M_V's multitude of bikes and adventures in the land of framebuilding

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.