Originally Posted by halfbiked

'joining strength' is something of a misnomer. The 'joining strength' comes from the material you use to join things, combined with the surface area of the join. With brazed frames, the joining material is the solder - silver, bronze, whatever. The surface area is the contact between the tubes - so it should be easy to visualize that thicker tubes have more surface area for the join. With a weld, the joining material is typically the same as the tubing material - i.e. steel or aluminum. The joining surface is the depth of penetration of the weld. With a butted tube, there is more depth available for the weld to penetrate. Also note that some aluminum frames were glued together. Same principle applies: the more material available for the join agent to 'grip' the stronger the joint will be.