Bending Moment

The tubes in a bicycle frame are subject to both axial forces (along the length of the tube) and bending moments (which tend to curve the tubes). These forces vary depending on the weight of the rider and on the rider's position, torque output, and the road surface, but they are always carried inside the bike frame in some combination of axial and bending loads.

From a strength perspective, the most critical part of the frame are its joints. Depending on the construction method, the tube connecting process can create stress concentrations, thinned tube sections, or heat affected zones that would weaken the joint.

Tensile Load

In this example, both the axial force and the bending moment tend to create tension in the lower side of the tube, this side sees the higher total stress. In the upper side, the bending causes compression, which tends to cancel the tensile axial load, lowering stress.

Undercut and Poor Weld Penetration

If these two tubes are simply welded together, the weld should be of similar thickness to the tubing wall. If the weld suffers from either undercut or poor penetration, the resulting joint is significantly weakened.

Proper Weld and Silver Brazed Lug

Even if the weld is performed properly, there is a "heat-affected zone" with different grain structure than the bulk tubing. A silver-brazed, lugged joint has no chance for undercut and creates no heat-affected zone because the joining temperatures are lower. In addition, the load is spread across a larger surface area in a reinforced section of the joint.