Examining the Science behind Bicycle Performance
Joint Stress in Bicycle Frames
The lugged frame has found itself relegated to a dedicated niche market. For metal frames, lower cost, infinite geometry choices, and ability to weld aluminum and titanium has made welded frames dominate. Whatever your preference may be, the comparison of the stresses at the joints for these two joint methods provides some interesting insight into joint design of a bicycle frame.
It is important to remember the difference between strength and stiffness. The strength of a frame determines if it will break when you hit a pothole. The stiffness of a frame determines how much it bends when you pedal and how much it hurts when you hit the pothole. Stress patterns are only dependent on loading conditions and geometry (including angles, tube diameter, wall thickness, cross sectional shapes, etc.) For a given stress pattern, the material properties determine strain and deflection, or how much the frame will flex. Because the local stresses at the joints cover such a small portion of the overall frame, they are not a significant factor in overall frame stiffness. These local stresses are only important in the frames strength and durability.
Below is an FEA comparison of a lugged and welded down tube to head tube joint. The load is a torsion applied to the down tube with the ends of the head tube fixed. The arbitrary torsion load is the same in each case. The plotted stress colors are on a consistent scale.
The maximum stress in the welded joint is about double that of the lugged joint. But keep in mind this is a rough comparison. Factors such as lug design and weld fillet thickness will significantly affect the stress pattern in each case.
The lugged joint has two advantages as far as stress is concerned. First, it has added material in the stress concentration area. The lug material reduces the stress in the joint. Secondly, the edge of the lug is shaped and tapered to avoid a concentrated joint edge where stress would concentrate. Although, you can see a definite step in stress at the edge of the lug on the down tube.
This is not to say that welded frames are bad. Obviously it is possible to build welded frames that are strong enough. However, the lugged frame may be able to use thinner wall tubing if it were stiff enough. Looking at these FEA plots, you can see why many carbon fiber frames flare out at the joints to avoid stress concentrations.