Bike Think

Examining the Science behind Bicycle Performance

Joint Stress in Bicycle Frames



Joint Stress

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.

3 Responses to “Joint Stress in Bicycle Frames”

  • James says:

    Hi Gary. Awesome info here on your blog. I wonder if I could get your input on a question I have. I am going to be drilling a 5mm hole in one of my steel frame’s chainstays to route the RD cable for an Ultegra di2 setup. The hole will be drilled about 2-3″ from the end of the chainstay tube near the dropout. Do you have an idea if I should drill the hole in the top or the side of the chainstay? I’m not sure where the stresses are highest in this area.

    Thanks for any input you might have on this. :)

    • admin says:

      I cannot say for sure. The models I have looked at are simplified load cases for answering specific frame performance questions. The variety of load conditions that would be applied to the frame in real world application would have to be examined for your specific frame.
      Having said that, I would guess that drilling into the top would be better. The sides of the chainstay are more likely to be loaded with bending stresses from lateral loads on the frame. Up/down loads will apply more pure tension/compression to the stays. For tension/compression of these members, location of the hole will not have an effect.
      Again though, I have to add the disclaimer that I cannot say with certainty what the effect of a hole in your frame will be on it’s structural integrity.

      Thanks for your question.

  • Colenso says:

    “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.”

    In fact, it is the toughness plus the tensile strength, compressive strengths, and shear strengths (in all directions) of the weakest parts of the frame that are subjected to the highest instantaneous forces and torques (relative to the intrinsic mechanical properties of that part of the frame) that will determine if any part of the frame will fracture or distort permanently when you hit a pothole.

    A frame made from standard crown glass could be both stiff and strong. But because a frame made from standard crown glass is brittle and not tough, it will crack or smash when suddenly jolted in going over a pot hole.

    Toughness is not easily defined, let alone measured or estimated, in the way that stiffness and strength are easily defined. Toughness, however, along with resistance to corrosion and other age related changes in mechanical properties are as important as strength and stiffness in a bicycle frame that will not be discarded after every race or after every 100 or so kilometres.


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