Resiliency, Flexibility and Fatigue

Historically, titanium frames have been more compliant than most steel or aluminum frames, and this has given titanium a reputation for being inherently flexible. But the so-called flexibility of any material is measured by its elastic modulus (Young's modulus). And the three most common frame materials-steel, aluminum, and titanium-actually have similar modulus-to-density (stiffness-to-weight) ratios. Steel's ratio is only about 10% higher thantitanium's.

This similarity means that a titanium tube of the same diameter and the same weight as steel or aluminum will have similar stiffness. But of course, no one builds frames that way-nor can they, because modulus isn't the only governing variable. The other property that must be considered is fatigue strength.

Fatigue strength can be loosely defined as the level at which a material can withstand an infinite number of stress cycles. It so happens that titanium has exceptionally high fatigue strength. Since titanium can endure a higher level of stress without damage, bicycle designers can create resilient frames with less concern that flexure will cause failure.

Conversely, metals that have poor fatigue strength cannot be given much room to flex. Aluminum has the worst fatigue strength of these metals, and so aluminum frames tend to be very stiff-not because the metal itself is stiff, but because allowing an aluminum frame to flex will significantly reduce its service life.

For a simplified example of this phenomenon, compare two aluminum frames, one very flexible, the other very stiff. Assuming everything else is equal-rider weight, terrain, frame geometry, and so on-the flexible frame will fail from fatigue much quicker than the stiff frame. The ultimate failure of each frame is caused by the cycles of stress it endures, with the more flexible frame cycling through higher stress peaks than the stiffer frame (the greater the deflection, the greater the stress). The higher the stress peaks, the shorter the theoretical fatigue life.

Steel has much better fatigue strength than aluminum, so allowing the frame to flex isn't as much of a problem. But steel is twice as dense as titanium, so it is more difficult to tailor the stiffness of the ride without running into weight problems. Put another way, since titanium is half as dense as steel, more of it can be used to tune the ride by juggling tube diameters and wall thicknesses, while still creating a frame that is lighter than an equivalent made from steel. And if the 3-2.5 frame were designed to be as stiff as the same steel frame and weigh roughly the same, it could have roughly twice the fatigue life.

Thus, it is not resiliency per se that is the issue, but rather how the designer is able to exploit the fatigue properties of the material. Although the modulus-to-density ratios of the materials may be virtually the same regardless of strength or alloy, a bicycle's tubing diameter and wall can have a profound effect on the stiffness or resiliency of a frame-assuming the fatigue strength of the material allows this design latitude.

This model is simplified greatly, and there are many factors beyond material choice that affect fatigue life. The tube diameter, wall thickness, butted sections, surface finish, and tapering all influence fatigue life, as do frame geometry, weld quality, braze-ons, component choice, and rider style.

The net benefit of titanium's high fatigue strength-to-weight ratio is the ability to modify the tube geometries in pursuit of a lighter frame that is stiff as a steel frame, or, alternatively, designing a more resilient frame without sacrificing fatigue life.

Finally, it follows that given the freedom to modify tube geometries, a titanium frame can be stiffer than a steel frame, too, if that is the goal.