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Steel & Damping

Joseph Semonian, Chairman Bill's Father, writes on materials and design.

My father is the smartest man I know. All of my life, he has been an intellectual colossus awing me and everyone he meets with his sharp, clear, powerful mind. It is hard to convey his brilliance to one who has not met him. In the early 1950's, he was an engineer for the old NACA (which became NASA). During his stint there, he solved several engineering problems that had stymied the most brilliant engineers around the world for years.

We regularly have wide-ranging discussions about nearly everything under the sun. Occasionally, when he wants to explain his thoughts to me more thoroughly, I will have the good fortune to have a short essay in my fax machine from him when I come to the office in the morning. The one below relates to bicycles. Dad has given me permission to post it. Please don't be put off by the technical terms. If you read the whole essay, its meaning will become apparent. Here is an essay from my hero, my father.

A bit of introduction to this essay will be helpful. Early in 1994, I was telling my father that one famous tubing company boasted that by regulating the tensile strength of the various tubes in its tubesets, the tubing manufacturer could optimize the softness and the hardness of the ride. In other words, a 1200 newton/square mm .8mm thick chainstay would be stiffer than a 800 newton/square mm chainstay of identical proportions. It would convey that stiffness to the rider in both greater efficiency and harsher ride. Not having an engineering background and no reason to disbelieve such a well-respected company, I repeated it to Dad. My father's baloney detectors were set very high that day. "Not so", he said. What followed was a long series of faxes to tubing companies trying to see how this "variable stiffness" theory was arrived at, and what experimental evidence existed to support this view.

The answer that we got from the original tubing company that started this was that they believed it to be so, and that professional riders had told them that it was true. Gianni Gabella, then the head of Columbus, on the other hand, concurred with my father. In these discussions some other characteristics of steel were brought up. One was the question of shock absorption by steel frames, an obvious segue from the original question of "variable stiffness". The following essay by my father to Signor Gabella and me follows. The reader will note in the opening paragraph that my father has his tongue firmly planted in his cheek. Signor Gabella asked my father's permission to translate it into Italian and deliver it as part of a speech to Italian engineers and journalists.

 

July 12, 1994

Dear Messrs.

It has come to my attention that there is an urgent need to establish a common set of terminology. Inasmuch as this is the world's foremost authority on matters of engineering we will attend to this matter forthwith. Please refer to the schematic sketch below.

This is the schematic used in the elementary vibration problems where the mass, the spring and the damper are all separated although they may all be present in the same physical element such as a bicycle fork. This extreme simplification will serve to define terms.

Those who own English automobiles are already familiar with the term "damper". A damper is a device which absorbs work (a force acting through a distance) and converts this work (a form of energy) into a form of energy such as heat or non-elastic deformation so that this energy will not be available as work, that is a force acting through a distance. Consider, for example, a taffy pull, an oleo strut or the brakes on an automobile.

Next, observe the spring, shown schematically as a zig-zag line. When a force acts upon an ideal (perfectly elastic) spring through a distance, the work is stored as strain energy, a form of potential energy, and is returned as work during relaxation of the spring. This compliance (the inverse of stiffness) of the spring depends upon the configuration and the material properties of the spring. Consider, for example, a coil spring wherein the stiffness depends upon the configuration and the value of the modulus of rigidity (G) because the deformations are almost entirely shear; or a cantilever spring wherein the stiffness depends upon the configuration and the value of Young's modulus of elasticity (E), because the flexural deformation is manifest as longitudinal strains.

It is seen then, that the function of a spring is almost the antithesis of the function of a damper and that it is important that damping and springing be kept distinct. Also, consider the fact that metal structures are rarely designed to be used beyond the proportional limit (elastic limit), in fact most structures are intended to stay within the elastic limit by a large margin. Exceptions to this rule might be the crush zones of an automobile acting to absorb kinetic energy to protect passengers during an accident or the shear pin used to release a rocket only after the thrust has reached a predetermined level.

It is also important to distinguish between elastic limit and yield stress. By the time a structure has reached its yield stress severe permanent set has occurred and often the structure is destroyed while, apart from fatigue damage, many structures last indefinitely when operated within the elastic limit. In the case of steel structures the life may be extended even further by operating within the endurance limit if the weight penalty can be tolerated.

In summary, then, it may be concluded that high strength steel may be expected to provide a superb spring with very low hysteresis, returning almost all of the energy stored in it as strain energy as long as the elastic limit is not exceeded, but high strength steel cannot be expected to provide damping by means of elastic strains. However, steel springs coupled with appropriate dampers are used almost universally to provide desired compliance and energy absorption.