The importance of torsional stiffness
I am a big fan of Kevin Cameron, the motorcycling journalist who has written for American motorcycling publications for what seems like ever. I don't often get to see a copy of Cycle World, but I have enjoyed reading his books immensely. In one of them, an article of his from 1984 is reprinted, on the subject of con rods. The subtitle reads, "A well-designed connecting rod is a steel or titanium image of the stresses it must carry. A kind of art".
Certainly the latest con rods for high-level motorsport would perhaps open Cameron's eyes a little, but he would probably still see them as art. The relationship between shape and stress has been somewhat clouded by other important considerations. Con rods are not, on the whole, highly stressed parts/assemblies. Their shape is defined by providing direct load paths. Clearly, stress is a very important consideration in certain areas of the rod, especially where assembled and bolted rods are concerned.
However, much of a con rod's design is concerned with stiffness. A con rod has more than one stiffness and many natural frequencies. How many of these natural frequencies are relevant depends on their frequency and the operating speed of the engine, which serves to excite the rod, eliciting a response. Axial stiffness is an important consideration if valve-to-piston clearances are marginal. Many people push these clearances to the limit in order to achieve high compression ratios.
Torsional stiffness is an often neglected quantity; the torsional resonance of a con rod can also have an effect on compression ratio, as it affects the clearance around the circumference of the valve to the pockets provided. Large resonant amplitudes require large pockets, and these serve to lower compression ratio. Calculating torsional stiffness and natural frequencies can be as important for a con rod as a crankshaft. If a major torsional mode is excited by an engine speed within the operating range, we had better try to avoid constantly running at this engine speed, lest con rod torsional failures ensue.
In the past, designers of crankshafts for ships were required by law to undertake extensive calculations of the torsional response of the crankshaft/propeller shaft system to excitation, and to recommend engine speeds that must be avoided except in transient conditions. If we are going to design con rods for minimum mass, we ought to think about doing the same calculations for the con rods and their associated components. Some con rod companies who are expert in such things may ask for details of piston assembly inertia to aid them with their calculations.
On the internet you can easily find pictures of Formula One con rods, which range from the quite simple to the very complex. All are designed with considerations of torsional stiffness and natural frequency in mind. However, some of the more outlandish designs, which might appear to incorporate strange shapes and design features in the beam, are conceived to show the kind of features that may be produced for reasons of beam stiffening.
Written by Wayne Ward
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