The complex motion of a slider-crank connecting rod has motivated analysts to work in terms of an “equivalent link,” comprised of two point masses at the ends of a massless link, where one end is located at the crank pin and the other end is at the wrist pin. It has long been known that this limited model is not fully equivalent in the dynamic sense, but the practice persists and errors are routinely introduced into torsional vibration and shaking force calculations. The purpose of this paper is to expose this error and show the nature of its effects. This is accomplished by means of a fully correct analysis, based on the two point mass model extended to include a massless additional mass moment of inertia, and then examining the terms that the usual model omits. Numerical results are given for several actual automotive engines.

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