An empirical formula which has been shown to give good values for ring bending strain in small rings has been applied to infrared bending vibrations. The derived dynamics are of a ?pendulum? character, and reasonable agreement is obtained in calculating the C-C-C bending force constant from the empirical formula constant. Resultant anharmonicities are calculated from the ?pendulum? dynamics, and good agreement is obtained with experimentally determined levels up to the 060 level of CO2 and 040 for HCN.