Bass notes of pianos exhibit high degree of inharmonicity that means that frequencies of overtones depart from integer multiples of the fundamental frequency. Small amount of inharmonicity is tolerable for musicians and listeners, but higher ones cause unpleasant metallic tone. Reducing inharmonicity the first step is to investigate all influencing factors. This paper deals with effects of bending stiffness and nonuniform winding of wounded strings. As there is no reliable information in literature on bending stiffness data, a measuring procedure has been elaborated that is based on analysis of frequencies of overtones and the mathematical model of a vibrating string. Gained bending stiffness data opens the door to further quantitative investigations to reduce inharmonicity and improve sound generation methods of digital pianos.