This document provides flexible driveshaft devices. For example, this document provides flexible driveshaft devices that have bi-directionally balanced torsional stiffness properties. In some embodiments, the flexible driveshaft devices provided herein are utilized in medical device systems, such as endoluminal medical device systems. For example, in some embodiments the flexible driveshaft devices provided herein are utilized in endoluminal ultrasonic catheter systems.