A system of passive balancing devices could potentially be used to suppress vibrations in helicopter tailrotor driveshafts. Passive balancing devices for rotary shafts consist of masses restricted by concentric guides about the shaft axis. At supercritical shaft speeds, the balancing masses automatically adjust to counter imbalance due to uneven load distribution. The problem is highly nonlinear and requires comprehensive modeling to achieve satisfactory prediction of the balancing behavior. A frequency-scaled tailrotor driveshaft test rig was fabricated to test the performance of a passive balancing device and to validate a comprehensive model. The model includes balancing mass collisions and balancing mass interaction with the balancer track through friction. Experimentally, the passive balancing device on average reduced driveshaft transverse vibrations by 62% at steady-state. Models available in the literature predicted vibration amplitudes to within 68% of the experimental values. The new balancing model improved the prediction of shaft vibration amplitudes by a factor of 3.9 when compared to published models (18% vs. 68%). This suggests that friction and mass collisions cannot be ignored in passive balancer modeling and that passive balancing is a viable solution for suppressing driveshaft vibrations.