Traumatic brain injury (TBI) affects millions of people worldwide with significant personal and social consequences. New materials and methods offer opportunities for improving designs of TBI prevention systems, such as helmets. We combined empirical impact tests and computational modelling to test the effectiveness of new viscoelastic add-on components in decreasing biomechanical forces within the brain during helmeted head impacts. Motorcycle helmets with and without the viscoelastic components were fitted on a head/neck assembly and were tested under oblique impact to replicate realistic accident conditions. Translational and rotational accelerations were measured during the tests. The inclusion of components reduced peak accelerations, with a significant effect for frontal impacts and a marginal effect for side and rear impacts. The head accelerations were then applied on a computational model of TBI to predict strain and strain-rate across the brain. The presence of viscoelastic components in the helmet decreased strain and strain-rate for frontal impacts at low impact speeds. The effect was less pronounced for front impact at high speeds and for side and rear impacts. This work shows the potential of the viscoelastic add-on components as lightweight and cost-effective solutions for enhancing helmet protection and decreasing strain and strain-rate across the brain during head impacts.