Head injuries cause a large proportion of fatalities to road users particularly motorcyclists. Currently, helmets are assessed using rigid headforms and criteria related to acceleration measured at the centre of the headform. This system is not representative of the human head, which comprises a brain that moves relative to the skull in an impact. Finite element techniques were used in the design, development and validation of a new dummy head prototype named Bi-mass 150. It has been constructed using a Hybrid 111 headform and comprises two masses: a skull and a mass to represent the brain attached to the skull with a damped spring system. The novel feature of this device is that it can simulate the brain -skull relative displacement at a frequency close to 150 Hz. This is the frequency recorded in tests in vivo. The prototype was made by removing mass from a Hybrid 111 dummy head and fixing to the centre a metallic mass, which simulates the brain. The two components were connected by means of a plastic element such that the natural frequency corresponded to the 150 Hz measured in-vivo. The outputs from thirteen accident reconstructions, using an instrumented helmeted headform, were used as inputs to the FE model. The results are the first part of the process to establish bi-mass related injury criteria, and, in turn, to provide a much more human-like headform for evaluating and optimising head protection systems.