Recently aerodynamic devices have become larger to reduce aerodynamic drag. Some structural problems have been experienced when attached to lightweight cabin roofs. On-road and wind-tunnel tests are presented to document the forces generated by the devices to provide information to the truck and device manufacturers. Two-component transducers measured vertical and horizontal loads at device attachment points and for one test a truck front axle was strain gauged. Data were recorded under a range of road conditions, including rough roads and heavy braking, on a Mercedes Benz V2244 prime mover fitted with a Rudkin-Wiley roof fairing and an International Harvester Australia (IHA) “S” series prime mover fitted with a Fuelscoop roof fairing.The results showed that dynamic loads arising from the road roughness are causing structural problems rather than the aerodynamic loads. It was concluded that a low mass device with a carefully designed support system is required.