A stressed member engine is a vehicle engine used as an active structural element of the chassis to transmit forces and torques, rather than being passively contained by the chassis with anti-vibration mounts. Automotive engineers use the method for weight reduction and mass centralization in vehicles. Applications are found in several vehicles where mass reduction is critical for performance reasons, usually after several iterations of conventional frame/chassis designs have been employed.
Stressed member engines was patented in 1900 by Joah ("John") Carver Phelon and his nephew Harry Rayner. and were pioneered at least as early as the 1916 Harley-Davidson 8-valve racer, and incorporated in the production Harley-Davidson Model W by 1919. The technique was developed in the 20th century by Vincent and others, and by the end of the century was common feature of chassis built by Ducati, BMW and others.
The 1967 Lotus 49 is credited for establishing a solution copied by "everyone" in Formula One. The V engine layout in Formula One provides stiffness required to use the engine as a stressed member. This requirement is cited as a reason the rules committee changed from an inline-four to a V-6 configuration for the 2014 Formula One season.
^Albert Boretti (2013), "F1 2014: Turbocharged and Downsized Ice and Kers Boost", World Journal of Modelling and Simulation9 (2): 150–160, ISSN1746-7233, "An engine in a F1 car today is a stressed member of the chassis, meaning that it is an integral part of the car. V-type engines have gradually pushed out any other engine type because they are compact and can be constructed very rigidly without requiring further strengthening to the chassis to ensure stiffness"