The high X-bone frame
When designing any open car, the loss of a fixed steel roof as a load bearing structure naturally demands compensatory measures to maintain the body’s rigidity. Furthermore, in a finely-honed, high-performance sports car it is even more essential that the body provides a highly rigid structure, thereby allowing the suspension to work to its full potential, uncompromised. And, in terms of crash safety performance, it is vital to provide adequate rigidity to allow front and rear crumple zones to operate effectively.
However, adopting a conventional approach to the problem – that is, applying major structural reinforcement to the side sills and central floor – can all too easily increase the weight of the car to a degree that harms performance, fuel economy and emissions performance, while the rigidity rarely matches that of a closed car.
Honda’s engineers therefore opted for a totally clean-sheet approach and their solution offers rigidity and passenger protection equivalent to that of a closed body, but in a lightweight bodyshell.
At its heart lies what Honda has termed a ‘high X-bone frame’. While the bodyshell is in effect a monocoque, what sets it apart is the manner in which the frame rails and centre tunnel combine to form an exceptionally rigid body-chassis. The ‘X’ element of the high X-bone frame refers to a notably deep centre tunnel, box-shaped in cross section to offer good resistance to torsional loads, and a pair of ‘legs’ at either end running diagonally. Those at the front connect directly with the front longitudinal frame rails to cradle the rear of the engine; those at the rear abut the rear chassis frame rails.
