In general, when resins are used for outer panels of a vehicle, all of them are made of resin including its doors like General Motors Co LLC's "Fiero." Daihatsu probably wanted to make all of the vehicle's outer panels by using resin, considering that the color of the vehicle will be changed by replacing outer panels. But it seems to have given up the idea this time.
The outer panels are fixed to steel parts with Torx screws, which function as an anti-theft measure, compared with hex screws. But wrenches for Torx screws are widely used now. So, there is still a risk.
The floorboard of the new Copen was designed based on the lower body of Daihatsu's "Mira e:S." Because the number of doors decreased from four to two and the outer panels are made of resin, the side panels were newly designed. Under the floor, an X-shaped reinforcement was added, and a transverse reinforcement was installed in front of the X-shaped reinforcement.
The thickness of the reinforcements is probably 2.3mm, which is about four times thicker than that of the original vehicle's reinforcements. In terms of stress distribution, it is close to the "X-shaped frame," which was used before the development of the monocoque structure. The reinforcements were attached to the floorboard with M10 bolts.
Furthermore, Daihatsu added another reinforcement that connects two places near the points to which a torsion beam is fixed. The company aims to prevent the auto body from being distorted by a transverse force applied to the points.
As a result, vertical flexural rigidity and torsional rigidity were improved by 200% and 50%, respectively, compared with the previous model of the Copen.