The rear aluminium insert supports the section of the engine and creates sufficient cooling and ads stiffness. The front insert is used to alter flex as needed depending on each track. The inserts are connected with machined flanged threaded alu nuts from the top, so the carbon wont damage.
The chassis with both inserts is about 60 grams lighter than the same in aluminium.
Serpent brass weights can be used to position the weight in preferred places.