Assuming we're talking about the Xray T4 2017 - the car comes with a graphite chassis, with the exception of the US, where it was sold with the flex aluminium chassis. Both chassis (plus the non-flex alu chassis) are available as tuning parts. The graphite chassis is mostly used for asphalt / low grip while the alu chassis is for carpet / high grip racing, as it has less flex. So depending on what surface you're planning to run on, you choose your chassis.