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Flat or stepped flat surface on the bottom of open wheel and sports prototype racing cars. Theory has varied along with aerodynamic developments and regulations, from the sidepod tunnels of ground effect to the flat undertrays of the 1980s in various attempts to use aerodynamics to suck the cars closer to the bitumen, minimising the air underneath the car that could slow its progress. Today most such categories feature a stepped undertray with sidepods siting higher in the air than the centre of the car, usually mandated by series organisers in an attempt to limit vehicle performance. Also refers to flat surfaces extending behind splitters in sedan and GT based racing cars.