Side skirts have been the center of multiple controversies in NASCAR for several seasons, but a NASCAR-mandated construction change could limit the attention this year.
On Cup Series cars in 2018, the fender component will be made of composite material (the same being used for Xfinity Series bodies) instead of steel.
The change is intended to ensure that side skirts will hold their original shape unlike the steel side skits that could be deformed. Bending or flaring out a side skirt in front of the rear tire can provide more downforce and sideforce.
— NBC Sports —