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.
NASCAR outlawed the practice of intentionally modifying side skirts in 2015 after teams regularly manipulated the steel part during pit stops. But it still was possible to have a side skirt become flared out because of contact with the asphalt, which could result in a performance advantage (but without drawing a penalty; it’s believed there were no penalties for side skirt modifications last season).
The switch to a composite side skirt should reduce the potential for side skirts to be altered in a significant way.