DeFeet UnD Shurt Long Sleeve Baselayer. The DeFeet UnD Shurt long sleeve baselayer is made to wear under a cycling Cycling Jersey. Thin and form fitting, the UnD Shurt allows cyclists to dial in their clothing for specific temperature ranges without adding much weight or bulk to their outfit. Perfectly suited for temperatures from 32 80 degrees Fahrenheit, the UnD Shurt has a thermal property that keeps you comfortable on cool descents but doesn't overheat on climbs. Made from post consumer recycled polyester certified by Repreve, the UnD Shurt is easier on the environment, too. The original product in the DeFeet UnDline of base layers, the UnD Shurt has been around since 1997.