From the BUB Racing Rule Book 2013:
2. M. (IV) LEATHERS
Suits made of synthetic material are not permissible. The use of stretchable Kevlar and perforated materials in non-critical areas are permissible. Leather suits shall be one-piece design or joined by a full circumference zipper at the waist. Leathers cannot be too big or loose. Critical area (knees, elbows, forearms, shoulders) armor or 2-layers of leather is highly recommended. Due to invisible flames, riders’ of motorcycles burning fuels of Methanol content are recommended to wear Nomex underclothing or something of similar nature.