05-19 YAMAHA TTR230: Dr. D NS-4 Complete Exhaust (Stainless Steel/Aluminum) Every DR.D exhaust is built, developed, and tested by Doug Dubach. DR.D pours more time into track testing, with more experience, than any other company. DR.D exhausts are not mass produced, each exhaust is unique and each has been developed on the track to best improve the bike&#39;s power character. You won&#39;t find &#39;universal&#39; components, such as heavy springs and gaskets, in their construction because cutting corners, on the track or in production, is never beneficial. Take it from the man who has spent more testing time on a Honda, Suzuki, KTM, Kawasaki, and Yamaha than all others combined.