Our eccentric ENO line of hubs are made specifically for bikes with vertical dropouts. The majority of bicycle frames made today are made with vertical dropouts, however, converting the frame to a single speed presents the problem of how to tension the chain. Chain tension with vertical frames can be addressed three ways by using a chain tensioner, using an eccentric bottom bracket, or with our eccentric ENO. The eccentric ENO is an easy, clean, and simple answer to the problem. The eccentric rear hub allows the rider to adjust the chain tension by simply rotating the elliptical axle ends up and back in the dropouts. As the hub rotates back the chain is pulled into tension. Flip/flop design fixed/free. Made in the USA