It's been 40 years since the first BMX-specific double-diamond bike frame was manufactured. Ever since then, companies have been constantly been looking for some way to get an edge over their competitors, whether it be by signing up riders or introducing new technologies â€” generally via signature product. ESPN took a look back to determine the 20 most progressive signature BMX frames of all-time. What, no Standard Bullitt? Check the list for yourself here.