welcome to the site. the difference between the 70cc and 50cc bicycle motor is tourque or pulling power. the 70cc was designed for towing or carrying weight so if like me and a little over weight the 70cc is the motor to go with. the 50cc was made for riding and cruising and not towing etc. so depends on what you want. speed wise both do about the same. glad to have you with us and hope that helps answer some of your questions
