Charles Brown has created four great documents detailing many of the fundamentals and esoterica of recumbent vehicle design. 

• Bicycle frame design explores different types of recumbent bikes and some good criteria to consider when you are designing a frame.
• Steering and ride details what makes a recumbent bike handle well.
• Air drag formula provides methods to determine and reduce the air drag of unfaired recumbent vehicles.
• Putting It All Together takes the concepts discussed in the previous articles and summarizes them