how do i design a de dion suspension for a formula SAE car
trying to design a rear suspension using de dion rear axle
First, you need to know something about kinematics theory. Boring, but necesary.
You need this information for you to take the best decision about suspension system selection.
I recommend you this useful books in the next order:
Race Car Vehicle Dynamics (Douglas L. Milliken, Douglas L. Milliken).
Chassis Design: Principles and Analysis
Douglas L. Milliken, Douglas L. Milliken, Maurice Olley
(All of this book are kinematics theory. Its more difficult to understand than Race Car Vehicle Dynamics).
De Dion Axles generally has heavy components. It´s interesting, but i´ll pass.
I recommend you use PTC Creo Parametric. PTC is a sponsor for CDS Formula competitions (i assume). It´s a reliable and powerful software, specially in assemblies with multiple components like suspension system & drive train.