My Prefix is short for a 2 word phrase
My Infix can be as round as a every circle
My Suffix is a common preposition that consists of only 2 letters
My whole can make your head dizzy
Now tell me, what am I?
It should be
Spin. Prefix is SP for Starting Point (Credit to Kevin) . Infix = Pi which is needed to calculate a circle parameters. Suffix = In which is a common preposition. And spinning around makes you dizzy!
Prefix: I'll, short for I will
Infix: usi, not sure
Suffix: On, a common preposition
An illusion can make you dizzy