http://books.google.com.au/books?id=MgzUK0vM0PkC&pg=PA182&lpg=PA182&dq=theory+of+nutating+disc&source=bl&ots=T0H_7ELpcB&sig=Nq7w4IenryNeIJWpHg7TWc2mvaw&hl=en&ei=Q7GkScaEK4-1kAWujZ25BQ&sa=X&oi=book_result&resnum=4&ct=result