Well I am a great beyblade anime fan but here is a small trick that you may use to make your blade spin for longer. Try to make it somewhat heavy. For instance put a circular thick iron ring inside it and pull the rip cord with all you got! Making the beyblade heavy will increase it's moment of inertia and thus the beyblade will spin for longer time (according to the laws of physics). Also it increases it's defensive strength. It actually worked for me. 
dragoon.shubham

dragoon.shubham