A burn doesn't mean you're working the muscle well. Bicycle crunches are bull in my opinion; I've seen people do them, there's very little spine movement there; they just turn their heads towards the opposing knee. The best way to do hanging leg raises is bent knee and to bring them up as high as possible. Try to knee yourself in the teeth; it's not easy to explain but like the airforce, aim high. Have someone watch while you do it, your ass should fly foward and your spine should flex (yay, mission accomplished)