Centrifugal. As the wheel spins, the force acting on the valve is in an outwards direction. As the wheel speed increases, so does the centrifugal force (g) to a point where it overcomes the rubbery bit and the metal bit moves towards the rim. High enough wheel speed will give enough centrifugal force to exceed the shear limit of the rubber and off it goes. The force is quite large - if you consider those spinny things used in astronaut training - the person gets quite badly squashed down and pass out under single digit g. Assuming a 17" rim and a 10 gram valve, your valve will undergo somewhere in the order or 350-400g at 70mph. Just because I like stiring things up. It's not centrifugal force that makes your angled valves rip out, it's the fact the valve isn't strong enough to provide sufficient centripetal force to prevent it ripping out. Like a heavy weight spinning round your head on a bit of string, it's not the centrifugal force that breaks the string when you get too fast, it's that the string can't provide sufficient centripetal force to keep the weight spinning - the required force exceeds the tensile strength of the string so it snaps and the weight flies off in a straight line as there is now no centripetal force causing it to move in an ark.. Centrifugal force doesn't exist.