I ride a rocky mountain hammer mtb. I've noticed that when I descend down smooth (paved) slopes that my front wheel will begin to spontaneously oscillate, beginning slowly but quickly reaching the point where the oscillation is dangerous. It's self amplifying. I must add that this only happens when I'm relaxing with both hands off the grips and going around 30km/h. It does not happen at slow speeds and there is no noticeable warping in either the front or rear wheels. My question is if there are any other factors that can cause this kind of thing other than the wheels being warped? Mabye frame issues? Any engineers or physicists in the house?