a wheel assembly having a braking mechanism that is actuated by centrifugal force . the braking mechanism includes a rotating hub insert having a first pocket , a thread guard having a second pocket fixed relative to the frame of the wheel assembly , and an internal ball between the rotating hub insert and the thread guard . the internal ball can be flung by centrifugal force from the first rotating pocket to the second fixed pocket to cause a braking force from contact of the ball in the second pocket with the rotating hub insert . the amount of centrifugal force required to fling the ball into the second pocket can be calculated for rotation of the wheel greater than a certain predetermined speed .