At the bottom of the wheel, the weight acts downwards and the support force acts upwards. The centripetal force is given by S - mg, where S is the support force and mg is the weight.At maximum height, the weight acts downwards and the support force (reaction) from the seat acts upwards so the centripetal force is given by mv2/r = mg - R.When v2 = gr, there will be no support force on the person from the seat i.e. R = 0.The centripetal force F = mv2/r = mω2r