My attempt,
N2 for the y-direction reads,
Ny-mg=ma_y or,
Ny-mg=-mA(2π/λ)^2 v^2 cos(2πx/λ).
When the wheel loses contact, Ny=0,
-mg=-mA(2π/λ)^2 v^2 cos(2πx/λ)
so,
v^2=g/(A(2π/λ)^2 cos(2πx/λ)) (*)
but when x=nλ, cos(2πx/λ)=1 and a this corresponds to local maxima on the cosine...