A vehicle moves in a horizontal circle inside a vertical cylindrical wall.

Forces: Normal force N provides centripetal force (N = mv^2/r). Friction (f = mu*N) supports the weight (f >= mg).

muN >= mg => mumv^2/r >= mg => v >= sqrt(rg/mu)

Minimum speed: v_min = sqrt(rg/mu). Below this speed, the vehicle slides down.