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 => mu$mv^2$/r >= mg => v >= sqrt$\frac{rg}{mu}$

Minimum speed: $v_{min}$ = sqrt$\frac{rg}{mu}$. Below this speed, the vehicle slides down.