For the motion of a motorcycle along the walls of a death well of radius r,
The normal reaction(R) from the wall supplies the force necessary to obtain motion in a circle.
ie R = (mv^2)/ r
For the body to stick on to the wall, the pull of gravity downwards must be balanced by the frictional force.
If the coefficient of static triction between motorcycle and wall of well is u,
Frictional force fs=u R
Minimum speed will be the one for which the maximum frictional force exactly equals the gravitational pull on the mass.
Lower frictional force will not be strong enough to provide the balance.
Maximum frictional force fs=u R
so for the motorcycle to stick on the wall
maximum frictional force=gravitational pull on the mass.
Hence the above gives the minimum velocity of motorcycle to stick on the wall.