This frictional force can vary as needed up to a maximum of .
But how much frictional force is available depends on the speed, which means there will be a MINIMUM speed for revolution at a constant height above the ground. This minimum speed will be the one for which the maximum frictional force exactly equals the gravitational pull on the mass (since any lower frictional force will not be strong enough to provide the balance).
Higher speeds will also produce no slipping, but as the speed rises steering and control will become more difficult.
So when , then v is the lowest it can be (ie vMINIMUM ), for no slipping to occur. Which means the minimum safe speed is .