I get a minimum height of 2.5R. So if the circuit is 4 meters high (with a radius of 2 meters), the car would have to start 5 meters off the ground to fair do the loop. Of course, this assumes that there is no energy loss due to friction; You probably want to start a little higher to account for that.
But not too high…
In fact, why cut it? Why not start a lot higher and eliminate all doubt? The reason is that the faster the car goes, the greater the G forces the driver experiences on the circuit.
Let’s think about this: if you let a car go around the circuit at minimum speed, there will be zero track strength (Ft). You would feel weightless (zero gravity) for an instant. If the car is released from a height greater than 2.5R, its speed would be greater than the minimum at the top of the circuit. To continue moving in a circle, the gravitational force would not be enough. The track would also have to push the car down. This would create a g force greater than zero.
Let’s go back to the real trick video. Comparing the loop to the passersby, I assume it has a radius of 2 meters. The car is clearly released from a height greater than the minimum of 5 meters (let’s say it’s 8 meters). The force at the top of the loop (divided by the weight, to get it in g) would be 3 g. Is It is possible for humans to resist up to 20 g.so this should be fine.
But what if you go to the extreme? If you start too high and make the loop too small, bad things can happen. How about a height of 20 meters with a radius of 1.5 meters for the loop? This would produce a force of 21 g. It may look cool, but it can also kill you. That’s not fun anymore.