I get a minimum height of 2.5R. So, if the loop is 4 meters high (with a radius of 2 meters), the car will have to start 5 meters from the ground to just make the loop. Of course, this assumes that there is no energy loss due to friction; you’d probably want to start a little higher to account for this.

But not too high…

In fact, why cut it close? Why not start a lot higher and eliminate all doubt? The reason is that the faster the car goes, the greater the g-forces experienced by the driver in the loop.

Let’s think about it: if you release a car to go around the loop at minimum speed, there will be zero track strength (F_T). You would feel weightless – zero g – for a moment. If the car is dropped from a height greater than 2.5R, its speed would be greater than the minimum at the top of the loop. To continue moving in a circle, gravitational force would not be enough. The track should also press on the car. This would create a g-force greater than zero.

Let’s return to video of the real waterfall. Comparing the loop to the loops, I imagine 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 is 8 meters. The force at the top of the loop (divided by the weight, to get it in g) would be 3 g. It is possible for humans to withstand up to 20 gso it should be fine.

But what if you get extreme? If you start too high and the loop is 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 might sound cool, but it could also kill you. It’s not fun anymore.