Acceleration Stress: the coaster approaches the vertical loop, entering at a high rate of speed. Entry may create a heavy force on the riders as high as 5 g, but only for a few seconds--any longer and riders may blackout from acceleration stress. Modern vertical loops are tear-shaped, clothoid loops that have a tighter curve at the top, where the velocity is smaller, than at the exit and entry points at the bottom.
The larger radius at the bottom of the loop reduces centripetal acceleration, thereby lowering the overall g-forces, which naturally increase by 1 g owing to gravitation. Often the designer will adjust the radius at the top of the loop to create a centripetal force of 2 g, allowing the riders, after subtracting the value of gravitation (1 g), to experience normal weight levels while upside down. The queasiness associated with looping rides is owing to some internal organs floating upward, counter to their natural hanging position.