Here you go. If you have a straight track and then you have a circular loop you'll feel no extra force until the circular loop's start and then all of a sudden you'll feel a lot of force because you are suddenly turning around the arc. This is really uncomfortable so in roller coaster design they make the turns ease-in gradually. The track is mostly straight at first and rises quite a bit while the turning radius decreases. At the top the turning radius is the smallest for the tightest part of the curve. Then the track eases out to get you back to the initial lack of extra force. If you draw something like this you get a teardrop shape.