Roller coaster wheels are designed to prevent the train to the track while it travels through fancy loops and twists. Not A Perfect Circle When you go upside down on a roller coaster, inertia keeps you from falling out. This resistara a change in motion is stronger than gravity. It is what presses your body to the outside of the as the train spins around. Gravity keeps pulling you toward the Earth when you go upside down, but inertia pushes you against the floor of the roller coaster car. This pushing force is stronger than gravity. The loop cannot be a perfect circle, though. If it was, the pushing force would be too strong for safety and comfort. For that reason, roller coaster loops are elliptical. They are shaped like stretched-out circles. America's First Coaster The earliest version of the roller coaster was a Russian sled ride from the 1400s. It was called Russian Mountains. what's the main idea