Roller coasters allow people to experience simulated extremes of motion and movement that would otherwise be dangerous or outright impossible. To achieve these feats, including going upside down and moving at very high rates of speed, it is necessary to have several types of wheels on roller coaster cars functioning at different times during the ride. These wheels serve unique functions and are made of various materials.
Roller coaster wheels, regardless of which of the three types they may be, are generally made of the same materials. The main portion of the wheel is most often steel, which is coated with nylon, hard plastic or rubber on the outside. The main purpose of the non-steel coating is to avoid the unpleasant noise of direct steel-on-steel contact as well as to minimize heat generated by the friction of the wheels against the metal track.
Guide or or running wheels are the main wheels that roller coasters rely on to function. These are connected to the underside of the roller coaster car and roll adjacent to the inside or outside of the coaster's track (on the side). Guide wheels serve the purpose of directing the train in turns of varying degrees and prevents the train from flying off the rail or track.
Road wheels or friction wheels are attached to the roller coaster car and roll on the top of the roller coaster's track. These wheels are designed to keep the car from shifting to the left or right while traveling the roller coaster's track. Without them, the roller coaster would wobble towards each side.
Upstop wheels or underfriction wheels are attached to roller coaster cars and roll beneath the track. These wheels secure the car to the track and prevent the car from flying off the tracks during inversions such as barrel rolls and loops, as well as when approaching the top of hills and evening out or dipping quickly at higher speeds (and consequently negative Gs). This wheel is the one that made roller coaster inversions possible in the first place. Early roller coasters lacked upstop wheels and relied on a brakeman who rode the roller coaster as well. The brakeman would have to decide when the coaster was going too fast and apply brakes to slow it down and prevent derailing.