Roller coaster wheel assembly
On a roller coaster, the wheel assemblies are the point of contact between the cars carrying the riders and the track rails. They often consist of at least three wheels per assembly, but can contain more. The typical arrangement of the wheels are: Road wheels (also known as running or tractor wheels): Positioned on the top of the rail, bearing the weight of the train. Side-friction wheels (also known as guide wheels): Positioned on a horizontal plane, either inside or outside of the rails. These hug the sides of the rail, keeping the train centered. Upstop wheels (also known as underfriction, underlocking, or uplift wheels): Positioned under the rails to keep the train from lifting off the track.
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