Underfriction
![A roller coaster wheel assembly. The underfriction wheels are on the bottom. The three sets of wheels clamp onto the track.](/Images/godic/202502/18/Roller_coaster_wheels3308.jpg")
On a roller coaster train, the underfriction, up-lift, or up-stop wheels are a device to keep the train from jumping off the track under intense movement. The design was patented in 1912 by John Miller.
Underfriction
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