Underfriction

A roller coaster wheel assembly. The underfriction wheels are on the bottom. The three sets of wheels clamp onto the track.

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.^[1]

References

1. ↑ US patent 1319888, John Miller, "Pleasure railway structure", issued 1919-10-28 

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