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 1919 by John Miller.^[1]

[edit] References

1. ^ US1,319,888 (PDF version) (1919-10-28) John Miller, Pleasure railway structure. 

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