Most modern coasters operating on tubular steel rails have three wheels on each assembly: The large drive wheel above the track which bares most of the weight, an up-stop wheel underneath the track, and a smaller wheel mounted at a 90-degree angle from the other two wheels which holds onto either the inside or the outside edges of the track. Serpent uses a two-wheel configuration thanks to the large flange on the drive wheels.
©2013 Joel A. Rogers.