The retention wheels are just riding on the underside of the plywood sheets. If they had a jam at a track joint, those might tear through the plywood or break off, allowing the car to derail and fall off the track. Unsupported plywood edges are weak and not good working surfaces.
Here's a standard roller coaster wheel assembly, with six wheels.
Any 3? If the top two fail, the assembly drops from the rail, doesn't it? If the bottom two break off, the thing could derail in a negative g track part.
I would think that is only true if we consider that cars have more than one of these.
http://formlabs.com/en/company/blog/2014/08/27/from-3d-print...
The retention wheels are just riding on the underside of the plywood sheets. If they had a jam at a track joint, those might tear through the plywood or break off, allowing the car to derail and fall off the track. Unsupported plywood edges are weak and not good working surfaces.
Here's a standard roller coaster wheel assembly, with six wheels.
http://www.themeparkreview.com/forum/files/dsc_0207_3.jpg
Any 3 wheels can fail without serious trouble.