Thought I'd share some fine tuning I've had to do when making rolling stock items using the adjustable chassis with a 3D printed shell placed over the top.
I make the shells to fit over the cut down adjustable chassis...and I make them a nice snug fit. I design in a "shelf" in the inside of each end for the chassis to sit on so the bottom of the shell is flush with the bottom of the chassis. Here's a photo to show what I mean:
When testing the finished (in this case) carriage, it tended to derail on curves and I noticed that one of the bogies was catching very slightly on something when rotated gently. On investigation, I found it was catching on a minute burr on the edge of the chassis. When I checked more bogies, I found one or two tended to catch on the underside of the shell that was a fraction of a mm too low!
To cure both of these issues, I attacked the bogies and chassis with a brand new scalpel blade....and took a sliver off the corners of the edges of the underside of the chassis, and the outside top edges of the bogie frames. See the photo below. The burr was at point A, and one of the edges of the bogie frame is marked B.
A re-test after these mods resulted in no derailments!