A while back, I decided to try making side bearings for my trucks. I used .012" phosphor-bronze wire (wound around a 1/32" wire for a mandrel) to make the spring. They are anchored to the bolster with 1/32" rivets. The bolster is 3d printed SLS nylon. If I could ever get around to finishing a freight car, I could tell you if they work or not.
