Creality Ender-4 Linear Raills Y-axis Upgrade

Issue: V-wheels of Y-carriages are causing lots of issues: they are tearing off; If loose, they cause ovals instead of rings, wobbling and print artifacts.

Solution: Replace V-wheels of Y-axis by MGN12x300mm linear rails. Upgrade is rather simple with maximum use of existing mounting plates. All you need is to print four 9mm thick spacers: 2 for front idle pulleys, and 2 for X- and Y-motors to rise the belt up to the level defined by Y-carriages that are sitting on top of linear rail carri