What is the correct orientation for the X-axis belt?
I first thought to place the smooth side down to match the tensioner bearing, but then changed it to match the motor pulley.
Sadly, the Ender-3 assembly instructions were not clear on step 7, what the right direction is (teeth on inner side of the loop):
The Belt has teeth that need to engage the teeth of the hobbed gear on the motor, just like in the preassembled Y-axis. The teeth have to go aim to the "inside" of the loop. This way, the belt will not slip on the motor side and the movements will be repeatable.