Knowing what does not work can be more important than know what does.
Originally shared by Whosa whatsis
Like the Z-screw-end bearings, here’s another way to screw up your prints by adding a bearing where there shouldn’t be one. I pointed this out to the guys from Airwolf3d last year when I saw that they were doing it, but last time I checked they were still doing it.
This modification changes the geometry and thus the length of your belt as it moves back and forth. You should be able to feel the difference between having the pulley in the center (belt will be looser) vs having it near the end (it will get tighter). This change in belt tension means that your belt will stretch and wear out faster, and also that your steps/mm value will change slightly as the length of the belt changes, causing dimensionally-inaccurate prints.
http://www.thingiverse.com/thing:196676