Backlash Belt Test

The carriage has a short and long side (around the pulley) end of belt - if there is a difference in the gaps left and right side. Your belt is too elastic or broken. Or there is too much resistance. This backlash will causes problems with clearances. 

In spiral mode the upper side is printed in one direction while the lower side tensions the other side of the belt when moving the extruder.

To stabilize the print there is a top to connect the sides so it will bend less.

Print this for X and for Y 90° rotated, position the print near the motor on the bed so the short end is short as possible because that is where deviations are biggest.

Results

here you see signs of a bad belt (over tensioned causing resistance). The gaps on the left side (of the lower row triangles)  are much bigger than on the right, because the carriage lagged more behind when pulled from the other side. A loose belt will show similar signs as there is play in the movement. 

Wanted Results

 Here you see more equal spaced sides, but still not perfect.

(you also need to look perpendicular or just print 2 layer)