There is a very slight inward angle of the wheels on the fully 3D printed model. Looking at the construction the twist seems to be due to the weight on the wheels pivoting slightly over the 15mm/30mm links holding sub-assembly 1 and sub-assembly 3 in place.
The dimensions of the links should be verified as it may be too short on the flat side of the link.
There is a very slight inward angle of the wheels on the fully 3D printed model. Looking at the construction the twist seems to be due to the weight on the wheels pivoting slightly over the 15mm/30mm links holding sub-assembly 1 and sub-assembly 3 in place.
The dimensions of the links should be verified as it may be too short on the flat side of the link.