Have you tried to trace a line in space with the morph tool to make handrails while AC rotated the anchor point and tube surface while changing planes? This article will show you step by step how to avoid this problem.
1. Introduce a horizontal section
A horizontal section need to be set up just before the turn.
2. Choose starting point
It is important to correctly choose a starting point, otherwise it will be easy to create slanted tubes.
The tube will not have a twist if:
1. The given segment is horizontal
2. The segments before and after the given segment are parallel with each other
3. The given segment and the segments before it and after it are on the same vertical plane
The image below exemplifies how the above requirements are met and create a tube without a twist.