I've a dynamic linkage in 2D. Imagine something like an articulated bicycle suspension. I've used constraints to create the dynamic simulation. All points and lines have been drawn on the same drawing plane. Everything starts out working, and then out of nowhere, horizontal lines that are constrained in line and parallel to each-other (joined at a point), swap around that point of intersection (segment that was on right is now on left and vice versa). Same with vertical lines joined at a point??

Q 1: How do I set things up so that this doesn't happen?
Q 2: Once it does happen how do I fix it?
Q 3: Is there some way to drag a line even after it has been constrained?

Note: Let me explain more clearly. Imagine a two vertical lines joined together at a point. Both are constrained vertically. The top line is constrained in length to 1, the bottom line is constrained to length 4. Now lets say that I have also a horizontal line that is constrained to the position of the joining point of the above two vertical lines. Well what happens is that the vertical lines often end up swapping positions (long one on top, short one below), causing the position of the horizontal line to shift as well and completely screwing up the mechanical linkage I am designing.

????