I'm not very familiar with SolidWorks Simulation, but it seems like you are trying to enforce a specified radial expansion on your coil is that correct?
Did you actually slice off part of your coil to get the cylindrical face? If you did slice it, the lost material may have an effect on your results.If you just did a split line, then perhaps the face not really being cylindrical is causing a problem.
Are you actually simulating what happens when the coil is placed over a mandrel of some sort? In that case, I would actually use a tapered mandrel to enforce the expansion.
I'm actually wanting to know what force will be applied by a coil onto a shaft, for a shaft that has a larger diameter than the coil (so the coil has to open up).
I'm modelling it somewhat backwards by applying a force to the inside face of the coil and seeing what the inside diameter expands to. This actually works better for me as I want to achieve a certain force on the shaft and this will tell me the shaft diameter that I need to achieve this force.
I have created a slice on the inside of the coil to get the cylindrical face. It's very small though so I don't think it would have much of an effect on the results.