Case inner diameter
The case inner diameter is the inner diameter of the spring chamber. It determines the amount of space that is available for the spring to unwind into, as it unwinds from the axle. By default, the inner diameter is calculated so that the cross sectional area of the spring (that is, its thickness times its length) takes up 4 percent of the total spring case area.
If you think that the spring case is too big for the length of spring that you want to use, you can select Preferences from the menu, and set the Case Ratio to a figure larger than 4 percent (perhaps 10 percent). However, doing so will reduce the amount of distance that your car can travel, because there will be less room for the spring to unwind.