You are basically making a progressive spring. This is offered by a couple of different companies. I am not sure where to get these springs other than at A Main hobbies. That has been the only place I have seen them at the present.

Picture of the springs I am talking about.



Link to the detail page about them.

http://www.rcuniverse.com/forum/m_5677620/tm.htm

They have other models availible as well. A buddy used these on his MBX5 and it seemed to help quite a bit on the rougher tracks we run on.

If you are thinking something is wrong with the picture because the middle of the spring seems to be wider than the top or the bottom, your eyes are not decieving you. By increasing the diameter of the coil it will make the spring softer. So you have a dual rate spring like you are trying to produce above but there are no issues with alignment of the two springs or chasis droop.