Well, maybe I've misunderstood the mechanism, then. My answer only applies in this situation
Code:
  |                   |
  |NNNNN<O>NNNNNNNNNNN|
  |                   |
  +-------------------+
where the kingpin with the two springs on is held in a constant-sized bracket, and the wheel is attached at bearing <O>, which can freely move along the pin. Because of the linearity, isn't this just the same as Wikipedia's parallel-springs picture
Code:
|
|NNNNN/\
|NNNNN\/
|
except with one of the springs having a negative spring constant?

Peter