You need to add an blank-mesh - just make a copy of the upper part, delete all vertices there are in it (its just to have the origin/pivot point at the same place - also, mind to keep the mesh name noted somehow for the later scripting (just add a "BLANK" word to it)) and parent the original to this blank. Then make an additional movement script for the "copy-mesh", witch will have the height set as the wheel-height (see wheel data acquisition in the OT-Importer-wiki). - the idea is, you can have just one movement/rotation script on one mesh, so the blanks have to be created to combine the different movements/rotations - but keep in mind that all the parented stuff does them in accordance to it.
- So this one first makes the blank one go up and down thanks to the wheel-height data and then the parented one (the real object mesh - witch moves up and down too, cause its parented to the blank) has a separate script for the steering rotation (witch you already has).