As the test build model progressed, some major issues were brought about by the undertray. While conceptually more "realistic", the undertray construction as separate component from the chassis floor created fitment and attachment issues. The tub and sidebox assembly difficulties noted in Process 5 still need to be addressed as well.

Typical prototyp cars (the Sauber C9 and Eagle MkIII, for example) are designed with a central chassis monocoque and separate sideboxes all sitting on a separate floorpan and undertray assembly. The model assembly is to simply build up an entire undertray/floor/sidepods unit and drop in the monocoque. Tony Southgate's Jaguar designs (and his later Toyota TS010) use a full-width monocoque, incorporating the sideboxes and forward floor as integral stressed components. Replicating this in the model has proved to make it really difficult to build.

So while a long list of parts are rearranged and redesigned, the driveline's back-end is test-built. This assembly has to be fairly rigid as it will have all the rear suspension attached to it and will actually hold up the back of the car.