Note that the best thing that you can do is cover up the radiator for a 7% reduction in drag (in wind tunnel tests they taped over it.) Also, contrary to what we have always been told, making improvements to the REAR of the motorcycle did little to reduce drag.
I am not vouching for the veracity of all this, but all testing was performed by an engineering student at Union College in NY.
OoooooooooKaaaaayyyyyyy, we need to take this with a BIG slab of salt.
A partially faired motorcycle is about the most complex aerodynamic object on the planet. The flow is virtually all separated (not "turbulent") aft of the engine and rider. Forks, radiators and fairings create as much stagnation as they do streamlining.
Tails make the largest difference to separation drag and stability, visit
www.ihpva.org and explore the links. Recumbants with no front and elaborate tail fairings out-perform bikes with front fairings and no tails. I didn't see anything in the research about stabilizing the aft separation into stable vortex flow, let alone designing a tail that would prevent the separation like Bub-7. Making a slightly larger passenger seat pad buried in the separation shadow of the rider would have little effect, and he did find that. It shouldn't have taken CFD.
What the researcher did here was a simplified motorcycle model, probably of himself or a friend. To get the mesh of this model accurate enough to see all of the separation and other effects would take upwards of a billion nodes. To accurately mesh just the fork/brake/rotating-wheel interaction would take millions of model nodes, he didn't go that far, so the drag numbers are nothing more than WAG's. The conclusions are even more so: they contradict the well-researched 50-year database of HPV science. His work may still have value, and like any CFD researcher, he needs to resolve some of these blanket statements with the proven knowledge and data from real-world vehicles that have run for decades contradicting his conclusions.
In other words, get off the computer and tuft it.