The handling changes is more likely due to the different steering geometry rather than wheel inertia. Wheel inertia has a minimal effect until much higher speeds than we do on normal roads.
On a "normal" front fork a smaller wheel makes minimal difference to the steering geometry. The angles change slightly but castor distance def gets shorter. The telelever uses the front swing arm to cancel out the tele fork dive. With a smaller wheel there is less distance from the ball joint to ground so there will be less counter- action of the fork dive effect caused by the tele legs.
Who knows what the numbers are but that's my take on why the smaller wheel feels different. A wider back tyre also affects the steering as it can cause the bike to lean more for a given cornering speed. Contact patch moves towards to turn side so bike has to lean more.
It's such a fascinating subject... to the extent that without being ironic, knowing now what I didn't know in high school, I would have studied in this direction with pleasure.
Without access to (most of) the requisite knowledge, a testing lab, and unlimited tyre resources, I can sadly only go as far as my current equipment and the feeling at the seat of my pants can take me.
The '09 originally came to me with a 110/80-19 Battlewing on a 19" front alloy, and a 180/55-17 PR3 on a K1200S rear wheel. (Slow steering, but very calm and confidence-inspiring... never felt like it was up to anything sinister and flat-out refused to shake it's head.)
Next, I exchanged the 180/55-17 for a part-worn 190/55-17 Pirelli Diablo Supercorsa on a different K1200S rim (an experiment - successful, as it turned out - to stop the ASC malfunction which was plaguing the bike at the time). For a very slight loss of overall stability, the bike's turn-in became much quicker.
Next came a part-worn 190/55-17 Metzeler Racetec K2 on the original K1200S rim. And here I learned how subtle the changes caused by even two tyres in the same size, but made by different manufacturers. The Racetec gave even quicker steering, but lost a little more stability.
Then I took the plunge. With all else as-was, I ditched the 19" front and replaced it with an R1200S front wheel, using a part-worn 120/70-17 Pirelli Dragon Supercorsa Pro.
I didn't like the handling at all in the beginning. Low-speed turn-in, especially at high steering angles (such as parking-lot situations) and even turn-ins at suburban speeds, felt too quick and 'forceful', like the bike had a mind of it's own and wanted to double the steering input I was giving.
But the faster I went, the better it got. At freeway speeds, the bike felt much 'nimbler' and lighter on it's feet. It was possible to change direction more quickly, with less effort.
It took quite a lot of experimentation to get the balance right. Cold pressures had to go down from 2.5 Bar front and 2.9 Bar rear to 2.3 front, 2.7 rear. ESA preload was previously set to rider + pillion; I had to bring it down to 'single rider'.
Since I can't
practically go down to a 190/50-17 (unless some kind soul would like to help out with a final-drive swap), I'm still curious to see if I can claw back the last of the lost stability (while retaining the quicker steering) by exchanging the Racetec for a milder, more touring-oriented tyre like a Pilot Road 2 or Pilot Road 3 (still in 190/55-17).
Even before I got the Camhead, my mind was made up: I had to go Supermoto with this one, too. I still have the original wheels, but for some variety, I left the bike more-or-less completely standard, and used R1200RT rims. The front tyre is a 120/70-17 Pilot Road 3, the rear a part-worn 190/50-17 Pilot Road. (It's already partly squared-off, but I plan to use it until it reaches it's treadwear indicators, after which it will be replaced by a 180/55-17 Pilot Road 2 or Pilot Road 3.) Current cold pressures on the Camhead are 2.4 Bar front, 2.8 Bar rear. ESA settings are the same as the Hexhead.
The Camhead feels very different to the Hexhead. It has none of the low-speed stability issues the Hexhead had - I suspect this is due to the lower profile of the existing rear tyre.
However, at higher speeds, it wants to weave and shake it's head - which I suspect is being caused by the rear squaring off.
Also, the rear RT rim is aligned too far to the left of the Camhead's centre line - by at least 10mm, maybe more!
I want to fix this - possibly by having the mating face of the wheel machined down to suit.
It certainly wouldn't affect the final drive unit - presumably, the spacer would machined in a way that would allow the wheel to run perfectly straight in relation to the bike's centre line, meaning that torsional forces acting on the FD hub mating face would be no different to a standard wheel.
