Both play a role. The spring weight will affect the total amount of dive but the damping will affect the speed or rate of the dive. The ride height of the rear suspension will also affect fork dive too. Too much preload, as in the case with our stock shock, will cause too high a ride height and will encourage more fork dive as it moves the weight bias forward.
For my weight the stock fork spring is in the ball park for proper sag, around 45mm with rider in gear, and the preload adjuster in the middle range. If you have to go all the way hard or all the way soft to achieve proper sag this tells you the spring rate is off and should be changed.
For my weight, 180 pounds, the back suspension is a great illustration of a spring weight that is too heavy. I have my preload set all the way soft and still only have 31mm of sag. The rear sag should be roughly the same as the front but in my case the stock spring is too heavy.
My soon coming Nitron shock will take care of this issue.
We're on the same page.
Last year, I swapped the rear shock for a Yamaha R1 2007-2008 rear shock. Along with the shock, I used a Road Star Warrior spring. I do not have the numbers with me, but can tell you that I have set up the rear shock as suggested as base line in the R1 shop manual. I'm happy with it. It gives me a Dynamic Sag (rider ATG + loaded luggage) of 10.5 mm. IMHO that's in the area it should be. There is not as much range as in the front.
With the above R1 shock and the OEM Versys front spring set at 5 turns preload, the Dynamic sag was at 42 mm.
The above was OK but I didn't like the harshness of the front spring when going over small bumps, etc. on the road. I finally broke down and ordered a Hyperpro front spring for the Versys 650.
With 0 turns of preload, I get a dynamic sag of 52 mm. As per Hyperpro instructions, the right fork damping is set at 1.5 turns open.
I was hesitant with the zero preload and hurried to try this out on the road. It revealed to be quite compliant and handled the small bumps, etc. very nicely. I tried but could not get the forks to bottom out no matter how hard I hit the front brakes.
As mentioned previously I'm quite happy with this but might play with oil weights later on down the road and also right fork valving.
For the time being, I'll concentrate on trying to ride more than I have been lately.