I understand the later bikes have around 25 ML more fluid in the forks and some have felt the need to back off on rebound damping to compensate.
I also noticed one peculiar trait I found only one person referring to in the suspension section of the WIKI. If I go below 18 clicks (like 14-16 clicks) on the rear my bike actaully starts to ride harder, not firmer but harder, like it doesn't absorb bumps as well. Anyone else
find this also? Someone said it was due to suspension geometry. Also my bike will go over railroad crossings with hardly a blip even with 20 clicks but rides harder at 16 clicks.
The oil level is a reverse way to change the air volume in the forks and fine tune the resistance (spring plus compressed air). The effect of a lower air volume will only be felt at the end of the fork's compression stroke and should not have any impact on the spring's resistance during normal riding....just under heavy braking. I don't think a lower air volume would have any effect on rebound damping but if you were going to make an adjustment it would be to increase rebound damping since the spring and compressed air would rebound with greater force.
The preload adjuster on the rear shock is supposed to be used to maintain ride height/steering geometry with varying loads. Preload does not make a spring stiffer and it does not change the portion of the spring that you normally use when riding however it does increase the amount of travel you have left after adding the rider's weight and as such also can be used to prevent bottoming under a heavy load. Your example of of harsh ride over railroad crossings with less preload should only happen if you are actually bottoming your suspension over those crossings....which could be very possible riding 2-up but should not happen riding solo unless you are very large and/or have a very large and loaded trunk. Regardless of where your proload is set, the C14 shocks seem to work best with the damping (screw at the bottom of the fork) set 1/2 turn from full hard.