I wonder if the weight difference is negated due to the rotors being 20mm greater in diameter therefore the weight farther from the center of the axle...
Could be. I was happy to find that the whole change resulted in less unsprung weight as opposed to more. I couldn't get accurate weights of the individual components due to the limitations of my scale, but the 320mm rotors felt noticeably lighter than the 300mm ones, so it might be a wash in terms of the new larger diameter since most of the weight reduction seemed to come from the rotors anyway. My main objective was improved braking, which should now be in abundance!
BK
