The R Series wheel has a modified carbon layup to provide addition strength at the spoke/rim interface as well as a robust rim flange. The carbon is considerably tougher (designed to better withstand road hazards/extreme situations/ hard-landed wheelies etc.), but it also weights a bit more as a result, 6.20 lbs. (2.81 kg) vs. 5.65 lbs. (2.56 kg) for the #161326. You will notice that the load rating is the same for both wheels 330 lbs. (150 kg) this is because the load rating takes the strength of the hub into consideration also. Since the hubs are identical, the load rating must remain the same for both style wheels. Please see the link below for load rating safety and measurement information: Note: the R+ Series wheels are even more reinforced. Of course, they will be a bit heavier also, if identical sizes are compared.