I need to know the difference between a rear 18" rim (Drum brake) and a front 18" rim (Disc brake). I know the hub size between the drum hub and disc hub is different, but what is the physical difference? Is there any? It would seem to me the only difference would be the trajectory of the dimples due to the different length spokes required for a disc versus drum brake front rim, being that the hubs are not the same in diameter. Please help if you can. Thanks.