I agree that when clean the shielded bearings spin very free but it doesn't matter if you race off road, on road or carpet shielded bearings will need to be cleaned after nearly every day of racing for top performance while the sealed teflon bearings can take a lot of running before needing attention. If you add up all the wasted motor spray and time disassembling and reassembling your car you will soon find you are best off to throw the shielded bearings in the garbage when first building the kit and purchasing sealed bearings, at least that is the case for me as I race seven different classes and maintenance time is very limited.