When bearings press directly into the frame it is important that the bearings are of the highest quality and the bearings in the BB30AR are exactly that. They are highly polished so they are smooth to spin and last a long time. If you want the ultimate in smoothness then you can opt for our ceramic TBT bearings. The seals on the bearings protect the bearings and races from dirt and water that try to make their way in. To ensure the bearings last even longer we designed the bearings to directly contact the crank axle.