Originally Posted by d_D
Some isis bbs have bearings that are not attached to the shell. The cups fit directly onto the bearings. Without the cups when you hold the shell and rotate the axle the whole bearing can rotate with the axle. When you fit the cup it spans the bearing and shell holding both together. Now when you rotate the axle the outer bearing race is held with the shell and doesn't rotate with the axle. The extra friction is probably from the bearing seals. If you grab the bearing while rotating the spindle you should get the same amount of friction as with the cup attached.
Thanks for a quick reply. My SRX is exactly the model you describe; bearings rotate with the axle. And yes, the friction seems to be caused by bearing seals. But something goes wrong when the BB is mounted: Friction increases so much that the axle rotates with difficulty.