As a rule of thumb, the one with a cartridge bearing (on some pairs) goes at the bottom. This is because cartridge bearings can't float laterally and a journal bearing bushing is what the doctor ordered for that function, at the top.
Another rule of thumb: Some pulleys are offset from the axle's centre. This offset is to minimize the sideways pull of the chain on the bottom (tension) pulley when the owner cross-chains the gears. The offset should be to the right.
Another rule of thumb: Offset issues are easily solved if there are rotational arrows on the pulleys.
Another rule of thumb: Instead of saying top and bottom, some pulleys say Tension and Guide. Tension is at the bottom, guide at the top. Tension and Guide is the insider-speak version of drive-side and non-drive side and of course, totally unhelpful.
Another rule of thumb: Just feel for lateral float and put the floatiest one at the top.
Thus, by those definitions, I think the bottom one should go to the top. I contradict Raleighnut at my peril simply because the top bearing looks like a cartridge bearing to me. Has anyone seen where I put my specs?
