Good question. I think it is either because of the large combined torque of two riders on the tandem or maybe because I ride it alone when the stoker is not with me so the rear BB has to function as a power-transfer axel between the tandem chain and the drive train chain (try saying "drive train chain" ten times fast).

I was assuming a cartridge would not come loose as easily as the current BB does but maybe I'm wrong. I was thinking of putting locktite on the current BB and seeing what it does but I'm afraid I would have trouble loosening it again later (I haven't used locktite much and I don't know how tightly it locks - usually I'm more concerned with greasing things to prevent them from getting too locked to loosen later).

Another option I've started considering is putting a lock-washer between the lock-ring of the BB and the frame. I've never seen that done, though, so I'm not sure how good an idea it will turn out to be if I go with that.

Do you think the cartridge BB is just as likely to loosen as the current BB?