On the tandem I *have* to have a thin cowhorn to fit the adjustable stoker stem. Yes a thin bar is a problem- it presses on one thin part of your hand instead of "spreading" the load.

Currently I have the Kores plus old fashioned foam grips. The Kores are great at spreading the pressure out but however they require a very thin bar- even the tandem bar is too wide and the screws holding them togther are far apart.

Yes, fat handlebar tape is a cheap option but the best is to invest in a flattened top handlebar in the long run.