What does not compute? This is your standard mid-80's Shimano 600 road double with 52/42 Biopace crankset and a 6-spd 21T freewheel that I wanted to replace with a larger cogged one for climbing. I was looking for a 6-spd about 28T freewheel, but found this 30T 7-spd instead. I see no probably with using a 7-spd freewheel in DT friction mode, or am I wrong?