Originally Posted by
John E
I really dislike the proposed 13-17 jump at the top end. If you are currently using a 6-speed freewheel with standard cog spacing and a 126mm rear axle overlock width, consider a readily availble Shimano 14-16-18-20-22-24-28 seven-speed, which will give you a nice tight ratiometric progression and the wall-climbing low gear you crave.
I concur completely, with the possible addition that the wider Megaranges, 13-34 7-speed, give a highly usable spread in the first 6 positions. With a Duopar, the difficult-seeming shift from the 24T in position 6 to the 34 T in position 7 is a piece of cake. It's not a piece of cake with a Shimano 600 long cage. With that cluster and Duopar derailleur your 52-40 crankset does not have to be changed.