Join Date: Jul 2005
Location: Southampton, UK
Bikes: Gazelle Trim Trophy, EG Bates Track Bike, HR Bates Cantiflex bike, Nigel Dean fixed gear conversion, Raleigh Royal, Falcon Westminster.
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The chainring is indeed the front sprocket. On double and triple cranksets, these are held in place by 4 or 5 bolts. On some single chainring bikes (like Downtubes) it's possible to add a second chainring, and cmcanulty has done that, but not added a derailleur, just shifts it by hand. Alternately, you can simply buy a double (or even triple) crankset fairly cheaply on ebay. Consider whether you want to go up, or down from the gearing you have at the moment. What might be trickier is finding a front derailleur that will fit - folders use very different sizes of tubing to regular bikes. Looking at the D7, it's hard to see where you could easily fit a derailleur.
As suggested, there is another option - the SRAM Dual Drive. This mounts your cassette onto a 3 speed hub gear, which more or less does the job of a front derailleur. 2nd on this is a 1:1 gear, so you get the same gearing you had before, 1st is lower, for climbing, and 3rd higher, for going faster. This is not a super cheap solution however, since you have to not only buy the hub and shifters, but have a whole wheel built.