More compact double
1. New rear cassette with 36 teeth, presumably with a new rear derailleur and a longer chain.
2. New chainrings (44-26). The front derailleur might work (the step-size is the same), but a new crank will be needed to take a small ring. A couple of years ago you suggested ‘bolting an inner onto a Stronglight ST55 single chainwheel’, and Spa Cycles are still selling this crank.
That’s a good summary of the least-cost options. Option 1 is simplest and most certain to work. Any 9-speed Shimano MTB mech will work perfectly with a 9-speed road shifter. So don’t stint yourself: get a ‘Shadow’ type 9-speed mech that’ll let you fit a ‘29er’ cassette with up to 36 teeth. Sora quality would be Deore RD-M592-SGS. Or upgrade to SLX RD-M662-SGS or XT RD-M772-SGS. SGS means longest cage, which you will need even though you only have a double, because the cassette has such a wide range.
As for option 2: Spa Cycles do still supply that cheap and truly compact ST55 double, but you’ll need a new, square-taper bottom-bracket (seek Spa’s advice on spindle length) and must hope that your existing front mech will cope despite the lack of shift-assist pins on this outer chainwheel – and it being smaller than the 50 your existing mech is designed to fit over. Fortunately this bike has a clamped-on mech that can be slid down the seat-tube, but there’s still a potential problem with the outer ring being more sharply curved than the outer cage. This shape mis-match opens an unavoidable gap over the teeth at the back of the cage, through which a shifting chain may over-shoot, then jam as you pedal the overshot links forward into the narrowing gap! So it’s safest not to go too much smaller than 50. I think 46 should be safe enough and would put 28 on the inside. (The tooth difference isn’t the same, but that’s not vital except between the outer and middle of a triple.)
Another option is to fit a Sora triple crank on the same bottom-bracket. Look for a secondhand one, with worn-out chainrings, because you’ll be swapping all of them for something different! Instead of the outer, fit a Stronglight chainguard ring (from Spa Cycles). They come in sizes to fit an outer ring (flipped around to go instead of the middle ring) with 44 teeth, except not even Stronglight makes 130mm bcd outers that small. Middleburn do (or will) make a ring like that, and it’ll be a better ring, but pricey! With an outer guard to nudge an overshooting chain back into line, you don’t need to mind the gap at the back of your cage and can happily use an outer this small with your existing mech. And since difference isn’t vital for doubles (unless it’s so great the chain drops to the very bottom of the cage, which it won’t), put 26 or even 24 on the inside.
This was first published in the April / May 2014 edition of Cycling UK's Cycle magazine.