Remember a while ago I said I was going to change the crank? Well, this weekend, I did:
Now, lemme tell you something about this crank:
A typical crank consists of two crank arms, two chainrings, and five pairs (two parts each) of chainring bolts. The rings bolt to either side of an extension of the arms, the extension forming a spacer. Total number of parts: 14. Easy-peasy.
These are Sugino PX crank arms, a “modern” (well, they were modern in the ’80s!) take on the venerable French Specialities T.A. Cyclotouriste crank arms. Here’s the deal: typical crank arms have a bolt circle diameter (“BCD”–the circle that you can draw through the chainring bolts) of 110mm or 130mm or so. This limits how small the smallest chainring can be, because the teeth have to be able to clear the bolts. So the smallest chainring you can put on a crank arm with 110 BCD turns out to have (IIRC) 34 teeth.
The Specialities T.A. Cyclotouriste, on the other hand, uses a different arrangement. It has a very small built-in BCD (I think it’s 53.4mm) to which a large chainring (anywhere from 40 to 62 teeth) bolts directly. Then, through an arrangement of additional bolts and spacers, you can mount one, two, or even three additional smaller chainrings to the outer ring. The smallest I’ve heard of is 26 teeth, but they might come smaller than that! I chose to use a 46 tooth outer ring and a 30 tooth inner ring. This required Five sets of bolts (plus washers) to attach the outer ring to the arms, six sets of bolts and spacers to attach the inner ring to the outer ring, and of course the two rings and two arms. Total number of parts: 37. And some in rather hard-to-get-at orientations.
But it’s worth it, because there is less overlap between the gears you can get in the small ring and the gears you can get in the large ring–greater range. The big advantage? The range of a triple, the simplicity in shifting of a double.
To make all of this work with my existing derailers, I had to (1) install a new bottom bracket, (2) shorten the chain, and (3) change out the cassette from a 13-30 to a 12-27. This last was necessary to avoid some shifting problems, but even doing that, I now have a significantly lower (by 12%) climbing gear. I could get everything to work with the 13-30 cassette, but it would mean changing over toa long-cage derailer in back, something I’d prefer to avoid. As it is, this setup is very intolerant of cross-over gearing (there isn’t enough chain for using both the large chainring and the large gear at the same time, and there isn’t enough tension to use the small chainring and the small gear at the same time) and the wrong move could be very damaging. But then, those are combinations that I would only hit by a sort of extreme accident. Still, a trifle worrisome.
We’ll have to see how it goes!