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Just a few more tidbits I picked up along the way:
For 120/80 BCD spiders: The largest 120/80 chainrings that SRAM/Truvativ appears to make is 42 (120BCD) and 28 (BCD). Also note that SRAM uses special 10mm chainring bolts for 120BCD chainrings that screw directly into the chainring. TA and Stronglight, make chainrings that also have the bolts threaded directly into the chainring in sizes up to 45T. FSA makes 120BCD chainrings in 46T, 48T, 50T that use standard chainring bolt holes, and these can be fitted with standard chainring bolts/nuts. The largest 80BCD chainrings I've been able to find are 30T - but this should allow one to make the popular 46-30 chainring combination. A further caveat about the chainrings with threaded holes: The special bolts from SRAM are a fixed length, just long enough to thread through the spider into the chainring. If the chainrings need to be shifted inboard for derailleur/crankarm clearance, the bolts may not be long enough to insert chainring spacers. However, WickWerks makes aftermarket 10mm chainring bolts which are an extra 2mm long, which should allow a small amount of chainring offset. (the 80BCD chainrings use standard 8mm chainring bolts, and these bolts are available in a variety of lengths.)
104/64BCD: It appears that at one time, SRAM/Truvativ made a 48-32 chainring set for a double crank in these BCDs. While the 64BCD 32T chainring is still available, I have not been able to find the 104BCD 48T outer double chainring anywhere. Normally, the outer chainring in a 104/64BCD double crank mounts to the inner face of the spacer, and on a 104/64BCD triple crank, the middle chainring mounts on the inner face of the spider and the outer chainring mounts on the outer face of the spider. There are some 104BCD 48T chainring available that are meant as the outer chainring of a triple, which I have been experimenting with mounting on the spider inner face double crank, but I can't yet report results. When mounted this way, the chainring offset may be different, and their are no countersunk holes for the chainring nuts, which may protrude out far enough that they might interfere with the chain in some gear combinations. However, a 32T inner chainring is large enough that it completely covers the chainring nuts, so I'm hoping that there will be no interference.
And if you're wondering why I've been experimenting with tiny chainrings on a road bike, I've been setting up a bike for the New England hill climb series events. These are races up some seriously steep mountains. In particular there is Mt. Washington, which is not only the highest mountain in the Northeast of the US, but has one of the steepest continuous grades in the world - it averages 12% for 7.6 miles, with extended sections at 15%-18%. For this event I'd prefer to use a crank with the low Q factor (and low weight) of a road crank, but with a chainring as small as 22T. This will also allow me to use a standard road rear derailleur and cassette, for a compact, lightweight, and relatively simple drivetrain, and one that can be easily swapped back and forth between "regular" and "super-low" gearing with the swap of crank spider/chainrings. (Okay, I'll have to swap different length chains, too, but that's easy with a quick link, and also readjust the front derailleur.)
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