On my recently rebuilt 1973 Schwinn Sports Tourer, I replaced the original TA Cyclotourist 54-40 rings with 50-36, which IMHO make more sense on a touring bike. Unfortunately, the new rings cannot mount the unusual chainguards.
The original rings had chainguards for BOTH the outer and inner rings. The outer guard was presumably to protect one's pant leg from chain grease and perhaps to prevent the chain dropping off the outside of the crank if the FD was improperly adjusted. The inner chainguard evidently was to prevent the chain dropping between the two rings, which could easily happen with the very wide 54-40 gearing.
Today, I reassembled the original rings and guards before I lose any of the parts - if I had thought ahead, I would have just removed them from the arms as a unit.
Examining them more closely, both guards are bolted to the outer ring through a single set of holes. There is a spacer between the outer ring and the outer guard to space the guard out from the ring. But there is no spacer between the outer ring and the inner guard, rather the inner guard bolts flush against the inside of the outer ring, as it must to leave clearance for the chain to engage the inner ring. This means the outer ring must be sufficiently large the the chain engages it at a radius great enough to clear the inner guard. Looks like the outer ring must be at least 46T, maybe 48T.
Measuring the BCD of the chainguard holes in the outer ring, they appear to be at the same 151 or 152 mm BCD as holes for a Criterium inner ring. Indeed, the hole diameter seems the same as well, so that one could attach a Criterium inner ring in place of the chainguard, but with different length bolts. Note that there are two types of Criterium "outer" rings. When using a Criterium adapteur, both outer and inner rings are of the same type, with a 151 or 152 BCD. But when the adapteur is not used, the Criterium outers ring attaches to the arms at the same very small circle as the Cyclotourist rings, except that the Criterium outer is drilled at 151 BCD for the inner rings, rather than at 80mm as is the case with a Cyclotourist. Now, although I don't have a TA cyclocross chainset, I rather suspect that for cyclocross, one used a Criterium outer, but mounted the cyclocross chainguards on either side of the ring, using the 151 BCD holes that would otherwise be used for the inner ring.
But for a wide range road double like the original crank on my Sports Tourer, one needs and outer ring with three sets of holes: The very small BCD set for attaching to the arms, the 80 BCD set for attaching the inner Cyclotourist ring, and the 151 BCD set, identical to the drillings for a Criterium inner rinng, but used instead for the outer and inner chainguards.
Anyone ever seen such a triple drilled TA outer ring, other than OE on Schwinns? Anyone ever seen such a ring smaller than 54T? I'd like to restore the original chainguards, both because I think they look cool, and also because the inner one probably does really help prevent dropping the chain between the two rings. But I'm not willing to tolerate a 54T outer ring.
Regards,
Jerry Moos
Big Spring , Texas, USA