East Hill,
You have one of the bulged tubed ones also(you bought the bike)? It is odd that they would make something so eyecatchingly assymetrical, but they did.
I took a picture of the chain "top tube" near intersection. On the small front ring, small rear there is roughly 3-4mm of lateral room-roughly 2-3mm of lateral room on big ring small cog combination (picture is small/small).
1) Did they do it just to negate the tiny bit of touching you would occasionally get from bumps etc or between shifts? I took a close look at mine, and there is no evidence that they ever touched; no paint chipping despite plenty of evidence of casual use.
2)Did they narrow the spacing of the two top tubes. If they weren't lugged they could start closer together on the head tube.They would remain closer together the entire "trip" down to the dropouts. This would cost a really tiny loss of clearance-maybe .25mm on 3mm-not much!!
3) Did they use slightly bigger diameter top tubes?
The bike does give a nice ,compliant ride for no suspension and narrow 1 3/8" tires.
The pictures show that the lugs and lug work is good and someone, or some machine took the time to paint a silver line around them. Kinda odd on a cheap bike, but so is a chrome moly frame on your bottom of the line bike.
A different question-is Cr-Mo the name of a tubing manufacturer? I have the same type sticker on a 1989 HardRock Cr-Mo with the dash in between. Of course those are how the chemical names are on the periodic table are written, but there are plenty of other ways to you could indicate chrome molybdenum-(4130, chrome moly, moly chrome chrmo).Cr-Mo is the shortest, but I still wonder if that is a tubing manu. name.
Thanks,
Charlie