I just received in my possession an older Tachyon with 650c wheels (straight downtube, not the curved dude), but can’t quite locate the geometry online, let alone figure out what size this bike is, or the vintage.
I can’t figure out what year, but can tell you that it got the turquoise paint slash/brush-swoop below the cursive-text “Tachyon” on the top-tube. The serial number is 17552, if that helps.
Specifically, where do I take measurements from to determine the size (the headtube and seattube extend a bit taller than the toptube). It is about 21.5 inches (convert to 54cm?) from center of the headtube’s top to its horizontally-reached equal of the seattube, but it’s hard to tell if that’s accurate and it seems like it’s smaller than a 54cm. It is 650c, if that helps. The downtube seems, (center of bb to center of headtube) to be 24.75 in (convert to cm??).
Did you try the catalog archives at litespeed.com? Last time I looked, they had all of 'em available. If you measure the seat tube center to center as you’ve done (in cm), you should be able to locate the geometry chart for that particular model. Short of that, you could try getting in touch with the good folks in TN and see if they can locate the serial # in their records - I’m guessing that’d be a real longshot, though.