Hmmm, maybe I'm just showing my age. In the "old days" for classical bikes, size was determined by the seat tube length. For example, Bianchi (my first love) still goes by this dimension for classifying frame size:

http://www.bianchiusa.com/...-class-doiomiti.html

Have the rules changed for tri styled bikes?

My alu P3 is a 48 cm, which is the seat tube length. The top tube dimension is more of a virtual number based on the design. Cervelo still seems to use the seat tube length to designate frame size, though:

http://www.cervelo.com/bikes.aspx?bike=P32009

So, what am I missing here?

Edit: I think I found it. From the Cervelo site (below the dim chart):

Note 2: All our TT/tri bikes have a dropped toptube and shortened headtube to enable the rider to position the aerobars low enough for a proper aero position. This means that the frame size is no longer equivalent to the seattube length, so don't determine the size that way. Simply go by the size sticker, or measure the headtube and look it up in the above chart.

I'm guessing that this might have become standard procedure for most/all tt-tri bike manufacturers in recent years, yes? In which case, that 53 cm top tube in the OP may be closer to a "51 cm" size (seat tube equivalent) than a "48 cm" size.

Could some one resend me a copy of the original memo on that? Thanks.

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