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Frame sizing for TT bike(2 posts)

Frame sizing for TT bikebimini
Jan 7, 2004 7:00 AM
I'm looking at getting a new/used TT specific bike and have a question of frame size. I currently have a standard road bike geometery with a seat tube of 59cm c-c BB and 59cm top tube length with a relatively long stem (not sure of exact length). With the clip on aero bars I have the seat forward as far as it will go and could/should go forward another inch. Seat tube angle of about 74 degrees

With a TT geometry 700 cm wheel bike with an seat tube angle of 77-78 degrees and real TT bars like Cenelli Angle bars, will I need a different length top tube or seat tube
TT length comparison...C-40
Jan 7, 2004 9:46 AM
Since you only know that your current STA is "about" 74 degrees (pretty steep for a 59cm frame, are you sure?), it's not possible to give an exact answer.

Each degree of additional seat tube angle will lengthen the effective TT by 1.2cm. A frame with a 78 degree STA would only need to have a TT length of 54.2cm to be as long as yours, with the saddle in the same position relative to the bottom bracket. In reality, you probably could not move the saddle that far back with a 78 degree STA.

Another way to think of this situation is that every additional degree of STA will move you forward about 1.2cm.