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I've been trying to figure out why my 2007 Madone 5.5 SL has it's bottom bracket behind the seat tube(16mm..from Trek). Wouldn't the bottom bracket behind the seat tube make the top tube longer(sit back more according to bottom bracket position). My 58cm Madone has a top tube of 57.2, would the 16mm bottom bracket setback make it a 58.8cm? Does any other frame makers place the bottom bracket setback from the seat tube?
 

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James said:
I've been trying to figure out why my 2007 Madone 5.5 SL has it's bottom bracket behind the seat tube(16mm..from Trek). Wouldn't the bottom bracket behind the seat tube make the top tube longer(sit back more according to bottom bracket position). My 58cm Madone has a top tube of 57.2, would the 16mm bottom bracket setback make it a 58.8cm? Does any other frame makers place the bottom bracket setback from the seat tube?
Actually, the seat tube is offset forward, since the center of the BB is the point of origin for frame measurements.

The TT length is measured from the TT /seat tube intersection point, so the TT length is not 16mm longer than advertised.

What is in question, is how the seat tube angle is measured. It's unlikely to be the actual angle, ignoring the offset. It could be between the center of the BB and the TT/ST intersection point, or it could also be figured to some other point, closer to the saddle height.

Cervelo does the same thing, listing all of their frames as if they all have a 73 degree STA, even though they may have offset seat tubes. Go to their website and look at the definition of reach. While it sounds like a great idea, it's also got a big flaw. You can only compare reach values for ONE stack height. If you consider a different frame size with a different stack, corrections have to be made to the reach, otherwise the difference in reach is not the same as the difference in stem length required.

http://www.cervelo.com/bikes.aspx?bike=R32008#G

Trek has also starting listing reach values.
 
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