Originally Posted by
Andrew R Stewart
Well, you'll be close to the rake with this method. Since the BB doesn't sit at a right angle to the steering axis as the front center line crosses it the offset being measured will be a tad longer then the rake. It's simple trig.
An eye that has spent decades looking at bikes and their specs can see general aspects of design with only a careful look at. So while the exact dimension might not be able to be seen a narrow range of likely size is not hard to discern. Andy
It will be accurate to half a millimeter. If you want more accuracy, measure from the bottom tooth of the small chainring.
57mm is "neutral trail" - which is often the goal with racing bikes.