Originally Posted by
melikebikey35
But they affect how your body interacts with those numbers. Different seat tube angles will put your body in different places in relation to the crank, and you may not be able to make up for that with saddle setback.
For instance, the reach on my two bikes are exactly the same, and the seat tube angle differs by only 0.5°
Yet, I have to have the seat slammed all the way forwards on one, and all the way backwards on the other (both 25mm offset posts), in order to give me the same fit. Same can be true with HT angle, only less drastically due to the short distance.
The whole point of a bike fit and its dimensions are based around the BB centre. The same as stack and reach numbers.
How the frame geometry allows you to achieve that fit within normal parameters of stem lengths, spacers, and saddle position on the post is why stack and reach numbers are the best for comparison.
The only down side is how stack affects reach numbers which has been pointed out in a previous post and easily compensated for by a simple calculation.