Originally Posted by
DaveSSS
To compare the reach of two frames, you should correct the reach so both are compared at the SAME stack height. This is simple to do. Just subtract 3mm of reach for each 10mm of stack height difference, from the frame with the shorter stack height. With an 8mm spacer on the BMC, to make the stack height the same, that's makes the reach about 39.2, so it's still about 8mm longer and one size shorter stem would be needed.
While this example is trivial, many people compare frames with a 2cm difference in the stack and don't make the reach correction of 6mm. That's enough to be noticeable.
For those wondering about the amount of correction, it's just the cosine of the HTA times the stack height difference.
Excellent info indeed Dave.
Now with all things taken into account, if an 8mm spacer and one size smaller stem is all that is needed to bring the 57cm BMC closer to geometric equilibrium with the C'dale, would the 54cm frame be the better choice after all? There is nearly a 40mm difference in stack height between the 54cm BMC and the 56cm C'dale resulting in an comparative reach measurement for the 54cm BMC of 37.7cm. In order to emulate the fit of the C'dale on the 54cm BMC frame it would take nearly 40mm of stem spacing and a size longer stem. To boot, I would be extending far more seat post to get the proper saddle height as the seat tube of the 54cm frame loses 4cm as well. Given that my goal was to achieve the relaxed fit of the C'dale on the BMC frame, would the 54cm be the one to try it on?
*disclaimer*
I understand the importance of test riding and proper fitting for ultimate fit guarantee. I am more intrigued by the mathematical side of this conversation now more than anything.