I am having a new bike built (Orca OMX) and am trying to replicate the geometry of the bike it is replacing (Tarmac SL6). This will be my road bike. I like a stretched out position but generally ride from the hoods.
The proximate question is, how much if any spacer height should I have the shop put in below the stem? The need to be as correct as possible arises because the steerer tube on the Orca must be cut so as to leave no spacers above the stem \ cap. In the past I've left a 5 mm spacer atop the stem just to have slack in case I want to go to a less aggressive position later. Here, I have to be right from the start. Also, changing the stem to a different stem angle is not an option; the stem is proprietary (-8 degrees).
Assume I can't get a fitting ... that's not a deal killer IMO, as the Tarmac geometry was based on a fitting and I have had consistent #s throughout my road bikes. Most notably, I know that my ideal saddle height is 75mm. Whenever I test ride a bike I have the saddle height set at 75 mm, and it works across brands. As to other metrics, I am a major reach and stack fan.
As I see it, I should start with the same 75mm saddle height, as that number is fixed to my optimal leg position. Then, the extent to which the geometry is aggressive depends on the stack (center of bottom bracket to top of head tube) + spacers below the stem. The difference between the saddle height at 75mm and the (stack + spacer) height dictates how upright vs. bent over I will be in while riding atop the hoods. (This is all things being equal...reach matters too, and we can assume it is about the same. Also, I worry less about reach as the saddle setback can be adjusted to optimize that.)
Thus, crudely speaking, the Tarmac stack is 565 mm, there was a 10 mm cap above the headset atop that, and then I had a 5 mm spacer below the stem. The sum is 580 mm [plus the stem height, which I will call a constant] as the height at which I rode.
The Orca stack is 7 mm greater, at 572. There is a 7 mm cap above the headset and a mandatory 8 mm high angle spacer into which the stem sits, oriented at the -8 degree angle. The total is 587 mm.
Thus, without adding any spacers to the Orca, it will be 7 mm less aggressive, in that I will be riding 7 mm higher and the drop from the saddle height will be 7 mm less. (I can live with that but don't have any choice given the mandatory cap and angle spacer!)
Is this the correct way of thinking about the question? If it is, while I could add a 5 mm spacer below the stem to be conservative in not cutting the steerer tube too much, that would decrease the aggressiveness \ drop from the saddle even further and put the new bike pretty far from the Tarmac. My gravel bike is less aggressive (580 mm stack + 20 mm spacers) and that's okay for that bike, but bikes that are too upright \ have too tall a head tube and stack put me to sleep.
Just in case, the two bikes also are different in two other metrics that may matter. Per both companies' web sites, stack is measured from the center of the bottom bracket to the top of the head tube. The center of the bottom bracket on the Orca is 2 mm lower than the Tarmac (i.e., the bottom bracket height is 2 mm higher on the Tarmac). Also, the head tube angles have a .5% difference (73 degrees on the Orca, 73.5 degrees on the Tarmac). I regard these deltas as rounding errors, perhaps wrongly. I don't see how they affect the saddle height, which is my main starting point -- especially as the seat tube angle is the same for both bikes (73.5 degrees).
