I am trying to wrap my head around your fit example.
Let’s try an example. Let’s say that your fit simulator, with stem
right down on the simulator’s plate (analogous to your stem sitting
right on your head tube top), shows you need a frame with a stack and
reach of 58.7cm and 43.5cm respectively. Let’s say you’re considering
a Calfee for your customer. You have to account for the Calfee’s
external headset. This headset, plus perhaps 1.5cm of spacers you
might want under the stem, is that 3.5cm of “angled stack” described
above. This 3.5cm of stack “angled back” at 73 degrees drives the
stack of the frame you’ll need down 3.3cm (3.5cm x sine73 = 3.3cm). So
the frame’s stack you’ll need is really 54.4cm, not 58.7cm. This 3.5cm
of stuff above the head tube top “shortens” the rider’s cockpit by
3.5cm x cosine(73) = 1.1cm. So the actual reach you’ll need on your
hoped-for Calfee is 44.6cm instead of 43.5cm.
Isn’t 58.7- 3.3 = 55.4 or is there another centimeter that is being
minus for something else?
Sometimes when you get out on the road, you find that you may want to alter your position a few mm. When you start to engage more of your core, that can make some difference in terms of what feels best out on the road vs. what felt right on the fitbike. So if you go with no spacers, you can’t go down that 2.5 or 5mm to see if it’s tolerable out on the road.
15mm of spacers also gives you greater room in terms of changing your aerobars. Want to replace your Visions (30mm of stack) with a 3T Brezza (50mm of stack)? Well, you’ll need all of that 15mm of spacers plus another 5mm from maybe a -10deg stem instead of -6.
Having somewhere between 5-15mm of spacers gives you some room for equipment and positional changes that you might want to make once you actually get out on the road. The fit bike is great, but it is not exactly the same as riding on the road.