I’m building a Gen V Merida Reacto with EXS Aerover bars. The bars were supplied with a dedicated headset cover for the Reacto as well as spacers:
Firstly, I’m used to headset covers not being included in published frame stack and head tube length figures from manufacturers, but as the head tube measurement of this size S frame is given as 128mm in the geo table, the head tube itself measures 120mm, and the headset cover (both the Merida one and the EXS replacement) measures 8mm in stack, I assume that the headset cover is being regarded as part of the head tube for geometry purposes. So far so (relatively) simple.
However, there is also a spacer supplied with the EXS bars that is obligatory to use because it is shaped to fit the bottom of the bars (the top one in the photo). This measures about 6mm. In the picture above there are additionally 20mm of “normal” spacers (4 x 5mm). The stack of the stem is stated as being 35mm, and indeed the part that clamps the steerer seems to measure as such (although the slightly recessed top cap adds another layer of confusion). But if the 6mm shaped spacer is necessary to fit the stem, is it possible that it’s included in a nominal stem stack height? I’m assuming not, simply because the actual stack measures 35mm, but I suppose it’s possible that the effective stack (relative to the stem height) could be 35mm even if the actual stack height including the spacer was 41…
Basically, can I assume that the stack added in the above setup to the published 128mm frame stack is 6 + (4 x 5) + (0.5 x 35) = 43.5?
(Edit: just to pre-empt a possible response - I’m aware that stem angle also has to be factored into total stack, I’m just talking about stem stack here).
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Why are you halving the 35mm stem height? Adjustment space? Otherwise I think you’re on target. I would assemble after marking and do a sanity check though.
My understanding is that stem stack itself is always halved to allow comparability when making these calculations. It means you are effectively measuring to the middle of the bars once you get the figure for total stack. If you have two stems and one has a 30mm stack and the other is 40mm, the 40mm one puts the bars 5mm higher than the 30mm one (not 10mm), because the stem emerges from the middle of the clamping area, not the top of it.
So there are two stack measures, headset stack and size stack. Which are you looking to compute?
I’m ultimately trying to calculate total stack, so that I can replicate it between different bikes. This is basically frame stack plus the total stack added by the headset, spacers, stem stack, and the effect of stem length and stem angle on bar height.
I have a spreadsheet that can do all of this (as well as corroborate published frame stack using the frame’s other measurements). The problem is simply knowing whether the EXS Aerover’s claimed stem stack height assumes that the top 6mm spacer is included or not. On the one hand this seems unlikely as the stem clamp area does actually measure 35mm, but on the other, it seems odd to have a 6mm spacer that is in effect a necessary part of the stem installation and not have it included in the stem stack figure.
Ahh I understand. That’s a good question. I’m inclined to say the spacer is included but that might be an email to the mfg to be sure.
In the past it was easy. The head tube was a certain length, the headset added a certain amount of stack (usually about 10mm), and the stem stack was just the length of the clamping area. The centre of the handlebar would be at a position determined by a line running from the centre of the stem clamp to the centre of the handlebar clamp, this line being the length of the stem (110, 120 or whatever), and running at an angle determined by the stem angle and HT angle.
Now with integrated bars there isn’t even a bar clamp, the bar itself usually isn’t round, and the stem is no longer a straight cylinder but can be any sort of weird shape. You have to trust that the figures the manufacturer provide for stem stack, length and angle end up placing the bars at the same height they would if they were quoted for a conventional straight stem of the same stack, length and angle.
I suppose ultimately you can set the saddle height precisely and then check saddle to bar drop.
I got a Tarmac SL8 last fall and while the dimensions of the frame, headset cover, spacers and under-stem adapter were clear, the dimensions of the bar-stem were ambiguous. I ended up creating a CAD drawing and importing a side view image of the bar-stem (scaled using the drop of the bars as reference) to compare with another bike to be able to select the total spacer height and stem length to match what I wanted. Not perfect but very very close. That said, it was a pain and wish brands made available drawings with accurate dimensions.
