Hey!
I have the option to purchase an Aethos expert di2 2022. I’m 173cm, 141 lbs. Unfortunately, there’s no real solid option for test riding anything because of the bike shortage. My gut is to go with a 52 BUT specialized recommends a 54 (my size medium TCR advanced is roughly a 54 and has always felt a little large for me). Any input from those how have been in my shoes would be appreciated!
The best starting point is to find your current bike (year and model to get the right starting geometry of what you have and know), and compare using tools like the following:
Unfortunately, knowing your height is only a small part of the sizing equation. Legs vs torso vary between riders of equal heights, and can lead to a different frame size being appropriate. Considering you have a gut feel on your current bike, take that and check against the optional sizes to inform your decision.
I’ve been trying to understand Specialized sizing recommendations. I’d ride a 56 but they recommend a 58. If I built a 56 it would have an 11 or 12 cm stem. Specialized ships their 56/58 with a 10cm stem. I’ve been wondering if this is a design/fit philosophy on their part.
I could make the contact points exactly the same on both a 56 or 58 so they both could fit. It kind of comes down to whether I’d want a slightly longer wheelbase.
I would do what Chad says and check the stack and reach of your current frame and then from there calculate what you’d think would be the perfect stack/reach. Compare both of those to the Aethos geometry.
Just get the same size as you would for the Tarmac…identical geometry. Problem solved. 
Like Chad said, your height is just one piece of the puzzle. We need more details before anyone can make a recommendation. Also second his recommendation for using geometry comparison sites.
Wondering if you went ahead with an Aethos and how you feel about your sizing decision. I bought a 58 to replace a 58 cm SL4, and despite the geo chart making it seem almost identical, and despite it feeling fine riding around in the parking lot at the LBS, after a couple hundred miles it really feels too small. On this new 58, I already swapped the 110 mm stem for a 120 and I want another cm of reach, but Im not really that psyched about putting a 130 cm stem on it. I’ve dropped a cm or so of stack as well. I’m pretty bummed about it…
I did buy it and it’s the best fit out of the box I’ve had. Great bike.
Could you have much shorter reach bars on the Aethos compared to your old SL4? The Aethos seems to come with 75mm reach bars. If you had 85 or 90mm bars (much more common 10 years ago), that could account for wanting a longer stem.
What size did you get, and what were you on previously?
Good point, and actually I broke the stock carbon bar on my SL4 two years ago and put on whatever the specialized carbon road bar was at the time. “short and shallow,” I believe. So I think it’s probably more or less the same bar.
I got a 52, recommended size was 54. Previously rode a TCR size Med. TCR always felt large underneath me. Thé Aethos feels perfect. I would perhaps get wider handlebars in the future as I have broad shoulders for my size, but all and all I couldn’t be happier. The frame itself feels exactly right for me; stem I believe is 90mm and I like the reach. I dropped the bars by about 5mm, and TBH I can’t recall where my saddle is but it’s quite high up; maybe 2 inches from minimum insertion.
I’m in the same boat.
173 cm height, 80 cm inner leg length.
Mind telling me what your inner leg length measures?
Would help me quiet a lot!
Thanks
Fascinating. I’m 173cm as well, 135-138lbs. Have a 54 Aethos with a 90mm reach as well and fits great. Do you have a good bike fit? Retul?
Did you put in the dimensions on the Specialized website?
I put in my height and then the measurement of the knee joint to the ankle joint.
Tarmac- Recommended size is 56
Crux- 54
That is as it comes from S. 100mm stem.
Hump
Ps I am 5-9