Aero bike sizing

Equipment
1

Hey all

Looking for some advice as I trying to figure out sizing for my next build.

I am 179cm with 84.6cm inseam (measurements taken from bike fit). 3.6 W/kg and hoping I can get that to 4 by September. Early 30s and should be doing some strength/flexibility work which I lack.

I currently ride a Giant Defy from 2022 in size ML. I’m trying to decide between a Giant Propel in Medium or ML. The Propel has a much more aggressive fit than the Defy. (I basically have to choose a Giant frame).

Fit/spec data:

  • I run a -10deg, 100mm stem on the Defy ML, and 17-18mm of spacers excluding the headset cap. Have 38cm c-c bars and have about 4.2cm saddle-to-bar drop. This was set up by my fitter for comfort/address a hip issue. Very comfortable (150km easy) but I find myself frequently wanting a stretch out a bit more i.e. more reach.
  • Propel Medium size has -40mm stack (vs Defy) and +7mm reach.
  • Propel ML size has -20mm stack (vs Defy) and +11mm reach.

Test rode Propel in both Medium and ML but unfortunately I couldn’t control bar width and stem length. The Medium I tried first felt great, very responsive, wippy, but a little bit cramped behind the neck when in aero hoods position (above spec with 100mm stem and I think 38 or 40 bars..). The ML felt like steering a boat in comparison (110mm stem in hindsight) although I could get lower.

I’m really stuck with how to choose which frame beyond going back to my fitter this year and trying for a more aero fit. I am thinking of ‘worst case’ replicating my current fit data and then trying to work into a more aero low/long position over a period of months.

What I’ve worked out, using velogicfit (comparison here) is that if I max spacers (45-50mm in showroom) I am only about 8mm lower on the Propel-Medium compared to my current Defy setup. If I put a 0-deg stem on the Propel-Medium I’d have a higher stack than my Defy. I’d hope I can knock 10-20mm off my current stack with some strength and flexibility work.

Conversely if I use the Propel ML then I have loads of space to play with the stack but I’m concerned that I might need to shorten stem (90-100mm). Handling is also a concern because stock spec 110mm stem it really felt like driving a boat. I’m not sure how much was stem/barwidth and how much was frame size (ML). Also, vanity, I am only a few grades into the seat post so it doesn’t look great :sweat_smile:

  • Am I overthinking it for the size?
  • Should I err to larger (ML) and get closer to replicating my very relaxed fit?
  • Am I right in thinking that the smaller (M) size with (e.g. 100mm stem, 45mm spacers) will handle better than larger size with shorter stem (e.g. 90mm stem, 30mm spacers)?

Build info if at all relevant:

  • DA 12 speed group set
  • Propel M or ML
  • Farsports 2026 Evo C 5/4
  • Narrower stock bars to start, possibly Tavelo RISE bar or similar
2

I’m pretty much the exact same size as you, and I’d go with the M/L.

You won’t want a 0° stem on that bike, and I think that a 546 stack for your height is probably a bit low.

When testing the bikes, did you leave the headset spacers under the stem?

The 4mm difference in reach is pretty small, so I’d be surprised if you actually needed a smaller stem.

While seeing a lot of seatpost has been the “cool” look for a long time, that’s changing. Look at most of the fast pros these days. Handlebars have gotten a lot higher, so significant saddle-to-bar drop is less common.

Yeah, the seat tube on the M/L is a little long, but the top tube is pretty much horizontal on that bike, and I’d rather have a little more seat tube than a bunch of headset spacers. :man_shrugging:

3

Thanks Eddie that’s helpful.

Yep - headset spacers were kept on and add 5-10mm of stack but it’s the same for all frames so I didn’t include it. In fact I didn’t adjust the cockpit at all; didn’t want to take the “p” given I wasn’t planning on buying the bike from that dealer so only did a 20 min loop on each.

Do you think maybe the handling weirdness of the ML (110mm stem, prob 40-42 bars) compared to the M (100mm stem, prob 38-40 bars) might solely be down to it being that bit too long and wide for me in that config? Steering felt very slow and sloppy, not as responsive as the M config, and it has me a bit concerned!

4

Maybe unrelated, but I felt that my bike handling (hence also descending) took a hit when I moved to an aero bike. I think it’s due to being more stretched out because I eventually moved my saddle significantly further to the front (due to pedalling mechanics and cleat positioning) and my handling magically improved

5

Out of interest, why not get the latest Defy, as it sounds like it will suit your position better, and you still have 17-18mm of spacers to remove if you want to improve your position?

With a good wheelset and an integrated bar stem, the Defy won’t be very far off the Propel in pure drag. Factor in a better rider position and it might be ahead?

7

I was originally going to build on the new Defy frame but I really want to try something different! I’m keeping the current Defy in case I have issues.

This is likely to be an interim build which I run for a year before deciding whether to change frames. Just so happens I have to choose a Giant frame for this setup.

8

Depending on which model year you’re looking at, I think both the M and M/L have 40cm bars.

The M/L likely has a longer front-center, which affects handling a bit. I’d be surprised if a M/L wasn’t sharp enough for you, but it’s a subjective thing..