Question re: Stack, Reach + speed & aero

For the bike-fit specialists and the numbers-oriented folks out there:

Here’s the situation: I have a custom road bike, and I was fit by the frame builder in person, so it has been pretty much perfect . However, they built the system around a 110mm stem with a -6 degree angle and 30mm of spacers. The reason for this, they told me, was that it would give me future flexibility to lengthen/shorten the stem and lower the stack in the future as my riding style might change. The bike was built in 2018.

For the last few summers I’ve been thinking about getting a bit more stretched out as I’ve gained flexibility, core and riding strength. Recently, I bought a 120mm stem and installed it at a -6 degree position — after giving it a month of riding, I came to the conclusion that it wasn’t working out. The handling was more sluggish, I couldn’t get the drops to feel right and I felt much sketchier descending.

Because it was a expensive stem, I thought I’d flip it into the +6 position and see how it felt … and other than being completely appalled at how it looked aesthetically, it really changed the entire feel of the bike in many positive ways. The handling was much livelier, I felt more solidly situated on my saddle, and keeping myself centered over the bottom bracket was much easier (thus descending felt rock solid).

However … it’s still a little to high for my personal taste and riding preference. I found an online calculator the tells me if I remove a 10mm spacer while keeping the 120mm stem in the +6 position it will increase reach by 6mm while only increasing height by 16mm over the original 110mm stem in the -6 position.

I think that really might be the happy medium position.

Given that my goal in setting out on this experiment was to get in a little more aggressive and aero position (while still being comfortable), would that position with the extra reach be completely negated by the extra height due to the stem angle.

In short, have I just slowed my bike down, technically speaking?

Or does everyone think I’m nuts and I should slam my stem with no spacers and a -17 degree down angle? :stuck_out_tongue_closed_eyes:

If your bars aren’t internally routed, then it should be pretty quick and easy to try the space removal, no? In that case, just try it instead of over thinking it.

It sounds to me like the issue is weight distribution more than anything. Having too much weight on the hands will cause your symptoms. As you lean forward you also close your hip angle, so you might need to lower your saddle a bit or even tilt it to compensate. You still might not be able to breathe and lay down the power if you go too low. Being more aero is not the only important factor in being fast.

Personally, I find making the drops easier to spend a lot of time in is makes me faster than having a more aero position that I can’t sustain.


Flipping your stem like you did pulled your weight forward. If you get a shorter stem and push your saddle back a bit, you can go low and keep your weight distribution close to your old position

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Tagging you in case you can weigh in here – seems like this topic is right up your alley if you have a minute :+1:

I missed the and have to give it a read to see if I can help.

OK, I need to do this in a clear timeline to discuss the changes and your experiences

  1. 110mm x -6* on 30mm spacer stack = initial setup per fitting and custom build

  2. 120mm x -6* on 30mm spacer stack = first change that feels sluggish and sketchier descending

    • Presumably from lack of steering “snappiness” or maybe you are too far forward?
  3. 120mm x +6* on 30mm spacer stack = lively handling and better saddle position

    • Presumably the shorter and higher reach allow you to “sit back” more on the saddle vs reaching for the long/low setup in #2 above?
  4. 120mm x +6* on 20mm spacer stack (10mm lower) = proposed next step

Comparing 3 & 4:

Comparing 1 & 4:

I think it 1 to 4 is a worthwhile change and easy enough to apply and give a ride I’d try it. Depending on what you think of the feel, you can take half steps by using 5mm or smaller spacers to juggle the height for a fine tuned setup. Chances are good you will find something that works and keeps your expensive stem in use.

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You nailed it … my question is, does the aero penalty from the height negate the aero benefit from the increase reach? Or is it negligible?

Aero is tricky, but overly simple… the 1 to 4 change is potentially a give and take that may not hurt you. If you “stretch” a bit more, you may be more “aero” even with the 6mm taller height. In reality, 6mm functional height change is likely “noise” in the grand scheme.

So much relies on how your body rests in each position. Things like your arms, shoulders, back curve and such all play into your aero, so it’s all but impossible to make any firm claim based upon these “minor” stem changes. If we were talking deltas of 15-30mm height and reach, and looking precisely at your body in each position, we might be able to make some guesses. But without a wind tunnel, it’s all far beyond guessing in many ways.

I just can’t make much of a claim with so little info. Even when I have people in my studio, aero discussion is covered with a huge * for all the comments.

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