Above the stem
 

What is the approved measurement for the amount of spacer above the stem?

As my core strength is improving I’ve started to move the stem down on the steerer tube, right now I am at a 10mm spacer above and 23mm below. I don’t want to cut the steerer more than once and think I’ll probably lower it some more but a don’t want to commit a BF faux pas.

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If you are positive you like your position and positive you will not be selling the bike, enough for one small spacer, 5mm.

At 10mm you are close enough that only a meanie would say “down periscope” or “chimney” to you so I would leave it alone for now.

Most would say no spacer above or a 5mm spacer above at the most is the perfect look (and I tend to agree).

That said, I always stick with a 10mm above just in case I ever fancy/need more height at some point. Stick with what you've got. If you do lower later on, you can cut then.

I moved my stem down and put the spacer over it. I don’t mind the look and it’s isn’t hurting anything by being there.

A 2mm spacer is generally enough, and no spacers will work perfectly if the top cap is designed for it; the important bit is that the top cap can push "against" the stem.

Anything more than 5mm is unnecessary, and tall spacer stacks above the stem prevent the compression plug (depending on it's length) from supporting the stem from inside, so with a spacer stack you will be clamping unsupported carbon, which is a bad idea and not how it was designed to work.

Not sure this makes sense to me, the steerer had the stem at the top with just enough gap to allow for proper preload and 33mm of spacers below. If I rearrange the order and move a 10mm spacer above the stem with a long enough compression plug what difference would it make? The compression is 40mm long and reaches below the top bearing.

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We both ride slammed stems and all mechanics I ever spoke to when I get these chopped suggested to leave a 5mm spacer on top.

Most top caps simply won't work (they won't preload the bearings) without a few mm of spacer, although you can easily find aftermarket top caps with integrated spacer and some bikes come with one. Furthermore, not all compression plugs reinforce the very top of the stem. If you don't know the details, leave a 5mm spacer on top, and you guarantee everything will work correctly.

If your bike comes with a top cap with an integrated spacer, or the stem and top cap are designed not to need one (many Specialized bikes) there's absolutely no need for 5mm spacer on top.

Many compression plugs are not very long, and don't reach below the top bearing. What you have is great.

Actually, a tall spacer stack will only be a problem if the compression plug is not long enough With 50+mm plugs available, it can probably be worked around. People ;just need to look for them if they need them.

No spacer above means the stem is tightened and the top of the steerer tube is impacted by the tightened stem. That seems less than ideal.
Setting it up so the entire stem is below the top of the steerer tube means the stems full clamping force is completely on the steerer tube and none of it is pinching and pushing in the top of the steerer tube.

...or am I misunderstanding what you posted?

If we want to talk about structural integrity, the area where the bolts are must be supported adequately from inside by a compression plug for everything to be ideal. The reason why it's safer to recommend everyone leave a 5mm spacer are compression plugs like this one:

https://cimg5.ibsrv.net/gimg/bikefor...846c4990ca.jpg
Not ideal

You'll notice the top (black) part of the compression plug doesn't actually expand, so without a spacer on top, the top bolt of the stem will, depending on location, clamp a partially unsupported bit. This type of steerer plug is also why a stack of spacers above is unwise if you haven't checked what sort of compression plug you've got; it's really short, so if you put 20mm of spacers above, the bottom bolt will clamp an unsupported bit. It's not a very good design, but it gets used.

This one is completely fine structurally without a spacer on top, and is longer meaning that with a spacer or two on top you'll still be fine nevertheless:
https://cimg4.ibsrv.net/gimg/bikefor...dce177a9fa.jpg
A much better design.

FWIW the second is the type of compression plug I have.

The wall thickness on my steerer tube is also at least 3/16s” thick carbon.

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Being very old and riding a fairly amount of drop, I have left myself a cm in case of a sudden attack of decrepitude.

If you are happy with just putting the spacers you take off the bottom to lower the stem on top, then there is no need to cut the stem at all. It's not like I'm going to pass you on the road and that's the thing I'm going to notice about you and your bike.

But if you do have to cut it, and your steerer is carbon fiber, then the general advice is that the steerer should be at or above the top of the stem slightly and a spacer on top the stem. A 2 mm spacer is more than enough if it allows enough slack to be taken out of the headset bearings without the steerer tube touching the cap.

As long as your compression plug covers the stem area, you're fine. Aesthetically speaking, the less the better, that's for sure.

Obscure trivia: Cannondale's "SI Integrated" carbon forks & headsets & expansion plugs etc (the hardware for which iirc is OEMd by FSA) explicitly state that you should not use any spacers above the stem. I haven't looked at their design to figure out why they are so emphatic about this point, but it's slathered all over their installation guides and owner's manuals enough times that I'm sure there's a reason either their design team, or their legal team, insisted this be stated unequivocally.

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If this is the design:
​​​​​​https://images.bike24.net/media/366/i/mb/95/d5/51/cannondale-kp017-si-expander-by-fsa-1136668.jpg

I can see why. The expansion plug is very short so any spacers above and you are claimping an unsupported CF steerer, and the mechanism is made to preload the bearings without spacers.

I have a Cannondale Synapse and my manual states the same. I will explain why below.

Exactly. What you linked to cannondale-kp017-si-expander-by-fsa-1136668.jpg (366×366) (bike24.net) is the bottom version of the Cannondale Si Compression Assembly on page 8 of the manual (for the 2012-2016 Synapse Carbon / Hi-Mod). 129387_oms_synapsecarbon_en_low.pdf (cannondale.com). I have the top version (on the same page). In either version, the top cap is more than a flat cap; it also has a cylindrical portion which extends into the steerer past the top stem bolt. Thus, if there are spacers on top of the stem, this top cap may not extend low enough to support the steerer against the top stem bolt. So, if I were to move spacers above the stem, I would have to switch to a different expander plug which extends low enough to support both the top and bottom stem bolts.

The Synapse also allows a generous 55 mm of spacers under the stem; I am using 50 mm (with a -8 degree stem). I have not seen any other bike with a carbon steerer which allows > 40 mm of spacers under the stem.

However, interestingly, in the manual 2017-2020 version of the Synapse, oms cd bk rd synapse disc en r1 134911.pdf (cannondale.com), the Compression Assembly does not support the top stem bolt, only the bottom stem bolt. See page 10.