lyle.coop

I'd like to gain some height on my handle bars, but currently my steerer tube is just proud (1-2mm) of the top of the stem (40mm stack height). I've read that the top of the steerer tube needs to be at minimum, level with the top bolt of a 2-bolt stem. Now that was for aluminum or a chromo-fork. That could be different for a carbon steerer tube because of the location of compression plug used to connect the top cap and the steerer tube.

My question is can I add a spacer under my stem to gain a little height?

See pic.

cuevélo

If you want more height, flip your stem instead. That will affect your reach as well, but it will be a starting point to try out.

Stem Comparison Tool | yojimg.net

aliasfox

My understanding is that you should always have a spacer above your stem for a carbon fiber steerer. The issue (as I've read) is that if you put the top bolt of the stem too close to the edge of the CF, you may end up crimping the unsupported, open end of the steerer. With an alloy/steel steerer tube, you could always have your stem be 2-3mm above the top of the steerer and be fine.

But yes, in this case, flipping your stem is likely a better option for minute changes. I currently run with a 10mm spacer above my stem, and a 5mm spacer below my stem, which allows me to go 5mm up or 5mm down from my current position. I had 20mm extra sticking out before (for a total of 30mm above the stem), but that was definitely excessive.

Broctoon

They say the steerer is more likely to crack if you don’t have it protruding at least 10mm above the top of the stem.

So that’s what I did with the last one I cut (the only carbon one I’ve ever gotten new).

I bought a fork with the carbon steerer already cut, and to get the stem height I want, I positioned spacers so there’s 5mm of protrusion. Figure at least it’s not flush or below the stem top. I’m going to see what happens. (I know, I like living on the edge.) So far, my bike has not exploded.

chaadster

I don’t run any steerer above my stem; I always cut just under flush, but I also use use a long expander plug, specifically Deda plugs, so the entire clamp area is supported.

Assuming use of such a plug in this case, yeah, you could probably add a 3mm spacer, but what are you going to gain from 3mm? I wouldn’t bother. I’d change the stem to get the rise I wanted.

lyle.coop

Yeah 3mm isn't gonna make a difference. For now I'm going to flip the stem (90mm, 10 degree) and move some of the spacers under the stem to above the stem.

Broctoon

For preventing cracks, it’s probably more important to ensure there are no irregularities inside the stem’s clamp area, like burrs or trapped debris.

datlas 

No. Flip stem upwards to start as suggested above.

genejockey 

Doesn't he need a spacer under the stem so the top cap is acting on the STEM, not bottoming out on the steerer? Or am I missing something?

SoSmellyAir

That was what I thought too, that the top of the stem (or the spacer above the stem) must be about 2 mm higher than the top edge of the steerer, so that tightening the top cap preloads the steerer tube bearings.

SoSmellyAir

My bike does not permit spacers above the stem, because the carbon steerer tube is supported by the metal top cap (at the top stem bolt) and by an expander plug (at the bottom stem bolt), and the distance between the top cap and the expander plug is adjustable (to account for the stack height of the stem).

topflightpro

It used to be that carbon steertube forks commonly suggested putting a spacer above the stem. That is no longer really the case, and many manufacturers now stipulate their should be no spacers above the stem. It really depends on the model and manufacturer.

OP, you can probably add a 2-5mm spacer under your stem and be fine. It looks like you actually need another spacer to give you the ability to compress your headset with the top cap. Though, as already noted, if you need to raise your bars, flipping the stem is a great idea. It will shorten the reach though. This site is great for seeing how your reach and drop will change by flipping your stem: Stem Comparison Tool | yojimg.net

aliasfox

Hmm. I don't think I've seen a setup quite like that, but it doesn't sound like a standard compression plug. Good to know that something like that's an option though.

chaadster

No, not if preload was set with a spacer on top of the stem. Once preload is set and the stem bolts tightened, the top cap and the spacer below it can be removed, which is what I’d assume we’re looking at in that picture. I’d probably run a top cap— the kind with an apron which overlaps the steerer— rather than run with the uncovered look, but for the ultimate weight-weenie, “a gram saved is a watt earned,” I suppose.

genejockey 

Given the nature of the questions the OP is asking I would not make that assumption.

chaadster

You think they’re riding around with a loose headset? The question was about adding a spacer below, not adjusting the headset preload.

genejockey 

Those are not unrelated.

chaadster

No…

bahula03

For "standard" carbon steerer/threadless setup, having the top of the steerer 1-2mm below the top of the stem is correct. The sometimes needed spacer above the stem is there to make sure that the compression from the top cap is following the correct "path" and that the bottom of the top cap doesn't contact the top of the compression plug before there's adequate preload. Given how much variance there is in the design of this stuff, if you aren't sure, the easy/smart thing to do is ask the company that made your bike

That seems like an...interesting design. Makes me wonder why they didn't just go with a compression plug long enough to cover the length of the stem clamp.

Kimmo 

As long as there's compression plug bearing on the steerer behind both bolts, you should be able to get away with whatever, but I'd be uncomfortable with anything more than 2mm below the top of the stem.