I’m about to install my first carbon fork. Are there any issues with having the steerer tube extending ~5” above the headset?

Use a lot of spacers and position the stem where it is most comfortable. Hang extra parts on the extended section like lights and phone or computer holder. It could be a benefit. Smiles, MH

Yes, 5" seems at least an inch too long.

For example, Cannondale says no more than 55 mm of headset spacers (including the top bearing cap) under the stem, and most stems (that I have seen) are under 45 mm tall. That adds up to 4" at most. Most other manufacturers (that I have shopped) allow even less maximum headset spacer stack height. You should clarify with your fork manufacturer the maximum headset spacer stack height for that fork. An expander plug that extends past the bottom of the stem is highly recommended.

yes.. it will look ridiculous, and it just might break.

i broke a nitto drop bar while cranking hard up a very steep hill, and across RR tracks... with 48/19 "low gears".. instant crash.
luckily, it happened at maybe 5-7 mph, and i was young enough to heal rapidly.
i went to a 46/23 "low" soon after that crash.... hawg heaven.

With carbon steerer tubes, the process is a bit more involved than just installing enough spacers to position the stem at the right height.

Huh?

Can I cut the steerer with a standard tubing cutter, or do I need something else?

wrt the expander plug, do you mean below the top of the headset bearing?

I may have been born at night, but it wasn't last night....

I have not needed to cut mine. But it seems spending a few more dollars for a CF-specific hacksaw blade would be prudent.

No, I meant an expander plug that extends past the bottom of the stem, unless your fork manufacturer requires an even longer one that extends past the top headset bearing.

Both of the above points presume that your steerer tube is also CF; is it?

Go with what the manufacturer recommends. Generally it is 40mm of spacers max for most but your manufacturer might be different.

For cutting use a proper Carbon cutting blade and wear proper protection. Do it safely or just have your local shop do it and save yourself the hassle. Carbon fiber splinters are NO FUN!

Short answer is YES.

The steerer simply isn't designed for cantilever that far. Simply put, the leverage offered by the 5" lever arm effectively doubles the potential stresses on the tube.

That said, it might be OK depending on the rider and how he works the bar, for example when climbing. So it's up to the rider to make a judgement call.

FWIW the question in and of itself implies that the frame is too small. The OP should consider a taller frame, or one with a sloping top tube to raise the head enough stay with conventional stack height.

NO to the tubing cutter if you mean the type used for plumbing tubes. Your CF steerer will tend to delaminate each layer as you cut through them. Use a fine tooth saw and a guide. Plenty of info on the net and YouTube vids.

I thought that might be the case, hence the question.

The frame is an XL Lynskey; definitely not too small. I haven’t installed the headset yet, so the xtra height is an estimate.

Great advice. Thanks!

Any reason I shouldn’t use a stem spacer as a guide?

I don't know what a "standard tubing cutter" is - if you talking about one of those clamp-on things that you use to score around and cut copper pipe, then emphatically no. If you mean one of those steerer cutting guides, then sure. I cut CF steerers using a new 32 teeth/inch hacksaw blade (in a hacksaw, obv.). I do it outside to avoid inhaling fibers, and I keep dust down by wetting everything down with a hose periodically. I have a folding worktable with a couple of rubber-lined clamps to hold the fork. I use an old/disposable threadless stem clamped loosely to the steerer as a guide, and keep the saw blade against the top of the stem. Measure twice, cut once. Hold the saw lightly and let the blade to the cutting as you draw it across the steerer - don't lean on it or force it, or you'll end up splintering the CF. When you're most of the way through, rotate the fork 90 degrees to finish the cut, so you don't splinter the last bit. When you're done, use sandpaper (I use 220 grit) to clean up the cut edge.
While pretty much everyone agrees that the expander plug internally reinforces the CF steerer against the external clamping force of the stem, some folks also maintain that an long expander that extends down past the stem provides structural support for the stretch of steerer between the bottom of the stem and the top of the headset. I'm not sure I buy that, but certainly ensure that the expander is level with the stem to minimize any possibility of the stem crushing the steerer.

Because it would move when you are cutting?

I have seen suggestions to use a couple of band clamps from one's local hardware store, which is a better idea than using one or more stem spacers.

Looks like I won’t need to cut it after all.

Thanks for everyone’s advice!

Your fork should have a spec which either specifies a maximum number of spacers between the upper bearing and the bottom of the stem or the total height of the steer tube between the upper bearing and the top of the stem. For a carbon fiber steer tube, it's usually those specs are 35-45mm and 75-85mm respectively. Just contact Lynskey if the fork didn't come with any specs or instructions.

That's where you should look for info, not here.

My experience is strictly with carbon fiber steer tubes.

You use a new, sharp, fine tooth hack saw. They make saw guides for this purpose to ensure the cut is square, and they're not expensive, but I've always just carefully done it free hand. I wrap a band of masking tape around the tube, mark a line around the entire tube for the cut line, and carefully cut keeping on that line. It comes out nearly perfectly square, and I make fine modifications of the cut with a fine file - if necessary - to square it up. But it doesn't have to be gnat's a$$ perfect. I've also tried a miter box as well as the "two hose clamp" method, but didn't think it came out any better than carefully cutting along a line as described. The saw guide is probably more fool proof. But I never bought one and every time I cut a steer tube, I think it's the last so I don't bother getting one. That was about 5 steer tubes ago.

When I'm done with the cut, I lightly and carefully file or sand the outer and inner edges of the cut to lightly dull the sharp edges and reduce the chance of splinters. Look on Youtube for instructions

Oy, the suspense is killing me.

You want an abrasive cutting disc on a Dremel, or a hacksaw blade coated with fine carbide grit. An ordinary hacksaw risks tearing fibres off the tube and a regular tube cutter as used for plumbing will crush the composite.

I haven't tried the dremel - I just might next time (if ever need to again). That sounds like a good idea. But I've used a new/sharp fine tooth regular hack saw several times and haven't had any issues with tearing or flaking fibers.

It might hit you in the face when you're in the drops.

Got it done - Easy peasy! No need for cutting!

crashing hurts, and that extra-long carbon steerer can snap off, causing you to bleed and bruise. smh.

oh, and walking your fancy bike home is more brutal than the physical pain, IMO.
Carbon Fiber is good at tension and compression load resistance, not so good at bend-load resistance....the bending induces shear forces in between the various layers of material, and can cause layer separations. the epoxy resin breaks down between the fibers...
it's an engineering thing.

another thing to consider... some folks just stack spacers on until the stem height desired is reached, then stack more spacers on top of the stem clamp, then put the expander plug in and crank it all down... this can cause the steerer to crack too.
the plug needs to be located inside the stem clamp to strengthen the carbon steerer against expansion cracking.

want to see how little resitance to bending a carbon bike frame has? squeeze the top tube in the middle section... but don't squeeze Too Hard!
that is why everyone gets so concerned about clamping a CF bike in a Workstand....the tubes crush really easily. crushing is a form of bending. Now.. apply heavy torque to a sticking Bottom Bracket with a big wrench... the top tube, in the stand's clamp, BENDS some, in an s-curve.

Another super helpful post by Koyote. As if the handlebar stem wouldn't be at the top of the stem, however long it is.

Don't use a tubing cutter. Use a hacksaw with a fine blade and then finish off any rough edges with a file. You want the expander plug at the same height as the stem where it clamps onto the steerer. This makes it much less likely that over-tightening the stem will crush the steerer tube.

try the disc... i use my pneumatic die grinder tool with a 3" thin cutoff disc... wear a mask, tho.. CF dust.
work slowly and carefully.

the grit-covered hacksaw blades work well too..