Lennard Zinn has a lengthy answer for what is far from a simple question regarding stack height.

Headset spacers

Dear Lennard,
I have seen several references that suggest that 30mm of spacers below the stem is the maximum you should have for a full carbon fork steerer tube. I have seen others that say 40mm would be the maximum. What is your position on this matter?
— Richard

Dear Richard,
Even though people want a blanket answer to this question, there is no way such an answer can be given because it depends on so many things.

Obviously, stress on the steerer and how many spacers can safely be used depends on the construction of the steering tube — its wall thickness, types of fibers incorporated and their orientations, the resin used, and the compaction and complete wetting of layers with resin.

It also clearly depends on the weight of the rider and his or her position on the bike, which determines what percentage of that weight is resting on the handlebars. Another critical dependency is how long the stem and handlebar are, as greater leverage puts greater stress on the steerer. The shape of the stem clamp and sharpness of its lower edge also affects the stress on the steerer.

Finally, the spacer height that can be safely used depends on the type of riding the bike will be subjected to. If the rider pedals it gently on smooth roads, the answer will be different than if he rides it into curbs and big potholes or off of semi-truck loading docks.

I have a philosophy on this that is colored by the fact that I design and build both custom and non-custom bikes for very tall, and often very heavy, people. I don’t want to ever have a steering tube fail, and, for that reason, we use a long, glue-in aluminum insert inside the top of carbon steerers on the bikes of big riders. Only then am I confident enough to use a tall spacer stack on the fork steerer.

Tall riders who do not have a custom frame that fits them often have a big spacer stack of as much as 100mm under their stem in order to get their handlebars high enough. They are playing with fire, in my opinion, if they have a carbon steering tube and don’t have one of our inserts inside the steerer. Breaking a steering tube is a guaranteed crash. And even flexing the steering tube, when flex is an issue for the entire bike and fork for any tall, strong, heavy rider, reduces efficiency and control.

We go to considerable expense and extra effort to install our glue-in insert system in the carbon steering tubes of forks that go on bikes for tall and heavy riders, as it provides a greater margin of safety for big riders. We have never had a customer break a carbon steering tube, and we intend to never have it happen; going to all of this extra trouble is our way of further ensuring it doesn’t happen.

Most carbon forks come with a simple expander system that is about an inch long. We feel that it doesn’t give us the same certainty of longevity of the fork steerer, nor does it provide as much stiffness to the steerer as ours does. Those expander inserts ONLY support the steering tube directly under the areas inside the stem clamp. Worse, if the user has a tall spacer stack above the stem, there may be no support at all inside of the stem clamp. And, critically, even if the expander is in the right spot inside of the stem, it provides no support of the steering tube below the stem. That may be enough, and for most riders it probably is, but I insist on a bigger margin of safety for a really big rider; I want to reinforce a long way down inside of the headset, even if the rider uses a bunch of spacers below or above his stem, or both.

Long ago, we had True Temper make special carbon forks (called Alpha Q Z-Pro) for us for tall bikes; they had 450mm-long double-thickness steering tubes. Despite that extra thickness, True Temper supplied, and we used, a four-inch-long, glue-in insert for them (it was an Alpha Q insert for a 1” steerer, rather than for a 1.125” steerer; because the steering tube wall thickness was so great, that’s what fit).

When True Temper quit making carbon fiber bike equipment, Ben Serotta, who had bought the Reynolds Composites factory, made carbon forks for us with 450mm steering tubes. When Serotta shut its doors in 2013, ENVE made us rim-brake forks with 400mm tapered steerers. Now, ENVE offers the Gravel Road Disc fork with a 400mm tapered steerer. Both of these steerers have the same wall thickness at the top as all ENVE forks and come with a standard expander plug. Since I have never heard of an ENVE steerer breaking, maybe I’m being overly cautious, but I don’t want to mess around when building bikes for 6-foot-10, 350-pound riders; consequently, we get special glue-in inserts made specifically for these forks.

The Alpha-Q insert was four inches (100mm) long and had a star nut pounded down into its bore. Wheels Manufacturing, which is conveniently located near us, now makes us a five-inch-long (125mm) aluminum sleeve insert with integrated thread inside for the top cap bolt. We glue it in with JB Weld epoxy.

We first sand inside the steerer, blow it out with compressed air, and wipe it clean inside with a clean rag soaked in rubbing alcohol. After epoxying it in, we leave it sit for 24 hours before adjusting the headset and tightening the stem clamp.

We sell that insert separately.

For a while about nine years ago, Isaac made a 60mm-long expander plug, which I think is also a more secure method than standard expanders, but I don’t believe those exist anymore.

Anyway, that is a very long answer to your question. If you are a lightweight rider who sits up very high on the bike with the handlebar much higher than the saddle (so that little of your weight is on your hands), and you ride on smooth terrain at relatively moderate speeds, you can probably get away with a very tall spacer stack (probably as tall as you want) on almost any carbon fork while using the standard expander plug under the stem clamp. But if you are heavy, ride with a long stem, and your handlebar is far below the height of your saddle, you should be cautious about using more than, say, 25-30mm of spacers. Same goes if you ride your bike fast on rough terrain.

I just don’t think I can give a blanket answer to this question that is more specific than this.
― Lennard