Originally Posted by
ljsense
When I'm ready to publish the numbers from the lab, I will, but in the mean time, I suggest removing the top cap from a mountain bike or a fat bike with a carbon steerer, and riding it for a few days over rough terrain and medium sized drops. Make sure the stem bolts aren't over torqued. You may be surprised to find that your headset has loosed. The stem cap is actually producing pretty significant downforce if the bolt holding it is torqued, as most specify, to around 4 Nm.
I've had similar real world experiences with carbon steerers and the stem slipping. One was a road bike and the other a mountain bike with a rigid fork. In both cases, the expansion plug didn't have enough surface area to prevent the stem(and expansion plug) from slipping and causing a loose headset. On my road bike, I cured the problem by switching to a better expansion plug. My friend fixed his MTB by epoxying the expansion plug into the steerer. The old adage of not needing the top cap after tightening the stem does not necessarily apply to carbon steerers.