Originally Posted by
Helldorado
What I have read and seen on-line: For carbon steerer tubes, the top of the tube should be no lower than the top of the stem clamp. This avoids asymmetrical forces on the fragile carbon steerer tube as one tightens the stem bolts and which could crush the end of the tube. A spacer keeps the end cap from touching the top of the compression bolt cap; if they did touch, it would be difficult if not impossible to secure the steerer tube inside the headset.
Yes, you are correct, for carbon steerer tubes! The above mentioned Park Tool article says "
When dealing with a carbon steerer tube, it is critical to have the steerer tube go all the way through the stem, in order to reduce stress on the end of the steerer tube. It is then necessary to have a spacer on top of the stem."