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Why?
If the stem ends in empty space, it tends to close up to a smaller diameter than the steerer over that empty space. That makes the bore of the stem into a cone, which is an inclined plane driving the stem up the steerer, which makes the preload screw work harder than it should to maintain headset adjustment. If the preload screw precesses in it's thread, which is will given half a chance because the load on the stem tends to rock the cap back and forth, the stem will walk up the steerer and you end up with a loose headset. Reducing preload screw precession is also why I recommend tightening the preload screw to a higher tension after securing the stem clamp.
Also, and this is a particular concern with carbon steerers which don't handle compression as well as metal ones, having the steerer poke through the stem reduces the maximum hoop compression stress on the steerer.
gbj_tester-
well explained. I'm having 10mm above the stem and 20mm below spacing wise. I believe theres also a max spacer height for carbon steerers of round about 30mm below the stem from some carbon fork manufacturers.
(settled on 5mm above on the bike shops advice)
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My top cap has a lip. A 1 1/8 downward protrusion that locates it snugly into the stem, so it stops the stem from being pinched more by the top bolt than the bottom.
So if that's the only reason I could run without a top spacer for moar HHSB points?
BernardRenault
Hefty
I know this to be a thing but have never thought about why.
So...
Why?