I'm about to have my carbon steerer cut after experimenting with the stack height. There is conflicting information online on whether or not to leave space for a small spacer on top of the stem when cutting carbon steerer tubes. Apparently, different manufacturers have different recommendations (Cannondale, I think, says not to leave a spacer on top). My bike is from Giant and I can't seem to find any manual that specifies this issue. Of course, flush looks better, and I see a lot of WorldTour team bikes that have it flush so it must be okay(?). What do you guys think?

Spacer on top. More secure that way. Can you cite where Cannondale tells you not to do that?

Flush.

This thread will probably go on with opinions from both sides.

I am no expert, but I believe that there a couple of pieces to this. The spacer cap and the compression plug. I think it is the compression plug that plays a major role in supporting a carbon steerer where the stem is attached. The length and location is important so that the stem doesn't crush the steerer. If leaving it flush prevents proper positioning of the compression plug, there could be a problem, likewise adding a spacer could do the same.

There are tons of threads on this out there. This video is quite interesting, especially towards the end, as it shows the impact of the compression plug on the OD of a carbon steerer that, for me, is pretty eye opening.


John

The design variables of your steerer tube, cap and stem may not be the same as mine. I'd do what the manufacturer of the steerer tube and fork recommends above all others. Next the manufacturer of the bike, then your local trusted mechanic, then the advice of anything else you find or get. At anytime, you can choose like Fleetwood Mac, to go your own way.

I’d email Giant.

The only generic advice that makes sense is to do what it takes to make sure the expansion plug is in the position to support the clamping force of the stem. If that requires the pre-load device (that screws into the expansion plug) to be flush with the stem, do that. If, like my most recent fork, it requires about 5mm spacer above the stem, do whatever makes sense for that situation.

I think the fork I'm talking about came with those sort of instructions.