Spacers on top of stem
 

I just bought a new frame and a fork and will cutting the carbon steerer tube of the fork. I am not sure what exact length to cut it in order to be comfortable but I need to leave it longer than too short to start. Is it ok to stack extra spacers (10-15mm) over the stem on a carbon steerer just until I figure out the proper height and I am ready to cut flush with the stem?

It shouldn't hurt a thing, but it's best to make the final cut leaving 2-3mm of steerer above the stem, so a 5mm spacer is always on top. It helps reduce the chance of cracking the top of the steering tube from overtightening the stem.

Once you tighten the stem, the excess above it does nothing so will not hurt anything (unless it's long enough to bump your chin on).

I have left spacers on the top of my stem on all my bikes until I was ready to cut the steerer. Doing this is absolutely okay.

Some manufacturers specifically advise you to not leave any spacers above the stem except for temporarily while you are fitting the bike. The reason they give is that if you have spacers above the stem, then the part of the steerer the clamps down against will not be supported by the top cap. This might be just some cover-your-ass thing, but I know Cannondale and Specialized warn against it.

I assume you know this, but whatever you do, don't make the top your stem/spacers "flush" with the steerer tube. The top of the steerer tube should be 2-3 mm below the top of the stem/spacers.

*scratches head* this don't make no sense. the steerer and top cap never come in contact in any setup.

Some have a sleeve that goes into the steerer. For example:

http://bikeforums.net/attachment.php...hmentid=142194

The instruction manual for my Easton EC 90 fork specifies this. Personally, I think it looks like hell but I'll make the aesthetic sacrifice in the name of safety.

Equally puzzled and agree the above statement makes no sense. However, to modify your posting slightly; "the steerer and top cap never come in contact in any setup" if the steerer is cut to the correct length or sufficient spacers or stem height are provided above the top of the steerer.

I've never read any manufacturer's instructions that caution against having spacers above the top of the stem. In fact, I've have seen several that recommend cutting the steerer long enough to require a spacer above the stem in the interest of having the stem clamp supported full height.

Here are two examples where the instructions specifically state to not put spacers above the stem:

http://cdn.specialized.com/OA_MEDIA/...n_Guide_r2.pdf
http://www.cannondale.com/usa/usaeng...plement_en.pdf

In each case either the top cap or compression plug is designed to support the steerer tube from the stem clamping force. I would bet that the Reynolds fork that goes with the compression plug I linked to above has the same warning.

I don't know who makes the OP's fork, but I'm just pointing out that some manufacturers advise against putting a spacer above the stem.