I’m in the process of building up a new road bike (Cannondale Supersix) which comes with a carbon fork steerer. I’ve not had a carbon steerer before hence the question/concern!

I’ve had a play about with the stem height and think 30mm of spacers bellow the stem is about right. Normally I’d leave a good few spacers above the stem and ride around for a few weeks while making adjustments. However, I’ve noticed that in the user manual online C’dale state that the maximum bellow is 55mm BUT no spacers above the stem. Clearly this goes against my idea of riding around for a few weeks before making the final decision. I can only assume that this is something to do with the funny expanding bung that is used instead of a star nut having to be pressing against a section of steerer that is supported by the stem clamping force.

So, my question is…will I die/crack the carbon steerer if I leave a nice stack of spacers above the stem in the short term before deciding on final height and getting it cut down (as I’ve always done with alloy steerers)? Cheers!

I’d tend to think that if it says no steerers above the stem specifically, it says so for a reason.
It may be that they just don’t want to allow people to leave millions of spacers above only for someone else to come along and move the stem up (Why else would you leave >55mm of spacers in total, after all). Have you tried contacting Cannondale to check?

I’d strongly advise dumping the stock fork bung and getting an aftermarket version. For example m:part (Madison brand) with a smear of carbon paste in the form steerer

The stock bung is notoriously poor at staying put (even with carbon paste)when trying to tighten the headset, it’s not a cdale part but the same generic item used by many brands.

I’d also recommend swapping out the stock spacers, my supersix Evo creaked and headset came loose until I realised the stock carbon spacers were ‘relieved’ internally leaving 3 minimal points of contact with headset cap and stem body. I put some regular carbon spacers on and creaking stopped, no more loosening

No issues running a spacer or two above the stem until it’s dialled. I’ve done that with all my road bikes including my supersix Evo. Typically 35-40mm is enough to play with until you are confident to trim.

Also, be very careful how much you tighten the stem, the steerer is very thin, and I have seen them cracked.

@coatesy

good point, goes without saying you should be using a torque wrench when tightening any light weight components whether aluminium alloy or carbon fibre

this is the fork steerer plug I recommened (the photo shows one rebranded as specialized)

Torque on the fixing bolt was recommended as 7-9nm (read the instructions that come with the specific product), headset compression using stem cap / bolt normally takes only 2-3nm if headset (may need shims) and spacers are setup properly

this was my Supersix evo showing stem spacers above and below

Great advice there thanks. Its a frame only build so no cannondale spacers to replace. I’ll look to get one of those fancy steerer plugs, look much more adequate. As for using a torque wrench…I’m a bit OCD/paranoid when it comes to anything carbon so always use one!