The FSA head set compression bung suggests a spacer above the stem
"It is recommended to have minimum 2mm steerer tube extended above the stem with a headset spacer (not
included) on top of stem to preload headset and secure stem to steerer tube properly. "
https://shop.fullspeedahead.com/en/files/index/download/id/90c31726193366031700722438038db1319/
Then many sites suggest not needed as the bung sits exactly where the stem bolts are without any spacers, so like this with all x5 spacers below...
EG.
"This has been the subject of some lengthy threads lately.
Cannondale have always taken the same view as this from the earliest days of threadless forks to today, during which time their expander plug type has changed. they say to cut the steerer 2-3mm below the top of the stem."
I've always gone with a spacer above and hang the aesthetics of it. I am not a fork manufacturer.
If it bothered me I'd check with the importer re: my specific make/model and see what they say re: warranty. If they'll warranty no spacer above then it's probably safe.
I'm pretty sure that the instructions that came with my Thomson stem indicated that a spacer above the stem was required with a carbon steerer (not so with a metal steerer). To be honest I'd go with a spacer above just to be on the safe side.
-- EDIT --
Actually, Thomson seem to require a spacer above the stem for all steerers (so I was wrong) - X4Instructiohttpsns.pdf
Cheers, i've just found the cannondale blurb which suggests no spacer above
that top cap design is good as it supports the inside of the steerer right to the top and the compression is a good way down the tube where the spacers are.
Would having the expander where the spacers are allow the carbon to be pushed out a little? As opposed to the expander being between the stem bolts?
The spacers aren't tight on the carbon steerer unlike the stem, which would stop the carbon being pushed out?
You shouldn't need to do the expander up so hard that it would exert enough pressure to stress the steerer, it only has to grip enough to enable it to put some pressure on the headset assembly. Is it slips when you tighten the top cap then you can use grip paste on the inside.
As for the spacer, I always aim to have the stem clamping the steerer for its full height but if the warranty says it's ok to have it 3mm below then it should be ok
the top cap only preload the bearings, the stem does all the gripping. as long as the steerer is 1-3mm below the top of the stem you can put as many or few spacers above as you want. after all once the bearings are set and the stem tightened to torque you could run the bike without a top cap altogether if uou wanted as it does nothing but look pretty and stops you impaling yourself on the stem/steerer top.
Heres the compression bung, with a spacer above it definetely sits more central to oppose the forces of the stem
With a spacer on top (not fitted in pic)
Putting that spacer back underneath gives even less compression bung for the steerer to press against, and also exposes the top carbon weave to much more clamping forces. So logically it seems spacer above is better, unless you want more protection around the bottom stem bolt only.
and also exposes the top carbon weave to much more clamping forces.
the stem will have a specific torque that it's designed to do it's job at, as long as you adhere to that there shouldn't be and extra clamping forces
I've never been able to get mine to tighten properly regardless of where I put the spacers. Hateful thing.
As per the user manual images above, fitting the stem with spacers above voids your frame warranty. Whilst you probably won't die, I'd advise getting the steerer cut once you settle on a position long term.
Isn't the bung there to stop the stem crushing the steerer RD?
bullandbladder, I didn't realise at first they have two bolts, a 5mm allen to torque up compression fitting, then a 6mm allen to seperately preload the headset bearings.
mtbfix - I do prefer the stem being one spacer down, but yet to try it even lower for an even more aero position. So i'll definitely cut the steerer once settled.
Odd though that many sites say the bung is to stop stem compression forces from the stem bolts, as cynic-al says, yet with no spacer on top, most of the stem is hovering above the bung, and only the very lower bolt is covered.
Bringing this back from the dead - it depends on what headset came with your Cannondale. My Supersix came with a bung where the top cap is actually a 5mm spacer and top cap combined and is designed to slot over the top of the steerer. The Cannondale manual requires 2-3mm of steerer exposed over the top, then the cap locates over the top like a spacer would and then you compress down.
More over it depends on the top cap you are using. C'dale offer two which are compatible with the carbon steerers and one for alloy steerers
