So, with a new all carbon fork arriving for the fat bike, whats the best solution for fitting the top cap?

All my other forks are SFN’s but I believe carbon needs a different approach. I have a Hope Head Doctor in the spares drawer, is this OK to use? All the CF fork bungs I see on the inter web look double the length….

I’ve used these before:
Expandy thingy
They work well but have always struck me as a relatively heavy solution. If anyone knows of a lighter option, I’d be interested to see it.

Head Doctor might not fit due to the thicker walls of carbon steerer. Extra length needed to ensure that the stem clamping load is taken evenly on the steerer

Hed Doctor may not fit as carbon tubes tend to be thicker and therefore have a smaller internal diameter.

Beaten to it by dovebiker.

It it also longer to avoid crushing the carbon, something that might not happen with alloy steerers?

So the bung is all the way through the steerer clamp ?

Expanders are needed for the stem to clamp onto, not just to preload – hence their length. Giant do a lightweight version that is bonded into the fork once position is dialed in. I haven’t bothered to use mine.

A-ha. Understand now, thanks

I’m using a head doctor in a carbon steerer without issue.
Not saying it’s right, but it works fine.

No, stems do not need a longer expander to “clamp” on to. If you’re crushing a carbon steerer with your stem, then you’ve tightened your stem bolts way too tight.

I use one of these without any issue: http://www.ebay.co.uk/itm/J-L-Carbon-Fork-Headset-Key-Expander-Star-Nut-1-1-8-Super-light-6-8g-/122045526289?var=&hash=item1c6a7af111:m:miGkKtqUKZePYL7sqsJgtxQ

Canyon bikes don’t even have an expander in their steerer tubes!

Every day’s a school day. Good advice, thanks

No, stems do not need a longer expander to “clamp” on to.

Manufacturers like Specialized and Trek are very clear that the expander should be fully in line with the stem. Referring to whether or not the clamping force is sufficient to crush the stem is an over simplification and a red herring. You will commonly find similar guidance and accompanying diagrams published by the big manufacturers, e.g. the diagram below published by Specialized and that on page 8 of this Deda manual.

Manufacturers like Specialized and Trek are very clear that the expander should be fully in line with the stem.

Of course it should, but not to prevent the stem crushing the steerer tube.