Top Cap advice - Bearing load etc

daz_mtb

Hello guys

I have a 2011 synapse carbon with its seems a system intergrated or FSA SI top cap, the bike has carbon forks.

This is the existing top cap

http://cgi.ebay.com/Cannondale-SI-Expan ... 335f4c4d1c


Would this be compatible? not sure if I need to stick with the SI top cap.
http://cgi.ebay.co.uk/FSA-Carbon-Top-Ca ... 43a3f3d7df

Cannondale instructions and general internet advice has me confused
The current top cap has a hex hole in it, leading to a hex bolt through the hole, if this is tightenined it expands the unit, however there also is a higher wider nut (the top cap itself) what does this do? I understand it might be preload... what is this? how tight should they both be? I am scared incase I screw it up and lose my steering etc

Do you think the new carbon cap I want has these two adjustment settings? as from the picture I can't tell if there is a lower chamber with the lower adjusting nut.

Thanks!


PS in the manual it states not to add any spacers above the stem, I only want to add one 5mm, surely thats ok as long as the top cap has it covered and I set the torque with a torque wrench?

balthazar

The large hex fitting expands the nut, fixing it to the steerer tube. That will have a torque spec (though low- prob 5-8 NM). The other smaller bolt threads into that, using the top cap to push down on the stem, which pushes on the headset, and preloads the head bearings (this must be done with the stem loosened!)

That (smaller) bolt doesn't have a torque spec, because it doesn't fix anything to anything else: it's just an instrument for setting head bearing preload precisely. Once tha preload is set, tighten the stem attachment bolts (to spec) to fix everything. You can snug the top cap bolt up a bit more at the end to prevent it vibrating loose, if you need.

The cap you linked to is probably fine, though there can be incompatibilities due to variable internal width of fork steerers. Why do you need a new one?

Lagavulin

I found this fairly useful.

daz_mtb

Thanks being reading a bit more about it.

Think I have worked it out so the expanding top cap slots in, you tighten the lower hex - this to push out the blocks in the steerer giving grip. Once this is done you tighen the top bolt in the cap to "pull" all the bits in the steering system together - preloading bearings with force, I guess if this is too tight the steering gets hard to turn. I guess the top cap also stops the stem from breaking the steerer.

Just thinking of changing as the existing one looks beat up even tho its new!

balthazar

@daz_mtb – you've got it. It's a fairly delicate procedure, correct preload will be sensitive to half-turn adjustments. Make sure the system has no play (drop the front of the bike from an inch or two off the ground and feel for vibration), and that it swings smoothly from lock to lock.