I have an XTC with an integrated headset and a Thomson X4 stem.
With the stem sitting directly on the top headset I would have a distance of 5mm between the top of the stem and the steerer tube. Is this pushing it ?
Looking at the Thomson fitting instructions it would seem that they recommend 3mm.
Im currently running reba's which have a 185mm steerer. I use a 5mm'ish spacer with these however I've got my eye on some SID's which have been cut down to 175mm.
what's the worst that can happen?
5mm isn't much.
would the top bolt on the stem be above the top of the steerer?
how deep is the stem?
5mm is fine
Yep fine I reckon.
shirely thats only 5mm before you tighten up the stem cap... non-issue in my book
just. But okay. You need 3mm clearance anyway for the top cap. Is the 5mm after you have tightened everything down? I normally find it comes up about 2mm when you get everything pre-loaded.
5mm??
I have and know lot of others that mount the stem cap (no spacers) straight on to the stem.
It's fine
As Andyl said you have a clearence for the topcap to preload the bearings. If this is already 3mm on your current 185mm long forks then you will have 8mm gap with 175mm long forks. This would put the top bolt on the stem above the steerer tube and give you uneven clamping. On a cutaway clamp like you get on a Thomson stem I wouldn't risk it.
Can you see where the top of the steerer would be on this image
[img] 
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5mm will probably be ok but i don't think you have got your maths quite right.
Current set up is 185mm steerer with 5mm spacer. Say you have a 2mm to acomodate the top cap going to 175 would give you 7mm from the top of your stem. With the open design of the Thomson x4 you may find this pushes the steerer below the top bolt to the unsuported area of the stem.
I may have miss understood what you have typed but to my understanding it is not going to be 5mm from the top of the stem which would probably be ok but 5mm less than your curent set up which may make it 7 or 8mm below the top of your stem which is pushing it.
[edit] damn you craig beaten to the post and he even sticks a picture in [/edit]
* wanders off chanting must type faster must type faster
as Craigxxl - it's very much stem dependent. I wouldn't be happy with 5mm clearance on a Thomson stem as the top part of the clamp will be not doing much. I tried similar on the road bike recently with an x4 stem... it looked wrong and I quickly changed back!
reading it again your measurements don't quite add up.
3mm current gap with 5mm spacer. (or 5mm ish).
But 5mm gap with -10mm steerer and no spacer.
I would try it as when you clamp everything up you often find a couple of mm. Also make sure the crown race is fully home on top of the fork. You really don't want the top stem clamp bolt point above the top of the steerer or it can go a bit cone shaped during clamping.
edit: looks like we've all just written the same thing! 😀
So is there 5mm steerer tube sunken in to the stem?
If so thumbs down. 🙁
haha the maths indeed are ropey, thats what happens when you post the minute you get up.
OK heres some accurate measurements:
headset face to face = 140.24mm
Stem height = 40.6mm
Assuming that the crown race pulls up when assembled so its flush with the outer face of the lower headset that would be a total height of 180.84mm or 181mm
The centre point of the top cap screw is 6.48mm from the top face of the stem so that would mean that only half of it would be clamping onto the steerer.
I think that make sense?
So, its actually going to be more like 6mm short of the top of the stem isn't it?
Any recommendations of a quality stem with a shorter stack height for XC ?
Or even a one with a better clamping option to make up for the shortfall of steerer length. Keep your eye on the classifieds as i'll flog the 0 x 70mm Thomson !
It's too much imo.
Sounds like you need a shorter stack stem.
These are 35mm stack so should do yer - http://www.on-one.co.uk/i/q/STOOCNC/on_one_ultralight_cnc_stem
stop trying me measure it and try it. 🙂
if no good new headset or forks. Or I think my Easton Haven stem might have a lower stack height than the X4. Will go check...
What depth on your stem from the top to the centre of the top bolt?
You may want to factor in the cost of one of these if you are determined to use the new forks.
http://www.chainreactioncycles.com/Models.aspx?ModelID=41211
it only has a 35mm stack hight.
Pushing it...
Your stem stack height is 40mm. Buy the forks, plus one of these with a stack height of 35mm:
Sell the Thomson stem and you'll still have spare change.
Sorted.
Cant try it Andy as I haven't bought the forks yet!!!
I'll whip the top cap off and mark where the current forks are. I know that the ones i'm looking at are exactly 10mm in steerer length.
my Easton Haven is 40mm just like my X4.
But I do have a perfect condition Superstar 70mm which is just like the On One CNC but in black with a red faceplate which is indeed 35mm.
Right,
current forks 184mm steerer
when pulled up with no spacers steerer protrudes 2mm above stem.
I've just contacted the bloke & the forks i'm looking at are exactly 176mm long = 8mm difference