Boxxer Forks and 156 mm crown to crown

OK – so this is rattling around in my head since I fitted the forks to my new (to me) DH bike. I made the mistake of reading the manual (or the mistake of reading it too late, depending on how you look at it 😀 ) and they advise 156mm +- 2mm from the top of the bottom crown to the top of the top crown. I haven’t got that and am pretty certain that there isn’t the steerer length to make it possible. I’m using the short top crown and even then it’s probably only 145mm or so.

Older manuals seem to recommend simply 156mm from the top of the bottom crown to the top of the stanchions (excluding adjustment knobs). I assume this is to make sure there is the right clearance between the lower legs and the lower crown at full compression and to ensure that the geometry is more or less respected, but is the spacing crown to crown important for something else?

Should I care that they’re not spaced at 156 mm or should I just set them up so bottom crown to top of stanchion is 156 +-2 mm? Please say the latter 🙂

Well, am assuming from the lack of reaction that either no-one else has read the manual and everyone has just done what I have or everyone else has read the manual and no-one has ever set them up like me. I’m confident there’s enough space to stop the legs slamming into the bottom crown so I’ll use it like that and hope that I die as late as possible and in circumstance entirely unrelated to my fork setup 🙂

I don’t know about anyone else, but I didn’t understand your question!

What has the distance between crowns got to do with the lowers hitting the bottom crown? You need a gap between the lowers and the lower crown of at least the amount of travel, but that’s obvious right?

Distance between crowns has nothing to do with geometry, that is regulated by the position of the lower crown with respect to the stanchions. Current Boxxer manual states that the distance between the top of the lower crown and the top of the stanchion should be 163 +/- 2 mm, but this may be different for older forks. It also states that there should be at least 2 mm of stanchion showing above the top of the upper crown.

The only other measurements that it specifies in the manual relating to fork setup is that if the distance between the top of your upper headset cup to the bottom of your lower headset cup is less than 146 mm then you should use the low top crown, otherwise use the high top crown.

I’d gamble the spacing is about (1) the lowers not hitting the crown on compression and (2) gaining maximum strength/support from the double crowns by spacing the crowns out as far as possible while not causing an issue with (1).

I have no idea if whatever measurement you have will accelerate your departure from this world and that’s probably also a factor of what and how you ride and whether you are built like a slightly under nourished rock climber or a Sumo Wrestler.

I don’t know about anyone else, but I didn’t understand your question!

🙂 Ok – I’ll have another go. I’m talking about the measurements on Page 7 of the User Manual I found here :
https://www.sram.com/sites/default/files/techdocs/95-4018-014-000_rev_a_dual_crown_forks_user_manual.pdf

I’d gamble the spacing is about (1) the lowers not hitting the crown on compression and (2) gaining maximum strength/support from the double crowns by spacing the crowns out as far as possible while not causing an issue with (1).

I think so too.

Looks like a bit of a misprint to me. Unless (and I have to confess I’ve never owned dual crown forks) all frames come in 2 headtube lengths – but I doubt that very much!

That does seem strange. The latest manual makes more sense – i.e. get the lower crown position right and make sure there’s at least some stanchion above the top of the upper crown.

Given you’ve already cut your steerer, I’d just make sure your lower crown is in the right position and not worry about the distance between the crowns – I can’t see any reason for this to be particularly important and it’s not something that is specified in the latest manual. Of course, what the exact distance between the top of the stanchions and the lower crowns should be for your year forks is a different question as it may not be the same as the 163 +/- 2 mm for the current forks.