Short version: it'll make fuck all (but not no) difference.Google pointed me to several threads on this, none of which gave me the exact info I'm looking for.
Situation: Current 26in Lyrik circa 2010 is giving me grief. Tempted to upgrade to current model Pike. I'm happy with every other part of my 26in wheel'd Nomad, but will realistically upgrade it to something 27.5 (possibly another Nomad) no earlier than a year from now, no later than 3 years.
So I'm considering getting a 27.5 Pike so that I can run it in the situation where the aforementioned 27.5 next bike is a build, rather than a full bike purchase.
According to Rockshox specs, a 26in 160mm Pike has an A2C of 542mm, the 27.5in 160mm Pike A2C is 552mm*, and my current 26in Lyrik is 547mm.
So in theory, I'll only be raising the front of my bike 5mm - which is fk all really, considering the bike is designed to run up to a 180mm fork.
My only question is, is there any downside to having the smaller wheel in the slightly longer lowers? Will the fork behave ... I dunno ... badly, from a torsion or flex perspective, with this 'unused' extra bit of lowers? My gut feeling is: no difference at all. But before I go spending $700ish, I thought I'd better off ask around.
(* these figures confound me - to allow for .75in/19mm more wheel diameter, they only have to increase the lowers by 10mm ???)
EDIT - I guess the other thing to consider is: are 26in forks going to start getting cheaper as the number of bikes needing them decreases?
Longer version: It'll have a slight effect on the steering geometry, more when you change to the future 27.5" wheel. The longer fork will have slightly more rake - forward offset of the axle relative to the steering axis, which with a smaller than designed-for wheel will make the bike a bit flighty, but with the bigger wheel to jack it up, the trail will come back to where it's designed & stabilise the handling.
As far as flex, no effect at all by running a smaller wheel. There is no "unused" bit of lower, the axle tying the fork tips together is in the same place whether you've got a 12" or 29" wheel in there (but the latter will probably drag on the bridge!). What's between the axle & lower bridge has absolutely zero effect on the fork. HOWEVER! All other factors being equal, the smaller wheel will actually make the handling stiffer, because there's less inherent lateral flex.