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I've just had an idea in the shower which could be the answer to a way to justify new shiny kit.
I'm currently the proud owner of a 2011 giant trance, 125mm/26in, the forks could do with a service but I've heard that the trance runs well with forks swapped for 150mm, which slackens the head angle etc. I'm looking to replace it with a 650b steed early next year, so I'm thinking I could put a set of 27.5 140mm revs on my trance, and move them across to the new bike when it arrives. Thoughts?
Measure the Axle to Crown on the 650b forks and add the extra for the 650b wheel (or not if you plan on running 26" with extra clearance) then compare to some 150mm forks.
Certainly early fox 650 forks were near on identical to 26" ones.
I know my x fusions on my other bike are compatible with both.
Abigale X Fusion can be run on both.
I'm running 650B 150mm Pikes on my Saracen Ariel 14X. Works fine.
Its come up a few times before on here...general answer seems to be yes, it'll be fine
Stop there. Theres a chip inserted into the fork boss that is designed to detect design infringement. It'll trip then cause catastrophic failure of the fork to prevent ruining the big wheel myth.
...then you WILL DIE
fit 27.5" forks but keep the 26" wheel in there, loads of mud clearance then aswell
[i]I'm currently the proud owner of a 2011 giant trance, 125mm/26in, the forks could do with a service but I've heard that the trance runs well with forks swapped for 150mm[/i]
Is that your model of Trance or a different one (2014 by any chance?).
And 650b forks have AFAIK a different offset.
Pike Axle to Crown / Tyre Diameter / Offset
26" 150mm = 532mm / 694mm / 40
26" 160mm = 542mm / 694mm / 40
27.5" 150mm = 542mm / 714mm / 42
27.5" 160mm = 552mm / 714mm / 42
29" 150mm = 561mm / 752mm / 46
29" 160mm = 571mm / 752mm / 46
Also Trek 51mm offset available in 29er flavour
All forms of beer tokens accepted 
My neighbour runs a 2011 Trance with Sektor Dual Position 150mm forks and it seems to ride and climb just fine.
Adrec, if you fancy experimenting without spending a fortune, I have a 650B RockShox Sektor for sale.
It's brand new, unused - it took it off my new Trance 650b.
Sektor Silver RL Solo Air
15mm Axle
140mm Travel
tapered steerer (175mm long)
Rebound adjustment, lockout
Black
In a recent review MBR reckoned they out-performed Float 32s.
Looking for around £200 shipped.
I run a set of 650b Pikes on a 2011 Enduro with 26" wheels for much the same reason. It works just fine and as mentioned above, gives plenty of mud clearance. There really isn't much difference, the 26" wheel doesn't look weird in the 650b fork.