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As title really.
Can you run 27.5 wheels in 29 dropouts and vice versa?
What does it do to the geo?
From memory I think the two effectively set the axel height relative to the frame at slightly different points to try and compensate for the slight difference in outer diameter of the two sizes and the different recommended fork lengths/travels.
In practice I imagine you should be fine using either wheel size with either set of drop outs, though if you run a 27.5 fork you might find your geometry numbers are a bit off with the 29 drop outs fitted in the frame (and vice versa)
edit: if you have a set of 29" forks and 29" drop outs I think the only geometry difference you would see when swapping to 27.5" wheels would be for the whole bike to sit ~5 mm lower)
i ran my previous gen with 27.5 dropouts and both sets of wheels, 27.5 2.8 and 29 2.4ish. not a big deal. fwiw 27.5 3.0 are somewhat larger OD than 2.8, at least in the tyres i measured - 3.0 WTB ditch finders and 2.8 rekon or DHR. i now have a replacement frame under warranty (the current one) that came with 29er dropouts and am equally happy swapping in and out wheels but this is the combo that will give lowest BB if you're worried about that. i run 175 cranks as i'm SS. you'll notice for about five minutes. IMV 27.5+ rear gives quicker turn in but hooks up more. i like the change it makes to the bike swapping, but am equally happy running either. 27.5+ front offers no advantages i could perceive and i don't bother with that any more. just mullet or 29er.
I’ve run both in mine with 29er dropouts. Works fine.
Thanks, was thinking that 27.5 dropouts with 29 wheels and 29 fork will steepen things a little, but I wonder if it's like 0.5deg, or less?
difference at the axle will be around 10mm so i'd say less than 0.5. i used to run the old one as you propose. it changed the bike in a way even i noticed but after 5 minutes it's just like riding a bike.