I've searched and searched and have found no info on this and I don't understand enough about spacing etc.
I've got a couple of 27.5+ hardtails, one has no wheels or forks currently and I want to see if it's possible to convert it to a fat bike...of sorts.
It's 148 boost spacing. So, what are the widest wheels/rims/tyres I can stick on this thing?
Seems a shame to buy a new fat bike if I can get in the territory with this "spare" hardtail.
Also, it's aluminium, so no cold-setting
The biggest issue will be space around the seatstay & chainstay bridges, as well as chainring clearance if you put bigger tyres on. With most plus bikes you won't get bigger than 3" tyres to work well. The front is easier, you should be able to get a fat fork and wheel fitted without any issues (depending on your frame).
I have both a fatbike and a 29+ - it's simply down to BB shell width and chainstay clearances as the skinniest fatbike tyre (3.8") is about 95mm wide on a 65mm rim. My custom 29+ has a skinny chainstay yoke design for big clearances and only has 95mm gap between the stays. Boost chainline is only 51mm i.e. you'd only be able to run singlespeed otherwise the chain will rub the tyre. The way that the first commercial fatbike got around this (Surly Pugsley) with a 135mm hub and 100mm BB was to use an offset frame design. Looks like you're limited to 650B+ tyres.
As @dovebiker says, it's down to clearances between drivetrain and rear tyre. It's a bit like trying to squeeze a quart into a pint pot. 29 to 650b+ sort of works purely by chance. I've a (supposedly) 2.8" 650b+ tyre in the back of my Solaris Mk1 at the moment, there's no way I'd get a bigger tyre in there. With a standard BB shell you are going to get chainrub, hence fat bikes using 100mm BBs, even then you have to be careful.
The front's easier but not necessarily cheap: fat bike fork and wheel and you are good to go as the diameter of a 4" fat bike tyre is roughly the same as a 29er tyre so your geometry won't change too much.
Thanks for the help guys, I guess I'll do something else with it like single-speed conversion.
Fat on the front is good on a rigid bike as the big tyre helps you roll over stuff even a plus tyre would struggle with - I ran my plus bike like this for a bit before going 'full fat'.