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Ideally steel
For 120mm fork
Clearance for Plus tyres (is this a given for Boost frames?)
Santa Cruz Chameleon fits the bill. Top bike. I have one built up in 29 mode with 120mm forks and it's great fun.
If they are still selling them you might get away with a Vitus Sentier vrx+. Boost rear for up to 2.8” wide tyres. Technically for a 140mm fork but still.
Also, Sonder Transmitter is similarly for big tyres but maybe a slightly longer fork.
Not sure about tyre clearance on an Orange Clockwork - might be worth a quick check.
Solaris max
Genesis Tarn, lovely made frame and rides well and a good price.
Selling a whyte 909 frame with a dropper. Boost spacing clearance for 2.8 tyres drop me a message if you’re interested.
Cotic Soul takes up to a 2.6 tyre and is a great ride
jamesgarbett
Clearance for Plus tyres (is this a given for Boost frames?)
Nope, a lot will be OK but you still need to check. Unless you accept 2.6 tyres that actually measure 2.4 as being "plus" anyway.
Sick?
Genesis Tarn, pace, cotic, Stanton, ragley all have steel offerings that broadly fit the bill.
Stanton Sherpa
27.5 or 29?? Steel, Al, Carbon? soooo may choices. plus? non-plus?
Pace. Gets you up to 2.6"
I've just built up a 127 boost, and it's a dream to ride. For me, it's got a brilliant balance between stiffness and flex in the right places.
Genesis and Sonder were also on the list too.
I think that the new Swift that's due in the Summer is meant to be boost; it'll take a tapered fork too so adding suspension shouldn't be a problem but I've no idea if it'll work with 120mm.
As a former Singular Hummingbird owner, I'd recommend waiting for the new Swift if you can possibly put up with the ants-in-your-pants excitement of delayed satisfaction. However, if you can't wait and a Chameleon doesn't float your boat, you might want to have a look at the Trek Stache - I bought a frameset last year when Jungle had sold out of Chameleons and haven't been disappointed. Worth noting that Jungle's customer service was exceptional, mind you. That's probably not helping much, is it? Sorry.