I wanted to add:
I dont personally think 650b is a bad think. Despite my skepticism in the beginning, I was ready to build one up myself, however the rim I wanted to use is only available in 29er 
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But, it should not replace the 26" wheel size. Thats just stupid.
Oh, don’t worry, 5 years from now people will be switching to 26" wheels, talking about how they provide more stability, a lower center of gravity, make it easier to huck the bike, etc. Americans are terrible at understanding when they’re being duped.
So do you think theres an enduro muni in the future as well? 
I wonder if you could fit 650B wheels in some 26" mountain bikes anyway without replacing the entire bike. The contact patch would be the same, only the bottom bracket would be lifted by ~2cm. People definitely are easy to dupe goes back to looking at shiny new unicycles.
QuAx might have stolen a march on the other manufacturers as the only provider of unicycle width rims in that size. Until/unless KH and Nimbus catch up, its either a QuAx or a narrower rim. It would be possible to release a rim only to be fitted into a 26 or 29er frame.
I had a try on a 650B but it didn’t feel at all special to me, even though the measurements and the tyre choice was identical to my 29er. Perhaps it might have been a reflection of the fact that the tyre wasn’t pressured the way i like or the schlumpf hub on it…
Velocity makes a ‘unicycle width’ 650b rim.
So they do, my bad. I would probably prefer a 40+mm rim to a 35 though.
You can probably fit 650B wheels in a 26" mountain bike now that everyone’s using disc brakes. Especially because most 650B tires are narrower than typical 26" MTB tires.
It’s not really special. It’s 38mm rim diameter larger than a 26" wheel; put a smaller tire on it and the diameter is close to imperceptibly different. There’s nothing inherently wrong with the size, it’s just unnecessary.
Even without a disc, it would only need a brake booster to realign rim brakes.
I wouldnt say the size is unnecessary. Maybe it should be used to properly fit people to bikes so that smaller bikes are 26" and move to 650B or 29" as the frame grows. This needs to be done more with road bikes, small riders should not be riding 700c.
Small riders are just fine on 26", though. My 5’4" wife rides a 26" Surly Long Haul Trucker (Big Apple 2.0") and it’s perfect for her. 650B is a horrible size for touring because of availability issues (whereas 26" is probably even better than 700c).
Actually only 25mm larger (hence why 27.5 is incorrect) - that is rim diameter, the difference in radius is only 12.5mm, or half an inch, hence why a 2.5" tyre on a 650B rim is the same diameter as a 3" tyre on a 26" rim (or a 2" tyre on a 650B rim is the same diameter as a 2.5" tyre on a 26" rim). Also the BB would only be lifted by 12.5mm if you used the same width tyres.
I’m not actually against the idea for unis, where different wheel sizes have more significance, but very put off by the way it is marketing driven in the bike market (and particularly by Giant’s marketing info, which is basically a load of lies).
The burning question for me is brakes. I’m about to buy another muni and think the 650b is a good compromise for my 26/29 indecision. With that in mind, it doesn’t look like MDC do a brake version
http://www.einradladen.com/webshop/584mm-275-Inch-Qu-ax-Muni
Unless that rim can take maguras? doesn’t look like that’s a brake walled rim?
I can’t find a disked version and am hesitant at the idea of a wheel-build at home. Does anyone know of a solution or am I being an idiot?
The frame is built to take maggies. The machining of a rim is an expensive extra which has apparently little effect on braking aside from noise generated.
You might be able to retrofit a disc using KH spirits and a d’brake but the frame has pressed housings and i dont know whether the d’brake could be correctly fitted. An alternative to the d’brake would be a KH or oracle 29er frame.
I am quite surprised they didnt release 650B QX unis at the same time.
That’s interesting, I never knew rims like that could work with a magura! Thanks for the reply, it’s opened up my search somewhat.
Anybody has found 650b rims fit for rim brakes? I found two mentioned on some forums but they are not available here.
Disc 650B: http://www.einradladen.com/webshop/584mm-275-Inch-QX-Disc-Muni
It’s a QX not Qu-Ax (Which is their higher-end version) so it’s a bit more expensive, but hey, it’s there 
You can add a disc brake with a Nimbus d’brake mount and KH Spirit Cranks to nearly every unicycle with ISIS interface.
The unicycle you selected has a steel frame with stamped bearing holders. Thus, you’ll need an extra adapter. This can be made of 3 washers od as a solid adapter.
http://www.unicyclist.com/forums/showpost.php?p=1584246&postcount=1167
And this is my solution with a solid adapter
In this thread we discuss the thing with the adapter/washers in english.