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650b road wheels on 26" MTB

Hi All,

I'm thinking of getting shot of my cyclocross bike and getting spare wheels with road (or CC) tyres for my mountain bike instead, but I have a few questions I am struggling to answer:

1) is this something that people do, or is it a stupid idea I should ditch immediately?

2) If I put 650b wheels on a bike that currently has 26" wheels on it, will the smaller profile of the road tyres (say 1.5 vs 2.25) compensate for the larger rim size?  ie will the overall diameter of a 650b wheel and 1.5 tyre differ significantly from a 26" wheel with a 2.25 tyre?  

3) are brake discs a standard size, or do I need to get the exact disc that is on my existing MTB wheels?

Question 2 is the one that I am struggling with the most.  Common sense says the 650b & 1.5" tyre would be the same size, as the difference in tyre thickness = the difference in the radius of the wheels, but common sense doesn't appear to generally apply to wheel and tyre sizing. 

Any insights gratefully received!




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