There was a recent thread on BROL that diverged into a discussion of wheel/tire sizes on the Xstream, where someone was wanting to put 1.5" tires on it, based upon some comments made a while back about the bike accepting wider tires.  To do this, obviously, you have to go to 559 wheels, since 1.5" tires aren't available in 650c...  but apparently, the stock fork on the Xstream is not wide enough to take this size tire.  Not having one, this came as a surprise to me, since the front fork on the prototype is the same as that on the StratusXP, as far as I can tell -- apparently the production fork is something different, that is narrower.
 
You'd probably have to change out the brakes as well, I suppose, to get the tire width to fit in between the pads.  I have put my 559 Thracian wheels with 1" tires on the XS prototype, and everything fits fine (and yes, Greg,   I adjusted my brake pad rim contact positioning...) -- although I guess if you needed to, you could deflate a 1.25" or wider tire, get it through the brake arms, then reinflate it!   Kind of like the classic puzzle of how you get a 14' tall semi truck under a bridge with 13' 11" clearance -- let some air out of the tires!     Although I do remember Ray talking about front brake clearance issues on his bike....

Quote from Opus the Poet on Jun 16^th, 2009, 3:17pm:

The difference in tires themselves (models, manufacturers etc.) and tire sizes tends to be much larger than the difference between the wheel sizes themselves. 650s are primarily used on women's tri bikes right now. Those and high-racers might be the only bikes that have an actual point in using them. Why do you ask?
 
I still laugh about the tri shop owner in Fred'burg telling me about decided to stock up for the 'women' just before Hell Week, only to have the highracers discover him and wipe out his entire 650 stock. :^)