Most (perhaps all) off-the-peg 650B wheelsets currently available in the UK are made for 27.5 mountain bikes. Rim widths vary, but that on Mavic’s XM 419 Disc has an internal width of 19mm, which is about the same as a regular 650B road rim. Fitted with a 650ﬁ32B tyre such as the Grand Bois Cypres, it should fit the frame in question with clearance to spare.
This would lower the bottom bracket by 10-20 mm depending on the original 700C tyre size, which may cause problems with pedal-to-ground clearance. A better solution would surely be to find the widest 700C tyres that would fit the frameset and forget about trying to shoehorn the mudguard into the same space. Trim the mudguard so it ends just above the chainstays, perhaps leaving a central strip for attachment to the bridge.
Be careful not to over-inflate tyres wider than the familiar 700ﬁ23 or 25C. 100psi is too much for a 32mm tyre, whether 650B or 700C. Not only is it too hard for optimal performance (see the tyres article on page 48), there’s an elevated risk of tyre blow-off. The stress in the walls of a pressurised tube at a given pressure is proportional to its diameter, so the ‘bursting’ load on the sidewalls and bead of a 32mm tyre inflated to 100psi is equivalent to a 23mm tyre at 140psi. For a 32mm tyre, around 75psi works well.
Cycle’s Technical Editor