Changing wheel size - what should I know? I own a recent tourer (Dawes Galaxy Plus) and would like to take it over some rougher stuff - muddy bridleways, gravel paths, that kind of thing. What holds me back is the tyres: 35-622 looks to be its limit with mudguards, perhaps 40-622 without.

I'm exploring the possibility of getting round this by using 650B wheels and knobbly tyres. I expect there is a lot to watch out for here, so I want to check my thoughts and assumptions are correct and I haven't missed anything:


Most fundamentally of all, I'm assuming the bike will still handle sensibly with fatter tyres, the fork will still handle the braking force from my discs, etc etc.
I'm aiming for swapping between the two wheelsets to be a five-minute job: just swap the wheels, no further adjustment required.
I think I'd be aiming for around 51-584 tyres because they're 19 mm taller than my present 32-622's, which exactly makes up for the 38 mm loss in wheel diameter. The web advises me these exist and are typically called 27.5" x 2".
I have easily enough horizontal space for wider tyres at my chainstays and fork, but only just enough at the seatstays. If a 51-584 tyre really is 51 mm, then I think I'm OK. I'm not sure a 54-584 (27.5" x 2.2") would fit. Does the 51 mm on a knobbly tyre include any horizontally protruding knobbles, or do they extend beyond the nominal width?
The new wheels will need the same hub spacing as my current ones: 100 mm O.L.D. at the front, 135 mm O.L.D. at the rear.
I have disc brakes, so I'll need the same size rotors (160 mm). I'll also need the lateral position of the rotor to be identical to my current wheelset, but I'm aware the tolerance is very low here and I'm not sure how this is measured, if it is standardised, and if it is easily adjustable (e.g. with shims)?
I'll need a 9-speed cassette like my current one. Somehow I'll need to make sure they have similar levels of wear, to prevent the chain skipping on one and not the other - presumably, frequent chain changes will suffice, though I hope not too frequent.
Last and definitely least, I'm hoping I can use my existing mudguards after a fashion, but they'll be rather narrow so perhaps this is a vain hope.


A much longer post than I thought - nonetheless, have I forgotten anything? And/or please tell me if I'm mad or misguided!
• Swapping the wheels usually needs a little derailleur and brake adjustment, no matter what. You still might need to adjust the high and low limit. And, reposition the caliper, if  hydraulic. If mechanical, most have the pad adj knob, and barrel adjuster.

• Since you're coming from a hybrid wheel, going wider is an issue with the seatstays. But, if you are riding in thicker mud, a narrower rear tire is better. In fact, they way it'll have to work is ideal, with a wider front, skinnier rear. The width measurement does NOT include the knobs. They could rub, if wide enough. But, usually a deep chevron style tread is better for rear mud tires.

• The 650B is coming back into mainstream, after a long-abandoned experiment. So, you'll have more choices of wheel and tire. It will be $$ because of the exclusivity.

• Many wheel sets will have 100/135 spacing. Make sure you get non-thru axle hubs, just the 100x9 and 135x10 QR axles.

• Make sure you get the correct rotor mounting system. Mostly, there is center-lock, which uses a cassette lockring, and then the 6-bolt design. 

• The rotors will be in roughly the same position between both wheelsets. I do not recommend extensive lateral dishing of the rotor. It's good practice to be handy at repositioning the calipers. Even taking off and reinstalling the same wheelset can still rub, no matter how perfect the brakes were before.

• Don't worry about similar levels of cassette wear. Replace the chain more often, and use the gears a lot. Nine speed chains came down in price recently. If the chain is worn now, most definitely. But, this discrepancy will happen with prolonged use of one wheelset. If you use both wheels a lot, it won't happen as quick.

• Narrow mudgards obviously won't work as well with fat tires! There could be some splatter.