Changing wheel size - what should I know?

Question

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!

Answer 1

• 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.

Answer 2

Great question! Looks like you've been very thorough.

• Handling should be unaffected, assuming your effective wheel diameter (including tyres) is similar. Same goes for braking forces - if you use rotors of the same diameter, the forces should remain the same.

• To make the wheel swap painless, I'd build the 650B wheelset around identical hubs to your existing 700Cs, and fit identical rotors. With a little luck, the caliper/rotor clearance on both wheelsets will be close enough. Perhaps someone with more experience with discs can chime in, but I'd have thought that the manufacturing tolerances involved would be similar to the effects of new vs. worn pads.

• You're right that you'd lose 19mm of ground clearance with equivalent tyres. You can certainly make this up with fatter tyres. Just beware if buying a tyre marketed as a 27.5" x 2" - I've never seen those on sale before. I don't think 27.5" is a name alias for a different tyre size, but I wouldn't take the chance. Make sure you can return them if they're not a proper 650B.

• IME tyre sizes don't include knobbles, so I'd probably downsize to a 42mm tyre just in case. As long as your bottom bracket has over 10 inches of ground clearance, you should be fine to drop by 5mm or so.

• Chain/cassette wear is something to watch. Probably safest to get a new chain, and new cassettes (same brand). Keep the drivechain clean, whichever wheelset you use, and it will help eliminate wear. Ideally you'd have two chains, one for each cassette, but I think that's overkill.

• If mudguard clearance is an issue, you could always split the guards and mount the two halves with separate brackets. I've done this more than once; it works like a charm. All you need is a hacksaw, drill, and a couple of spare 'L' brackets.

My only worry is rotor clearance. If luck permits, you may be able to fine-tune that with some very narrow washers on the mounting bolts.

Otherwise I say go for it. I've successfully converted a few bikes to different wheel sizes (27" to 700C, 700C to 650B, 26" to 700C, and 700C to 27") - admittedly none of these had disc brakes to worry about, but as long as you've got a handle on frame clearances and mudguards there isn't much that can't be worked out.

Good luck!

Answer 3

Good existing answers on how.

But I don't think a 2" knobby tire is the answer to "some rougher stuff - muddy bridleways, stony paths".

Cyclocross racers handle those conditions and do so on 32 and 35.

A 35mm tire will handle a stony path.

As for mud if it is more than a couple inches it is going to be sloppy. A 2" tire is not going to save the day over a 35 or 38.

It is still a touring bicycle with drop bars. Putting a 2" knobby on it is not going to make it into a mountain bike.

Stony path versus mud is a tread issue and exact opposites. Stony path you want a tight or small block. Mud you want wide space knobs. And for mud you need clearance as mud builds up.

I would start with some stony path tires on the existing wheels. Some soft 35mm or 38mm with a little bit of tread. Good wet traction as wet stone is slick. And there might be a language barrier - in the U.S. stone is big smooth rocks like cobble stone. Smaller crushed rock is gravel. Still the same basic type of tire but wet gravel is not slick.

For mud get yourself a set of 38mm aggressive knobby and try it out in the mud. You are not going to like the way they behave on road or stony path. Yes some 2" on a 650b will do better in mud but you are still not going to have a mud bike. If you want a mud bike then get a mountain bike.

Answer 4

I swap wheels between 700c and 650B on this bike when I go from summer to winter ( or as winter as coastal California gets. )

If I had disk brakes on both the back and front, it would require no adjustment at all. Disk brakes are relatively standardized and swapping wheels is pretty straightforward. The only issue I've ever run into is that not all rotors will work with all brakes ( Avid BB7's and Shimano ICEtech rotors don't work because the tabs hanging down from the brake shoes interfere with the alloy spiders on the rotor )

As far as the gearing goes, as long as the clusters are similar ( big sprocket within a tooth or two between clusters), no adjustment is required. If you are diligent about checking the chain for wear and replacing it at regular intervals, there should be no shifting problems.

The handling is a bit different, but not enough to matter as long as you use bigger tires on the smaller rims. Just for fun one day I put my 26 inch MTB wheel on the front, it worked. Looked funny, but it worked.