Learn me on 650b
 

I just got a new frame and it's set up for disk brakes. It's designed to take 700c wheels with up to a 700x40c tire. I'm building a set of wheels for it, and am considering going with 650b rather than 700c for the ability to run higher volume knobbier tires for trail riding. Is this a decent idea? Is it a terrible idea? Will I lower the BB too much? Negatively affect handling? Are there decently light, fast 650b tires out there for when I'm sticking to the road(probably in the 32mm wide range)?

32 is too small for 650b conversion on a bike designed to accept 40 700c tires. 38 would be a good minimum with 42 recommended as the best size.

650b is for when you want bigger tires with wheel diameter close to the same of the larger 700c wheels with smaller tires so the rotational inertia and "feel" of the wheels stays about the same but comfort and capability increase. There's no point in running 650bx32 on a normal sized bike.

It's a decent idea. Options are great. You should beta test the idea before building ... perhaps go to your local shop and see if they have a Cannondale Slate, etc, you can borrow the wheel from.

A 650b x 42mm wheelset is about equivalent to 700c x 23, but note: don't try to match the diameters of 700c wheels. It's apples to oranges. It's more a matter of finding the right sweet spot for air volume and tire contact patch for the riding you want to do. Besides, you will likely run into fork crown and chainstay clearance issues especially if you try to run a very fat tires with knobs, and if you are running a wide rim.

If it's strictly for trail riding I'd personally stick with 700c x 40mm. That's plenty of cushiness, especially on a wide rim. You can adapt to pedal strike somewhat, but I think it's annoying to have to push a bike uphill because you can't clear the tree roots at a slow pedaling cadence.

To answer your second question plenty of us have run 650bx42 Babyshoe Pass, Grand Bois Hetres, and the Panaracer offerings, and they fit your criteria for decently light, fast 650 tires and then some.

I have run 700s and 26s on my bike, but my 26s are much fatter than my 700s, so there was no lowering of the bottom bracket. I have to think it would be a noticeable change if you went to a smaller diameter and a narrower tire. Whether it's enough of a change to bother would depend on you. I find the height of my bottom bracket is noticeable primarily when I'm off pavement, and it sounds like your narrower tires would primarily be pavement tires. On the other hand, I haven't found any real benefit in going narrower with my tires. If I don't think I'll be off road, then I just look for a smooth tread, rather than a skinny tire.

The other issue you might face is trying to cover your desired range on one rim. I've run down to 32 and up to 40 on my 19mm wide rims. At the 40 end, it started feeling a little sketchy unless I kept them near the upper end of their pressure range. But I'm a heavy guy, and I'm sure that's a factor. It might not be a factor for you. Still, now that I know that I prefer a wider tire, I look for wider rims to match.

If it were me, and I wanted a narrower road tire for pavement-only rides, I'd build up two sets of rims. 700 rims with an internal width of no more than 19 (slightly narrower if I ever imagined going thinner than 32), and a 650b rim set that was a little wider, somewhere in the 20s, so I would know that I could maximize the tire width to limits of the frame.

For practical purposes, there are several really fast 650b road tires in 38, 42, and ~47mm widths. Panaracer Pari Moto, Panaracer Gravel King, Compass, Grand Bois, Soma. There's also a good range of knobby 27.5" tires 42mm and larger (the best selection is in the 50mm-55mm range wide).

The smallest size 650b tire you should try depends on your frame's BB drop. I'd recommend 38mm tire for 70mm (or less) BB drop, 42mm tire for 70-74mm BB drop. With 75mm or more BB drop, you should stick with 700c wheels on the road.

The largest size tire (for off-roading) will be determined by the clearance between your chainstays, seatstays, or fork blades. Measure it! Give yourself about 6mm clearance from the nearest rub point if you'll be riding in mud. If you like your paint, don't ride with less than 3mm tire clearance.

I agree with the above -- it's a good idea if you want to run fatter knobby tires, but I would stick to the 700C wheels for skinnies. :thumb:

French, 650 is the nominal size, ABC is the variety of tire rim combinations bigger rim thinner tire & smaller rim wider tire .

This past year the MTB Manufacturers adopted 650B, as 27.5..

I have a 26" wheeled Specialized FS mt. bike. It has a noticeably lower b-bracket then my Fuji steel mt. bike, to the point where I hit roots and stumps constantly.

It's really annoying to the point where I might sell the bike. Thus I would not be recommending a bike with a lower b-bracket for trail riding.

Thanks everyone! Two sets of wheels it is. I'll work on the 650 set later in life before I do the tour divide or something.

I'm not into big tires on the road. My normal commuter tires in the summer are 700x25 in a light-weight variety. Right now I'm running Vittoria Open Corsa SCs and they're great. My wider "winter" and touring tires are supposed to be light for their size (700x35 Vittoria Hypers) and I can still feel the difference, so I'm not really into the 35+ tires at all. It doesn't sound like the 650b dual use wheel-set would be for me.

Learn me some English.

Fair enough. But it's a commonly used phrase for many years now and full sentences don't translate to thread titles. I don't think "Would everyone be so kind as to teach me the differences between 700c and 650b wheel sizes?" would have fit.

Yikes, no coffee today? Can't count the number of times I see stuff with no punctuation or the brake, break, pedal, petal, peddle. Dude.