lungimsam

Was wondering.

Only thing I read/think of is:
Pros: More clearance for fenders and wider tires.
Cons: Harder to find tires for them. Slower than 700c??? Is the slower part only a concern for racers?

ironwood

They allow you to run a bigger tire on certain frames. A 38mm tire is more comfortable than a 28mm tire.

I like a wider tire because I ride on the bad roads around here. I find them better on dirt and gravel roads also.

A 650b rim is smaller, and might lower the bottom bracket. For example, The radius of a 650b x 38 Col de la Vie tire is about 10mm smaller than a700x32 Pasela, and the bottom bracket is 10mm lower. However the outside diameter of the Col de la Vie tire is about the same as some 700cx23 tires, so there is no lowering of the bb.

The choice of tires is limited, but I am satisfied with The Col de la Vies and the Soma B-lines.

The pros and cons have been discussed on the C&V forum; there is some more information on the Rivendell site, and on the internet -bob, and 650b groups on Google.

NightShift

You can do the conversion the other way too, from a smaller wheel to a 650b. Rigid steel mountain bikes can make nice touring bikes, but if you want narrower slicks you have a much smaller diameter tire as a result. A lot of 26" wheel mountain bike frames can fit a 700c rim, but not with a 32 to 38mm wide tire. If you can get the brake setup to work a 650 wheelset is a nice option.

Flying Merkel

I have a 650b Schwinn Mirada. The rear rim was tweaked to the point of no return. No cheap replacements to be had. Managed to fit a set of 700c rims. Got a near new set of 650b Panaracers if anyone is interested.

Almost scored a set of 650b mountain bike rims. They would have fit nicely on my Nashbar frame. Disc brakes makes things easier.

The only drawback to 650b is the rarity. Cost of rims & tires is higher and selection is lower. I don't think they'd be much slower in the real world than cushy 700c tires.

NightShift

650a is another option, but the choice of rims is really limited (really only one option). The tire selection really doesn't seem bad.

Retro Grouch 

BALDERDASH!

If you both ride in your highest gear and pedal at your highest cadence a big percentage of the time, 650b will be slower. Nobody, even racers, does that. Anybody else can simply to use a higher gear to be just as fast.

Hairy Hands

A 650b with a 38mm tire is the same circumference as a 700x23mm. A 650b will technically accelerate faster than a 700c but will technically not hold its momentum once rolling on the flats as well. A 650b wheel build vs a 700c build with the same components will be stronger and lighter, although not much but there is a difference. You can find lots of tires built in 650b, but finding one on the road if you flat may be difficult, so having a spare is a good idea. If you have cantilever brakes and the posts are set for 700c your screwed, you have to look for a custom frame in 650b sizing, or you need to find long reach caliper brakes, or if disc brakes then your good.

I think many ppl, including Jan Heine publisher of BQ will tell you they are faster and much more comfortable than a 700c. It will most likely change your gearing ratios, but that is easily fixed.

The main benefits seem to be that you can have big cushy comfortable tires while using a fairly standard frame size and it keeps the wheel circumference within reason.

Werkin

Look no further than Grand Bois Hetre, this tire is the best reason to go 650b on the road.

MileHighMark

Slower? I have two bikes--one with 700C wheels, and one with 650B wheels. The slower one is always the one that's being ridden by me.

NightShift

You won't be able to use canti post that are too high, but there are ways to use canti post that are lower; 700c to 650b would mean you need a different type of brake, but maybe not with 26" to 650b.
Paul's Components and Velo Orange both offer brakes with more reach adjustment, and an adapter plate can be made to increase the reach of a standard V-brake.

Hairy Hands

Well, speaking from experience I had a 700c Cyclocross frame that I wanted to use 650 wheels on but I was unable due to the brake post being too high as designed for a 700c wheel. I could have possibly removed the posts and used a long reach caliper brake, but I didn't like that idea. It seemed obvious to me that yes you could use a lower post but then that would make the frame designed for something other than a 700c wheel.

Maybe we are saying the same thing just not understanding each other.

NightShift

Hairy, the only part of your post I was contradicting was your statement that to use cantilever brakes you would "have to look for a custom frame in 650b sizing". We were saying the same thing about brake issues with 700c frames, but 700c isn't the only common frame size.