We have a Co-Motion Speedster and while touring last week I was kicking around an idea. So I thought I would bounce it off you all and get your input!
Currently have 700c wheels with disc brakes. Since I need to rebuild the rear wheel I was wondering about switching to 650b? Would like a comfier ride!
So????

Buy a full carbon fiber tandem . . . MUCH comfier ride!

Trying to be semi-retired!!$$$$

Fatter 700c tires would also give you that.

Running 32mm with fenders. No room for anything bigger.
Thanks!

I have converted a 700C tandem to 650B and are very pleased. If you want light weight flexible fast high volume tires then 650B is the way to go. Large volume 700C tires are also are more likely to cause toe overlap. Mine is rim brake so replaced fork as well.

Big tires are heavier but running 65-75 psi means smiling every time I read a thread here about discussing the best 700C tires to kill road buzz or suspension seat posts.

Just did the 650b conversion this summer. I tried to read as much as I could about the conversion but finally decided I just had to take the plunge and see how it worked out. Had the LBS build up an older mixed set of hubs (White rear (disc), Hope front) with Velocity Cliffhanger rims. With Panaracer Col de la Vie 650b x 38's (actually 584-40) they just fit our Erickson. Went to disc brake rear but rim brake front to avoid the need for a new fork. Had to replace the front caliper with a long reach but it works well. Quite pleased overall, and have had some great gravel grinding rides - rail-trails, dirt roads, etc. The one issue is that the Erickson has quite low bottom brackets to start with and the overall 1cm drop compared to the 700/28s is quite noticeable, especially since the point for us is to get off pavement. We both love the cushy ride, and getting away from traffic.

So Tom;

First do some measurements. How much clearance is there all around your present wheel/tire combination, and actual measurements of the current width and height of your present tires.

Calculate how much wider a 584 tire can be than the 622 tires you have now. If you can only get 1 or 2mm additional width, probably not worth it. If you can get 10mm or more additional width, then worth it.

Also note that you do not have to have the same size wheels and tires front and back......

As 2frmMI did; I recommend Velocity's Cliffhanger rims.

Did some measurements and I believe I could get up to a 47mm tire. My thought was to use my DT Swiss hubs but they 40h and after doing some searching a 650b rim is not available in a 40h.
So the idea will go to the back burner for now!

Do you really need 40 spokes for the smaller rim diameter and cushier tire? We're not small folks and I ride stoker at 190 pounds. We often tour with a full load and we have been very happy with 36 spokes in our 26" wheels.

I do not understand the trend to bigger and bigger tires. I still run 700 X 25's and sometimes run a 700 X 23 on the front of our Calfee and our ride is completely comfortable. I run 100 psi in the front and 105 in the rear with no problems with pinch flats. I do announce bumps to my stoker so she can be prepared for bumps. We ride typical county/township roads in central Illinois and do not see any need to go with bigger fatter tires. Maybe it is the carbon fiber as Zona mentioned.

I think our road conditions and our desire to not slow down over rough stuff causes us to enjoy soft wide tires.

1) If I announced bumps to that my stoker could prepare then it would be a constant stream announcements and I would have little time to breath.

2) For us 650Bs are very slightly faster. This also has to do with road conditions because I can see other bikes in a group we are riding with slow slightly while we do not when we get into heavy chip seal and rough pavement causing us to let up or brake.

3) Never a pinch flat. - We used to get pinch flats occasionally running 28mm gatorskins at 120 psi. Usually this was because I bombed through a road issue attempting to keep up our hard won momentum. Once got a pinch flat because of some gravel on a straight road. Happen to hit a larger piece square on. (Hey the gravel is big here too).

4) road buzz - what is road buzz? I can't be sure but I think the big tires work better than carbon frames for this because I never think about finding better tires, handlebars, or other components to absorb buzz but it seems to be a regular topic on this forum.

We road narrow tires for many miles and enjoyed it. The wider tires are real but marginal gain in enjoyment. That is about it.

Velocity Cliffhanger is available in 40H
Velocity Wheels - Hand Made in USA

VELOCITY CLIFFHANGER MSW 650B BLACK RIM - prowheelbuilder.com says they have them in stock.

Aluminum tandem bikes (& half bikes for that matter) really benefit from plumper tires. Our Santana Scandium only wears 700 x 32c Vittoria's (hypertour). 85-90 psi is all they need to be inflated to. The ride is so much nicer than any high dollar 28mm tire I've tried over the years. And we are able to ride faster with them, too (as waynesulak's comments attest). Another benefit specific to the Santana is the "pneumatic trail" effect. The twitchy, floppy steering typical of Santanas is lessened noticeably. If I weren't running rim brakes I would size down to 650b, too.

Our Paketa sports 700 x 23c yet rides smoother than the Santana on most roads. So, frame material really makes a huge difference.

The average road surface quality in San Diego county goes from poor to post-apocalyptic, with new pavement only just starting to show up in the last year. My aluminum framed "toss in the car" road bike rides best on 700 x 28c with clearance for the fatter stuff. I fear that if I were to put bigger tires on, I won't want to go back - don't want the conundrum of climbing weight vs. comfort. I only ride half bikes for 2-3 hours a week at most as opposed to 5-10 on the tandems.

In the captain's saddle, I have a fine ride on high pressure narrow tires. Unfortunately, I usually ride stoker and provide most of the power. Having the tuna beaten out of me, even on a fairly compliant steel frame, does tend to decrease my output, especially on rides over 200 km. I wonder what's not to like about lower rolling resistance and increased comfort, all for a tiny weight penalty. (No, I'm not going to give up ice cream just to make up for it.)

Pinch flats? In the many hundreds of thousands of miles I have ridden, I've had one pinch flat and my wife has had one as well. Those really are the least of my concerns. However, the fatter tires do seem to have a much lower rate of being flatted by glass shards, which is kind of critical around here (ground zero for drunk driving where tossing the bottle out while driving home is the norm, plus the county uses winter grit that seems to have obsidian in it.) The lower pressure seems to allow the tire to simply conform to the sharp edges rather than be cut by them.

Some day, if you ever have a bike that can take some truly wide tires, ride a route several times on skinny tires and then do the same on some high quality 48-54 mm tires. Don't be fooled by the apparent feel difference; the fat tires will absorb the high frequency road vibration and thus feel slower. Also, don't pump them up to their rated maximum; just pump them up enough to avoid pinch flats. Take a look at what they do to your average speed and make up your own mind. Sure, if you're doing a short, flat time trial the fatties don't make sense simply because of the aero penalty at high speeds and the fact that the ride quality won't make up for that. But in real world conditions, especially when traction makes a difference, they can be a bit nicer.

While on our ride Wednesday when hammering along at around 24 MPH, (nice tailwind) on a typical Illinois township road I asked my stoker if she was comfortable on the back seat. I asked her to rate the ride quality on a basis of 1-10. 10 being outstanding and 1 being terrible. Her rating 9.5, mine was a 12, our Clafee is very comfortable with Schwalbe One tires (700 X 25) and latex tubes. For us if it ain't broke don't fix it.

Indeed, Cliffhangers do come in 40h, as that is what we used for our rear wheel - 36h front. 40 spokes was not essential in my mind other than that was what the hub I had laying around. Probably more bomb-proof than this 320 lb team needs.

That is a sentiment that I very much agree with. To me a large part of having a happy life is realizing when you are have a good thing and resisting the urge to look for the "new and improved" version. Almost every bike mentioned on this forum is a good bike that a team can enjoy. I have similar feelings about disk brakes. For our riding rim brakes stop well. Disk brakes just fit in that category for me.

If there is a problem then search for a solution. If there is no problem then just enjoy the ride.

One tandem team I know has a gravel bike built up with 650B tires (I think that's what they are, not 700's, not 26"). They are not a light team, tandem is for rough ground, and they seem to be doing fine, so it can be done.

Well, for us it "ain't broke" and we didn't need to fix anything. We want something else. That "else" is to ride out of traffic and on rough surfaces. If you don't want to do that then I won't argue. Enjoy your 25's.

On an aluminum Cannondale with Schwalbe Ultremo 28 tires, the ride was surprisingly good. I'm sure the Gatorskins it came with would be much harsher. Stoker completely content with Thudbuster seatpost.
That said, one reason I got the bike was that the disc brakes and clearances allowed the option for 650b, which is the way to go if you want to do a lot of gravel riding. A 42mm 650b will have almost the same diameter as a 25 in 700c

Well 650B is a fatter tire like a 1.5" very French too ..

Taking Mr Peabody's way back machine.....

700a is an ETRTO 30-640
700b is an ETRTO 33-635
700c is an ETRTO 39-622
700d is an ETRTO 56-587

650a is an ETRTO 30-590
650b is an ETRTO 33-584
650c is an ETRTO 39-571
650d is an ETRTO 56-537

The more modern designations do not jive with the original intent of the sizing convention, which was millimeters for overall diameter and a letter designation for width. It is a method that is logical, but does not make sense giving the limitations of manufacturing.

How many people claim to be riding 700c that actually have c (39mm) width tires mounted?

We have one tandem with 700c wheels with 28 mm tire (maximum size that clears the frame and fork). Our other tandem has 650b wheels with 42 mm tires. The wide tires are really nice on rough roads and paths. We have plenty of room on that bike to install 700c wheels. I am not sure there is much difference in speed. If we ever downsize to a single tandem, the less versatile one is going to go.

I see the 650 vs 700 debate still rages. I remember Fred DeLong writing about the advantages of 650B tires in Bicycling magazine back in the mid 1970's. To which someone responded that 650B's were fine for people who were more interested in trying to find parts than riding their bikes. Fortunately, it can be a real debate now. (For the record, we ride 700 x 32 on our Santana)