Long Distance Competition/Ultracycling, Randonneuring and Endurance Cycling - 650b or 700 for Randos?

Do Randos prefer one o'er t'other?

If so, why?

Do [individual] Randos prefer one o'er t'other?Yes.

Why would you want to use 650b? The availability of them (tires, rims & frames) is poor compared to 700c, the vast majority of commercial frames are designed for 700c, getting spares on the road would be an issue.

650b is an old size, old and practically obsolete for road use, new and possibly up coming for MTB, although going by the 29er hype from a few years ago, and the number of riders I see with them; 26" will be here for a long time to come (in the UK at least, the US does seem to have taken much more to 29er).

Looking at current bikes available, the QBQ ranges (Salsa & Surly) use 26" on their smaller frames, most of the Thorn range is 26", which manufactures give 650b as an option?

I have seen quite a few wheel replacements on brevets, and with 650b that would be a ride-ending problem. I knew 2 people that needed new wheels on PBP. You could buy a 700c wheel at most controles.

There are plenty of people that use 650b successfully. I expect some of them will chime in shortly.

There are some advantages to the 650. Smaller rim = lighter weight, less rotational weight, greater acceleration, overall height of rim with tire is less than equivalent 700c, overall height of bike lower, and many say the handling is better with the smaller tire rim combo.

Negatives are: harder to find tires, must order almost exclusively on internet(expensive) and carry spares around with you, with few exceptions you will need a custom built frame which will limit you to steel (if you care about material choices), and are generally associated with retro type designs.

Cant think of any more at the moment. Jan Heine seems to be the biggest proponent and promoter of this.

I have a 650b bike that I will probably ride on some brevets and use for commuting. Need to get a spare tire. The rims I got (Velocity Synergy) seem to be relatively heavy in comparison to the 700C version. A lot of people have converted 700c bikes to 650b -- you just need longer reach brakes. If you use a larger tire, it doesn't change very much about the bike.

Unterhausen, I read your post further up the thread and I thought I remembered your were building a 650b a while back. I was surprised you did not have more to say about the subject seeing as you have one. Everything I know about the 650b is from reading Jan Heine's articles. I originally started out wanting 650's on my new build, but I also wanted a CF frame. When I found the frame I wanted it had posts on it for cantilever brakes. That killed my 650b idea.

My new bike is not completed yet, but one of the things that immediately is noticeable is the standover height. Compairing the new with the old bike the height of the bike is several inches taller. Both frames are 58cm. The only thing saving me is the new bike has a slanted top tube, Im curious how the height difference will effect my perception of things when riding it.

Another one of Jan's claims is that a 700c rim with a tall fat tire will be inherently more stable straight line vs the 650b with same combo, but less nimble in the turns. I would think the straight line stability would benifit a person when they cant hardly keep their eyes open.

I try not to ride when I can't keep my eyes open, but I think nimble is always better. If you get sideways with a more stable wheel, it's going to be harder to recover. But I'm not sure there is that much of an effect. If there was, it would be even more noticeable with 26" wheels, and I can't say that I have noticed it there.

I'll probably just use the 650b bike on 200k's and maybe a few dirt roads. Not going to be my primary bike. 650b used to be the preference for utility bikes. I guess it fell out of favor when those kinds of bikes were supplanted by mountain bikes, which is a shame. I think the 650b tire choices are going to continue to get better, but they will definitely be on the hefty side.

As a guy who rides a small frame ( 50-52cm, depending on the geometry ) and has both 650B and 700s, I definitely prefer the 650B for most riding. I get toe overlap on all of my 700's. From an aesthetic point of view, 700's simply look too big to my eye on small frames.

In terms of tire/rim availability, yes, there are fewer options. You can still get everything from puncture-resistent tires to fast "event" tires, though you're not going to generally find them in your LBS.

For tire performance - well, when I'm riding with my 11-year-old, who is on a frame not much smaller than mine but has 700x25's, while I'm on 650Bx38's, I still beat him by a large margin when we've tried roll-down tests on hills. Plus, it's rather nice than I can go from smooth pavement to gravel and not even have to think about it. ;-)

650B is definitely "faddish" at the moment, especially in light of the easy availability of 26" tires/wheelsets. I suspect it will stay a niche, but still entirely viable size for a long, long time though. It meets a nice need for all-rounder bikes that aren't quite met by most 26"/700's in certain frame sizes.

When we group up for a brevet here, one might see one or two 650B machines out of a hundred bikes. The rest will be 700c. Many carbon bikes, many TI bikes, several aluminum bikes, a few steel bikes.

I like my 650b bike. I think it comes down to tire selection ultimately. If you want to run very large tires, things get a bit awkward with a 700c size, both in terms of handling and construction. Since the roads our organizers send us over here in BC are pretty random, and sometimes not roads at all but trails, I like my 42mm tires a lot.

Of course I've done these rides on my racing bike too, it works fine. It's a bit more tiring and the chance of wheel damage is increased of course, but then there is also the advantage of the much lower weight when conditions are good. This is the primary disadvantage of 650b, the added weight of the larger tires.

Some people talk about availability as a disadvantage, but I don't think that's really relevant. It's no big deal to bring another tire, and ordering enough tires off the internet to keep a stock on hand is simply done. I suppose a catastrophic wheel failure will keep you 'grounded', but I'd hate to find out what kind of obstruction could destroy a nicely built 32 spoke wheel shod with 42mm tires at 55psi. I've certainly never encountered one yet that wasn't easily avoided, and I have to think if I failed to do so, walking and breathing would be my primary concerns afterwards.

there is definitely some comfort in having bigger tires. I remember before I got my current lights, riding at night with 25mm tires could be a little stressful because I was afraid of hitting a pothole at speed. I've been doing a lot of gravel road riding, and even 32mm offer a lot more margin.

there is definitely some comfort in having bigger tires. I remember before I got my current lights, riding at night with 25mm tires could be a little stressful because I was afraid of hitting a pothole at speed. I've been doing a lot of gravel road riding, and even 32mm offer a lot more margin.

Yep, I mostly ride at night due to my schedule, and even with a dyno light setup sometimes I get a little overenthusiastic and outrun them... straight into a pothole. ;-) I suspect that the ones that make me say "Ooops, that was dumb" on my 650bX38's would have me looking for all of my tooth fillings that had fallen out on my 700x23's...

I double flatted within 5 miles of the start of a 300k when I hit a couple of big potholes in the dark. Don't know if bigger tires would have saved that from happening, but it was pretty stressful. Even though I finished, I never fully recovered from that setback.

I hadn't really thought about it, but a friend of mine recently got a Bacchetta recumbent that's 700-size, but she mentioned a lot of the older Bachettas were 650- they were just now going to 700s across the line. The significance around here is that most of the 650's you'd see here aren't on upright bikes. Which also means tire availability should be a little better than you'd imagine.

Similarly, I saw a kid-size racing bike the other day, and it's hard to imagine that thing would be in production if the wheel size wasn't being used on some recumbent somewhere. Or maybe it's the mini-velo fad, but either way, it gets some odd sizes in production.

I hadn't really thought about it, but a friend of mine recently got a Bacchetta recumbent that's 700-size, but she mentioned a lot of the older Bachettas were 650- they were just now going to 700s across the line. The significance around here is that most of the 650's you'd see here aren't on upright bikes. Which also means tire availability should be a little better than you'd imagine.

Similarly, I saw a kid-size racing bike the other day, and it's hard to imagine that thing would be in production if the wheel size wasn't being used on some recumbent somewhere. Or maybe it's the mini-velo fad, but either way, it gets some odd sizes in production.

Bacchettas have been sold with 650C wheels (not 650B). 650C is ISO 571. 650B is ISO 584. Then you consider that 26" mountain bike wheels are ISO 559. Three wheel sizes with only very small bead seat diameter differences between them. Well, crap. If you took the tire availability in each of these sizes and somehow made them all the same BSD, then you'd have pretty much every possible type of bike tire and use covered. Unfortunately though, things are fractured, and will likely remain so. It may even get worse as time goes on. For a while there all road bikes were 622 and all mountain bikes were 559. The world made sense. Now, we have road bikes with 584, Tri bikes with 571, mountain bikes with 662 and 584 and 559, and recumbents with 622, 571, 559, 520, 507, 451, 406, 355, and 349. Well, crap.

recumbents with 622, 571, 559, 520, 507, 451, 406, 355, and 349. Well, crap.
and that's just your one bike

and that's just your one bike

Of course your joking, but I have, in fact, had at least one contraption using each of those wheels sizes except 571 and 349. But they're out there. :twitchy:

I have one bike that I switch between 700c and 650b. I like the cushier ride with the wider tires, other than that, I don't notice much difference between the two. My average speed is about the same with either wheel size. The main advantage to me of 700c is the greater availability of tires. I don't worry about wheel failure with the 650b. I've never been on a brevet where I felt like I could've found a replacement wheel of any size, and still been able to finish within the time limit.

Seems to me the real advantage of 650B is that you can fit a wider tire on a frame that limits the size of the 700C it was built for, assuming you can get the brakes to work. That lets you ride wider tires at lower pressure without getting a new frame.

It would, therefore, seem reasonable to make sure a new bike fits 700C.

Exceptions are people who ride smaller frames (can somebody cue up a chorus of "Short People" please?).

One other note is that most 650B tires are (a) expensive and (b) thin. Lower rolling resistance than a similar (>30 width) 700C, and probably lower lifetime as well. It's hard to find a 700C tire >=32 with thin sidewalls, for instance, because most of these tires are sold for tourists (carrying a load and not wanting a flat) or hybrids (don't want a flat).

Bike Fit > Wheel Size

Bike Fit > Wheel Size

Wow what a random comment.

I can see no advantage going from a 622 down to a 584. I can see going up from a 26" mtb application though.

Here is my reply for later use:
Currently on my 29er I am running a 622-60 Big Apple and the tire/wheel combo is huge and really, really cushy. It's a tubeless application and the 30psi is great for urban assault riding.
On my 700C road bike, going up from 25 to 28 tires was a huge advantage and I run them at about 80-90psi front/rear. The long distance bike I'm having built will use 700C wheels, but I won't ride anything smaller than 32 tires.
So...for me, no 650B. I like big wheels, the roll over stuff nicely and as far as one comment about being less nimble...I can flick those BA around just fine, so I don't buy that at all. The other comment about weight and acceleration is so over used and unfounded... minimal at best. I can spin up those BA just fine and once I get rolling look out.
As far as the other issue discussed about availability. I wouldn't let that stop anyone from going with the 650B.
If you're a shorter person, maybe 650B makes sense.

Wow what a random comment.

Not entirely random. Many folks look to 650B wheels as a magic bullet. I've ridden 700C and 650B, and they both have their pros/cons. I think it's more important to choose a bike on fit/function, and then accept the corresponding wheel size.

Not entirely random. Many folks look to 650B wheels as a magic bullet. I've ridden 700C and 650B, and they both have their pros/cons. I think it's more important to choose a bike on fit/function, and then accept the corresponding wheel size.

I am with him, proper fit should come before all else on a bike and if smaller wheels can help make the bike fit better I would certainly say go for it.

I would hope that in the long distance forum that proper fit is a given. Don't think I've ever seen someone with a bad fit on a brevet, i suppose it's inevitable though.

I would hope that in the long distance forum that proper fit is a given. Don't think I've ever seen someone with a bad fit on a brevet, i suppose it's inevitable though.

It is one of those things worth repeating till blue in the face.

I have seen a number of people come here asking about long distance riding that do not even own bikes yet. There are a number of very seasoned riders on here but there are certainly people new to the sport, many of them are just lurking and never actually post so it is good if something as important as proper fit is stressed.

It is one of those things worth repeating till blue in the face.

I have seen a number of people come here asking about long distance riding that do not even own bikes yet. There are a number of very seasoned riders on here but there are certainly people new to the sport, many of them are just lurking and never actually post so it is good if something as important as proper fit is stressed.

Exactly. I've seen plenty of photos of rando bikes where a Brooks saddle is slammed all the way back on an offset seatpost, and the rider still complains that they still can't get the saddle back far enough. But hey, the bike has the "right" wheels, hammered metal fenders, etc, so it's OK.

Exactly. I've seen plenty of photos of rando bikes where a Brooks saddle is slammed all the way back on an offset seatpost, and the rider still complains that they still can't get the saddle back far enough. But hey, the bike has the "right" wheels, hammered metal fenders, etc, so it's OK.

Well okay, but...what does wheel size have to do with fit?

Well okay, but...what does wheel size have to do with fit?

In some cases, people place more value on "correct" wheel size that frame fit/geometry.

In some cases, people place more value on "correct" wheel size that frame fit/geometry.

Out of all the folks I know riding 650bs, two of us are riding stock frames. Everyone else is riding full custom, and one of the two riding stock (me) chose the frame specifically for its geometry, including an unusually slack STA.

So...I mean I'm sure it's true in your circle of friends, but that certainly isn't my experience.

not buying the idea that fit is somehow related to 650b tires. Might as well say that eating properly is more important than tire size, because that is also true.

Most Brooks saddles don't allow a lot of setback, but if someone has a long setback post and a Brooks and still can't get the seat back far enough, they have a pretty weird body. Since the most widely used 650b tires have approximately the same diameter as a 700x25mm, I don't see how that compromises fit at all.

Everything I know about the 650b is from reading Jan Heine's articles.

Heine also prefers rather wide tires.

650b+wide_tires == 700c+narrow_tires. (Height wise, basically.)

If you have disc brakes, you could use either.

If you only intend to use narrow tires, 23mm, 25mm, or 28mm (maybe), there isn't much reason to choose the more-unusual 650b size over the much-more-common 700c size (unless you are worried about weight or are unusually short).

Well okay, but...what does wheel size have to do with fit?

With the same tire width, the 650b wheel has a smaller diameter than the 700c wheel. On the same bike, the standover height would be less with the 650b wheel. And the bottom bracket and pedal clearance would be less too.


not buying the idea that fit is somehow related to 650b tires. Might as well say that eating properly is more important than tire size, because that is also true.

It's related to (it's a component of) the vertical fit and bottom bracket and pedal clearance.

wheel_size + tire_size + frame_geometry == vertical "fit".

With the same tire width, the 650b wheel has a smaller diameter than the 700c wheel. On the same bike, the standover height would be less with the 650b wheel. And the bottom bracket and pedal clearance would be less too.



It's related to the vertical fit and bottom bracket and pedal clearance.

wheel_size + tire_size + frame_geometry.

Well of course, but you're talking about some extremely short folks that would benefit from the 650bs. I think that's a fairly specific topic, and if that's what we're talking about, some of these posters really need to clarify their posts.

Well of course, but you're talking about some extremely short folks that would benefit from the 650bs. I think that's a fairly specific topic, and if that's what we're talking about, some of these posters really need to clarify their posts.

I don't think the people here are "extremely short folks".

The thing that people are missing is tire width (tire width is part of the discussion).

There isn't much of a reason to use 650b if you are only interested in using narrow (23/25/28) tires.

If you are interested in using the very-wide tires that Jan Heine prefers, it might make sense to consider 650b. (I'm not sure if it would make sense to argue that 650b doesn't benefit Jan Heine!)

If a rider finds a frame's stand over height is too tall with 700c wheels, I would think that in most cases, the top tube and seat tube lengths would also be too long. Sure, they could decrease the stand over height with 650b, but it seems like the frame would still be too large.

As I said earlier, I have a frame that I switch between 700c and 650b. I like that I am able to go with wide or narrow tires, on one bike, as I see fit.

I don't think the people here are "extremely short folks".

The thing that people are missing is tire width (tire width is part of the discussion).

There isn't much of a reason to use 650b if you are only interested in using narrow (23/25/28) tires.

If you are interested in using the very-wide tires that Jan Heine prefers, it might make sense to consider 650b. (I'm not sure if it would make sense to argue that 650b doesn't benefit Jan Heine!)

Uh...is anybody actually missing that? Is anyone using narrow tires on 650b rims? Or even discussing it? I've never, ever seen that. 650c, yea, but not for a really long time.

'Round these parts, everyone's running Hetres. And for good reason, they are super!

Bacchettas have been sold with 650C wheels (not 650B). 650C is ISO 571. 650B is ISO 584. Then you consider that 26" mountain bike wheels are ISO 559. Three wheel sizes with only very small bead seat diameter differences between them. Well, crap. If you took the tire availability in each of these sizes and somehow made them all the same BSD, then you'd have pretty much every possible type of bike tire and use covered. Unfortunately though, things are fractured, and will likely remain so. It may even get worse as time goes on. For a while there all road bikes were 622 and all mountain bikes were 559. The world made sense. Now, we have road bikes with 584, Tri bikes with 571, mountain bikes with 662 and 584 and 559, and recumbents with 622, 571, 559, 520, 507, 451, 406, 355, and 349. Well, crap.Yeah, lots of redundant sizes. All the energy devoted to designing and promoting such an in between size over the past few years kind of irks me since I`d much rather see that same energy go into filling a gap. With 406, 559, and 622, you can have your choice of just about anything from 25mm to 50+, smooth or knobby, bombproof to silky smooth and uberlight. Why bother trying to squeeze in another into the reasonable "one step" space between 559 and 622 rather than filling the gaping hole in the 24 inch neighborhood? The world can obviously live without a 24-ish size that goes both skinny and fat, but at least it would give a useable option where there weren`t any before. But I know that all the enrgy going into 650B`s rebirth wouldn`t have ended up anywhere else that I care about, so I might as well stop whining and let people dial in their wheel diameters microscopically if that`s what makes them happy.

559-584-622: five point nine nine of one, half dozen of another, six point oh one of the next.

I think there is dormant demand for 650b since it was at one time one of the most common sizes on adult bicycles. So there are plenty of 650b bikes out there, they are just older utility bikes.

Yeah, lots of redundant sizes. All the energy devoted to designing and promoting such an in between size over the past few years kind of irks me since I`d much rather see that same energy go into filling a gap. With 406, 559, and 622, you can have your choice of just about anything from 25mm to 50+, smooth or knobby, bombproof to silky smooth and uberlight. Why bother trying to squeeze in another into the reasonable "one step" space between 559 and 622 rather than filling the gaping hole in the 24 inch neighborhood? The world can obviously live without a 24-ish size that goes both skinny and fat, but at least it would give a useable option where there weren`t any before. But I know that all the enrgy going into 650B`s rebirth wouldn`t have ended up anywhere else that I care about, so I might as well stop whining and let people dial in their wheel diameters microscopically if that`s what makes them happy.

559-584-622: five point nine nine of one, half dozen of another, six point oh one of the next.

The original French sizing system was one in which the combination of tire plus rim had the same outside diameter. So a 650B tire on a 650B rim would have the same outside diameter as a 700C tire on a 700C rim. The A, B, and C were width designations. Then people started making various widths of tires that resulted in different outside diameters.

Bottom line: a 650Bx42 tire is approximately what the 650B size designation originally "meant", and a 700Cx25 tire is approximately what the 700C size designation originally "meant" and they both have about the same o.d.

On a fork that is optimized for the pneumatic trail of the tire, handling will also be fairly similar.

The advantage of the 650B is that it has a great deal more air volume so it can be run at a lower pressure and absorb more road shock. As roads improved over the years, that became less of an advantage so people shifted to smaller air volume tires, that are also lighter and easier to sprint in a race. But for randonneuring distances, having the extra air volume helps you have a more-comfortable ride and perhaps avoid hand numbness.

Nick

Some people talk about availability as a disadvantage, but I don't think that's really relevant. It's no big deal to bring another tire, and ordering enough tires off the internet to keep a stock on hand is simply done. I suppose a catastrophic wheel failure will keep you 'grounded', but I'd hate to find out what kind of obstruction could destroy a nicely built 32 spoke wheel shod with 42mm tires at 55psi. I've certainly never encountered one yet that wasn't easily avoided, and I have to think if I failed to do so, walking and breathing would be my primary concerns afterwards.

What he said...

I've been riding for 30 years, and I've never ever had a tire fail badly enough that I couldn't put a new tube or boot in and get home. I figure the chances on having it happen twice in a single ride is remote... so I'm good with carrying a tube and a tire and a patch kit. And if you do need more tubes... 26" mountain bike tubes work fine and can be bought at nearly any store that carries sporting goods.

And if I crash badly enough to have to replace a rim, I probably won't feel much like completing the ride anyways.

The original French sizing system was one in which the combination of tire plus rim had the same outside diameter. So a 650B tire on a 650B rim would have the same outside diameter as a 700C tire on a 700C rim. The A, B, and C were width designations. Then people started making various widths of tires that resulted in different outside diameters.

Bottom line: a 650Bx42 tire is approximately what the 650B size designation originally "meant", and a 700Cx25 tire is approximately what the 700C size designation originally "meant" and they both have about the same o.d.

Well, you started off right. ;) The idea for the 650 "family" of tires was that they all had an outside diameter of 650mm, and used three different rim standards to accomplish that -- 650A used 30mm-ish tires on a 590mm rim, 650B used 33mm-ish tires on a 584mm rim, 650C used 40mm-ish tires on a 571mm rim. It's really more of a happy accident than design that skinny tires on a 700C rim (which were originally meant to use ~39mm tires to give a diameter of 700mm) give the same diameter as fat tires on a 650B rim.

But that's really a digression -- I think Mark was trying to say that fit should be given more consideration when it comes to comfort than the wheel size (since the vast majority of riders should be able to buy a bike that is comfortable and fits the desired tire width regardless of rim spec), but I've refrained from jumping in until now. ;)

This may be a really stupid question but I'll ask it anyway. How can using 650B wheels help with toe strike when the wider tires used are the same diameter as 700's with smaller tires (as mentioned above). Assuming the frame geometry is the same.

I don't run 650b's so I have no pony in this race but one arguement I always hear, because I sometimes use boutique wheels, is that if you use parts that are not readily available you run the risk of DNF'ing because you can't find parts to fix it easily. To be honest, I don't put much stock in that arguement. Probably because I've never been caught out and I've always kept my equipment in pretty new condition so I don't think that would stop me from running 650b's. I just might make sure I have a spare tire/tubs/sparks in my drop bags or carry them.

This may be a really stupid question but I'll ask it anyway. How can using 650B wheels help with toe strike when the wider tires used are the same diameter as 700's with smaller tires (as mentioned above). Assuming the frame geometry is the same.



The usual logic is this: A rider is using skinny tires, and wants to switch to wider tires. If they stick with their ISO 622 (aka 700c) wheels, they may run into toe overlap. If they switch to ISO 584 (aka 650B) wheels, they won't. In other words, the toe overlap with 42-584 wheels/tires is similar to 23-622, and far less than 42-622 - in fact, about 19mm less (the difference in the radius of a 42-622 tire and a 42-584 tire). Fenders can lead to similar issues. Of course, for short riders, they may have toe overap with either - they should probably use ISO 559 (aka 26") wheels, even though few bike companies seem to be interested in building ISO 559 wheeled road bikes for their smaller riders.

On a fork that is optimized for the pneumatic trail of the tire, handling will also be fairly similar.
By "optomized for the pneumatic trail", I take it you`re getting at the trail difference resulting from two vastly different tires on the same fork? I`m sure that can be measured, but personally I don`t notice it while riding. My touring/commuting daily driver gets 32-559 Paselas if it`s going to be spending a day or more on paved roads, 50-559 Big Apples if it`s going out for an unpaved adventure, 47-559 studded tires when ice is expected. They all work acceptably and the handling differences caused by the huge differnce in tire volume completely eclipse anything that I could imagine was due to minute trail differences.

For serious racers (who don`t have at least one dedicated race bike optomized for the wheel size they`re running?), it`s probably a different story, but that only accounts for a needle`s worth of riders out of every haystack`s worth on the road.

Again, the popularity of any sizes that I don`t use has no real negative affect on me, but I sure could think of stuff I would rather see on the market. The 559 road bikes that mbmuller mentions are an even better example of what would make me happy than 24 inch wheels I mentioned earlier. 584 still just seem like a silly half-step to me. What a whiner, eh?

EDIT (I forgot part):
>Bottom line: a 650Bx42 tire is approximately what the 650B size designation originally "meant", and a 700Cx25 tire is approximately what the 700C size designation originally "meant" and they both have about the same o.d.<

So what? If one (559 or 622) is too big or not big enough, why not just go to the other? The only reason I see that it matters to have the same tire OD is that handling thing^^. Other than that, doesn`t it make more sense to have the rim diameters the same in order to evade the brake reach issues?

Bike Fit > Wheel Size

Yes, but the question is about wheel size.

Exactly. I've seen plenty of photos of rando bikes where a Brooks saddle is slammed all the way back on an offset seatpost, and the rider still complains that they still can't get the saddle back far enough. But hey, the bike has the "right" wheels, hammered metal fenders, etc, so it's OK.

Is the fit bad because it's slammed, or because the rider is still complaining?

With the same tire width, the 650b wheel has a smaller diameter than the 700c wheel. On the same bike, the standover height would be less with the 650b wheel. And the bottom bracket and pedal clearance would be less too.



It's related to (it's a component of) the vertical fit and bottom bracket and pedal clearance.

wheel_size + tire_size + frame_geometry == vertical "fit".

Your equation is related to standover height. Vertical fit is also the difference between saddle height and handlebar height, not to mention actual saddle-BB height. At least as important is saddle fore/aft with respect to the BB (setback or "slam factor"), and distance from the saddle to the hbar however you like to measure it. The only fit attribute affected by wheel size is standover, and it has oft been argued to be less important than these other factors.

The benefit of at least fat 650s is smoothness over rough pavement and good stability and steerability over anything imperfect short of broken-field or woods riding. For skinny 650 there's an added opportunity to lose the toe overlap, but that can also be done with less trail, if you like low trail.

By "optomized for the pneumatic trail", I take it you`re getting at the trail difference resulting from two vastly different tires on the same fork? I`m sure that can be measured, but personally I don`t notice it while riding. My touring/commuting daily driver gets 32-559 Paselas if it`s going to be spending a day or more on paved roads, 50-559 Big Apples if it`s going out for an unpaved adventure, 47-559 studded tires when ice is expected. They all work acceptably and the handling differences caused by the huge differnce in tire volume completely eclipse anything that I could imagine was due to minute trail differences.

For serious racers (who don`t have at least one dedicated race bike optomized for the wheel size they`re running?), it`s probably a different story, but that only accounts for a needle`s worth of riders out of every haystack`s worth on the road.

Again, the popularity of any sizes that I don`t use has no real negative affect on me, but I sure could think of stuff I would rather see on the market. The 559 road bikes that mbmuller mentions are an even better example of what would make me happy than 24 inch wheels I mentioned earlier. 584 still just seem like a silly half-step to me. What a whiner, eh?

EDIT (I forgot part):
>Bottom line: a 650Bx42 tire is approximately what the 650B size designation originally "meant", and a 700Cx25 tire is approximately what the 700C size designation originally "meant" and they both have about the same o.d.<

So what? If one (559 or 622) is too big or not big enough, why not just go to the other? The only reason I see that it matters to have the same tire OD is that handling thing^^. Other than that, doesn`t it make more sense to have the rim diameters the same in order to evade the brake reach issues?

The tire volume difference is what causes the difference in pneumatic trail and hence total trail. Pneumatic trail results from the distortion of the tire contact patch due to driving forces (friction from the road is what makes the front wheel turn), such that the centroid of the driven contact patch is not the same point as the centroid of the stationary contact patch. That difference in position is the pneumatic trail. Tires of different volume and hence width distort differently, causing part at least of the change in road feel. Volume might have other effects that are significant in handling and steering feel, but I'm not familiar with any more.