Flexibility of exchanges between 700c and 29″, and between 650b and 27.5″

Question

TL;DR

How do I explain to a budding cycling enthusiast the flexibility afforded by switching tires between 700c and 29″, and between 650b and 27.5″ wheels/tires?

Rather than let the nice folks answering go through the trouble of writing 500 words, I went ahead and summarized below what I believe is correct. This way your answer will perhaps take far fewer words. But of course feel free to provide as much detail as you like.

Long statement

I would like to confirm that my understanding of the offerings (in 2021), for both road and mountain bikes, is correct.

1. The vast majority, perhaps all, road bikes sold today (in 2021) have 700c wheels.
2. The vast majority, perhaps all, mountain bikes sold today are labeled as having either 27.5″ or 29″ wheels.
3. The label 27.5″ is actually a misnomer, used for simplicity. It is more accurate to refer to it, especially when buying replacement tires/tyres, as 650b.
4. The label 29″ is likewise a misnomer. The inner diameter (the place where the rubber sits) of 29″ rims is precisely equal to that of 700c rims.
5. One can take a random 700c tyre and fit it on a so-called 29″ rim, but it would be a good idea to choose a wide tyre, say at least 32-45mm, rather than 23-28mm, because the rim will be wider. A 23-28mm 700c tyre will fit, but will not sit flush with the border of the rim.
6. Because of the last point, it is perfectly possible to get a 29″ bike, and replace the tyres, if/when one wants, with 700c.
7. The opposite—attempting to fit 29″ tyres on 700c rims—is misguided. The tyre will fit in the rim alright, but it is very unlikely that the tyre+rim will fit in either the frame or the fork.
8. Exactly the same applies with a 27.5″ bike. It's possible to use a 650b tyre, as long as it's a wide (30-40mm) one.
9. It is today no longer possible, or it is very difficult, to buy a mountain bike with wheels labeled 26″. These bikes are effectively obsolete, after a good run of 30+ years dominating the market.
10. One can, however, today (2021) buy a fatbike. Fatbikes have exactly the same rims as yesteryear's 26″, except that they are fitted with outlandishly large (50+ mm) tyres.
11. If one has a 26″ bike, it is (obviously) not possible to simply fit them with fatbike tyres to have a fatbike. The wheels may, with some difficulty fit, but even if they do it's pointless, because the wheels will never fit back on the bike frame or fork.
12. If you dislike the rumbling noise made by knobby tyres on smooth roads, one option is to get the modern hybrid. Yesterday's hybrids are reborn and are now christened—with a drop handlebar instead of a straight handlebar—gravel bikes (please don't flame me for this over-simplification).
13. It is possible to use a gravel bike on smooth roads without putting up with the rumbling noise. A modern gravel bike will have 700c tyre. It then suffices to use smooth 700c tyres.
14. There exist on the market studded tyres in all dimensions appearing above: 700c, 29″, 27.5″, and even 26″ (for as long as that size is supported).
15. Controlling a bike on slippery conditions is easier with a (mountain bike's) straight handlebar than with a (road or gravel bike's) drop handlebar.

I believe that every point above is accurate. Did I get anything wrong?

I asked recently a not-terribly-good question, then I asked a downright bad question.

I have then been asking people in my area verbally, and the consensus is that to ride on snow, whether it's firmly packed or loosely packed, a fatbike is necessary, and the present question is meant to give me enough knowledge to know the following: suppose that I give up cycling on snow (the third of the three pictures in the question just referenced), but continue to ride on snow conditions such as the ones appearing in the first two pictures (thin layer of snow—and possibly some black ice—on asphalt), would studded tyres on a 29″ bike do the trick. This question is mine to decide; I see that's it's somewhat subjective; I'm only asking to be armed with the understanding mentioned in the title line.

Answer 1

The vast majority, perhaps all, road bikes sold today (in 2021) have 700c wheels.

Majority yes, not all. 650B for allroad/gravel/rando, 650C for some tri/TT and small adult road bikes, 26" (559) for some other small adult road bikes of various subgenres, and all the 24 inch sizes for youth road and cross bikes - 507, 520, and 540 24" specimens all exist.

The vast majority, perhaps all, mountain bikes sold today are labeled as having either 27.5″ or 29″ wheels.

Enthusiast-level adult bikes, yes. Mountain bikes for small adults and/or the gravity/DJ disciplines still exist in 26". Youth mountain bikes sold today are still often 20", 24", or 26".

The label 27.5″ is actually a misnomer, used for simplicity. It is more accurate to refer to it, especially when buying replacement tires/tyres, as 650b.

27.5" is a newfangled name for 650B that was invented at the same time 650B mountain bike tires were. All nominal designations are in a sense misnomers. Their ISO designations (584 in this case) is the one and only name a tire and rim size has that reliably describes any physical dimension of it. All the other names always have various cultural or historical baggage attached. Once upon a time, the French system (700C, 650B, etc) had an element of dimensional specificity to it, or tried to, because it was originally a way of naming the diameter of the inflated tire in a bead-size-independent way, which was useful to frame manufacturers. But that only works if all 700C tires, for example, are the same fatness, which hasn't been true in a long time.

The label 29″ is likewise a misnomer. The inner diameter (the place where the rubber sits) of 29″ rims is precisely equal to that of 700c rims.

Yes, the relationship is the same as the above. The exact line in the sand between them is arbitrary.

One can take a random 700c tyre and fit it on a so-called 29″ rim, but it would be a good idea to choose a wide tyre, say at least 32-45mm, rather than 23-28mm, because the rim will be wider. A 23-28mm 700c tyre will fit, but will not sit flush with the border of the rim.

It's not even that clear-cut anymore. You can find rims that are or used to be designated as 29" that have 17mm or 19mm internal width, i.e. into the widths where it's not the worst thing ever to put a 28 or even a 25 on.

Because of the last point, it is perfectly possible to get a 29″ bike, and replace the tyres, if/when one wants, with 700c.

Yes. There are many cases where rough-stuff road type bikes (touring, clydesdale, etc) borrow components from 29" to useful effect. You can find examples of the inverse in the early history of 29" bikes.

The opposite—attempting to fit 29″ tyres on 700c rims—is misguided. The tyre will fit in the rim alright, but it is very unlikely that the tyre+rim will fit in either the frame or the fork.

Mostly yes this does not come into play much, but one could imagine a situation to the contrary. If a chunky 700 rim is on a trekking or touring bike with a lot of clearance and the knobby or studded tire at hand says 29x1.95" on it, for example.

Exactly the same applies with a 27.5″ bike. It's possible to use a 650b tyre, as long as it's a wide (30-40mm) one.

Yes, it is the same sort of relationship.

It is today no longer possible, or it is very difficult, to buy a mountain bike with wheels labeled 26″. These bikes are effectively obsolete, after a good run of 30+ years dominating the market.

See above. Mainstream enthusiast-level bikes in the most populous sizes, yes this is true, but there are still exceptions as listed.

One can, however, today (2021) buy a fatbike. Fatbikes have exactly the same rims as yesteryear's 26″, except that they are fitted with outlandishly large (50+ mm) tyres.

26" fatbike rims are 559 rims, but only realistically take fatbike tires, which are more like 85mm+. The bikes they go on would also tend to have ground clearance problems with anything but fatbike tires.

If one has a 26″ bike, it is (obviously) not possible to simply fit them with fatbike tyres to have a fatbike. The wheels may, with some difficulty fit, but even if they do it's pointless, because the wheels will never fit back on the bike frame or fork.

Yes.

If you dislike the rumbling noise made by knobby tyres on smooth roads, one option is to get the modern hybrid. Yesterday's hybrids are reborn and are now christened—with a drop handlebar instead of a straight handlebar—gravel bikes (please don't flame me for this over-simplification).

There are a lot of differences between gravel bikes and hybrid bikes, and hybrids didn't go anywhere. The gearing, geometry, rider positioning, and components are all different. The style of riding and rider they're intended for is quite different. There are some elements about how they came to be and their place in the consumer and marketing world that are similar or debatably the same.

It is possible to use a gravel bike on smooth roads without putting up with the rumbling noise. A modern gravel bike will have 700c tyre. It then suffices to use smooth 700c tyres.

You can put whatever tires you want on as long as they fit. There is also no equivalency at all between gravel or off-road tires per se and knobby tires. But yes, many gravel bikes are running around with smooth tires for all sorts of purposes.

There exist on the market studded tyres in all dimensions appearing above: 700c, 29″, 27.5″, and even 26″ (for as long as that size is supported).

Yes.

Controlling a bike on slippery conditions is easier with a (mountain bike's) straight handlebar than with a (road or gravel bike's) drop handlebar.

There's some subjectivity there, but generally speaking yes, flat bars and the rider positioning they tend to go with are better when lots of technical handling is involved.

I have then been asking people in my area verbally, and the consensus is that to ride on snow, whether it's firmly packed or loosely packed, a fatbike is necessary, and the present question is meant to give me enough knowledge to know the following: suppose that I give up cycling on snow (the third of the three pictures in the question just referenced), but continue to ride on snow conditions such as the ones appearing in the first two pictures (thin layer of snow—and possibly some black ice—on asphalt), would studded tyres on a 29″ bike do the trick.

Stud count and quality are huge factors in that, not to be underestimated. Drawing equivalencies as though all studded tires of X size do the same thing is a mistake. When black ice is a frequent concern, something like a Suomi W240 will give you much more grip than the W106 version of the same tire, at the expense of more clatter on clear patches. How much to trust what other people think in this instance might have a lot to do with whether you're in a part of the world where people have much actual experience with good studded tires. What kind of challenge you want to face when you ride a bike is another part of it.

Answer 2

4. The label 29″ is likewise a misnomer. The inner diameter (the place where the rubber sits) of 29″ rims is precisely equal to that of 700c rims.

I think that when you say inner diameter, you're thinking of the bead seat diameter. Briefly, your tire beads are what keep the tire locked onto the rim. The beads are designed to sit on a shelf in the rim. The diameter formed by that circle is the bead seat diameter.

For both 700c and 29" wheels, the BSD is 622mm. For both 650B and 27.5" wheels, the BSD is 584mm. I wouldn't say that 29" and 27.5" are misnomers. They do give an indication that the rim is wide enough that it's designed to take MTB tires and not road tires.

I don't have experience with mounting gravel tires to MTB rims, but my understanding is that they would probably mount fine - and if those MTB wheels don't use Boost spacing, or if your gravel bike somehow uses Boost spacing, then the wheels will fit the bike too. (Older MTB wheels may not use Boost spacing, so if you're searching for a pair of gravel wheels, you could consider older MTB wheels provided the front thru axle is the correct diameter or replacement endcaps can be found.)

12. If you dislike the rumbling noise made by knobby tyres on smooth roads, one option is to get the modern hybrid. Yesterday's hybrids are reborn and are now christened—with a drop handlebar instead of a straight handlebar—gravel bikes (please don't flame me for this over-simplification).

I wouldn't exactly say that this is correct. Hybrid bikes are flat bar bikes, but they do tend to be more marketed at the casual cyclist. You never had super high-performance hybrid bikes. Gravel bikes can be built as entry level performance bikes, and all higher specs than that. Gravel bikes will tend to have less upright positions than hybrid bikes.

Otherwise, you can indeed run a gravel bike on the road if you wish, but they all do come stock with lightly treaded tires. You can swap out to a pair of slick tires. However, bikes are generally designed around a certain range of tire sizes. Using a much smaller tire than the design specs will quicken the handling of the bicycle, and it may not be quite what you want. Basically, you probably don't want to put a 23-25mm performance road tire on a gravel bike. I'd probably do at least a 28mm tire. Some gravel bikes, especially the more MTB-like ones, are probably designed for a very wide tire, and they may be poorer candidates for substituting a road wheelset.

15. Controlling a bike on slippery conditions is easier with a (mountain bike's) straight handlebar than with a (road or gravel bike's) drop handlebar.

I don't believe it's how slippery the conditions are. It's more how rough the terrain is. As the terrain gets more challenging - tighter turns, steeper descents, more roots and rocks, you will increasingly prefer a MTB. The flat bar position is, I believe, better at this type of terrain. Drop bars offer easy access to a lot of different positions for smoother roads. Consider that paved roads can be pretty slippery when it's raining or snowing. Neither a drop bar nor a flat bar will make a difference there if you overcook a turn.