would a 26er make much of a difference over a 24er for XC?
Yes, it would, depending of course on what you are focusing on. There is a much, much larger selection of tires and tubes for 26"; you will be somewhat faster; and roll over obstacles a little better.
Plenty of threads on this, I’m sure, if you search.
Keeping it fairly simple: given a similar tyre, the 26 is around 8% bigger, which means around 8% faster on the flat. Muni speeds around 6-8 mph means an extea mile an hour at most.
However, the bigger wheel is less vulnerable to small irregularities in the ground, and rolls over things more smoothly. Therefore, average and “journey” speeds are much higher.
On the other hand, the 24 is lighter, and more manoeuvreable.
The 8% will matter on hills. It’s like changing a gear on a bicycle.
Various schools of thought: a 24 for hard hoppy droppy MUni, a 29 for fast cross country. Where does a 26 fit in? Somewhere between?! Choice of wheel size is dependent on what is available. Maybe the ideal size is 25 inches, or 27, but you have to choose between 24, 26 or 29 because those are available. Is 26 the ideal compromise, or is it neither fish nor fowl?
My MUni is a 26. If I replaced it, it would be with a 24 with a fatter tyre, I think. The fat tyre makes the effective diameter nearer to 26. But then a fatter tyre on a 26 would make it a bit nearer to a 28!
More tyre choices available for 26 than the other two sizes.
Buy one of each!
It really depends on the type of XC you plan to do and the type of trails you plan to ride. Different areas of the country have different style of trails and different people have different preferences for how they ride (aggressive vs. speed). So what’s great for me or someone else may not be what is best for you.
If you like speed, ride buff trails and don’t do much hopping to maneuver on the trail then a 29er would actually be the choice for XC. If you like more technical riding, rocky or rooty trails, and need to hop to get over logs then a 24 with a fat tire would be the choice. A 26" XC wheel is kind of the odd size out now. The 24" beats it for maneuverability and hopability and the 29er beats it for speed. If you go for a 29er for XC get something with the KH 700c rim. Nothing else is wide enough to make for a good XC muni wheel.
So it depends on the trails and your riding style. If you do some of both then get both the 24" with a fat tire and a 29er. Then ride the one that best fits the ride you want to do that day.
I have a 26" Muni, which I use exclusively for all my Muni/Offroad stuff, and it’s always seemed to me to be a more capable machine than the 24" I used for the 2 years previously. Maybe its my high strength, but I can use my much heavier 26 for all the technical stuff I did on my 24, and on sketchy stuff (wedges between rocks, dropping onto edges, riding in channels) it performs much better and is much more predictable. I’ve got a LM rim on mine, which increases tyre stability somewhat over a ‘standard’ 26.
I’m more than happy with mine, and would have no qualms in recommending a 26 to anyone who wants to do muni. For someone with a lighter build (I’m 6ft, 190lbs) it might restrict technical maneuverability, but I think its something you get used to rather than something that holds you back.
Loose.
I have got a 26" muni and think it’s great, I find that 24" uni’s seem slow and unstable by comparison.
I definately reccommend 26" as a size for muni, it might be heavier but it’s not an issue unless you are doing insane trials type stuff.
I always wonder why more people don’t ride 26’s as there is a better availability of parts and a wider variety of parts. it seems that 24" became a sort of standard size for uni’s at some point in the past so people are sticking with it for the sake of it.
I guess 24 has become a “standard” largely because of Kris Holm’s influence. The KH24 seems to be a benchmark Muni. (I say that based on what I’ve read here, not on what I’ve seen.)
Another problem with 24s: pedal strike. Given similar tyre sections and cranks, on a 26 a rock needs to be an inch higher before your pedal hits it.
On the other hand, if you choose cranks according to your leg length and riding style, then the 24 will be lower geared and it makes a real difference to control on steep downhills.
I ride a 26x3" muni (with 152mm cranks) and I’m very happy with it. I do mostly longish fastish cross-country riding, and I’ve thought about a 29er a few times. But there are some very rocky sections on the routes I commonly ride and I’d be afraid of pranging the rim without the big 3" tyre, although a 29er tyre would probably be fine in practice. I haven’t ridden a 24" muni much so I can’t really give a direct comparison, but my 26" does seem to pull away from people on 24" wheels quite significantly on anything but steep or technical climbs - I suppose it goes an extra six inches for every pedal revolution, so that’s pretty noticeable in absolute terms even though it’s less than 10%.
So I find the 26" to be a good choice for my riding style, but then I’m probably somebody who should be riding a 29 
Rob
Ish? The boy says “ish”? 
Or maybe that 12 that I saw in Nottingham the other day, then I might keep up with you.
I was meaning “ish” in comparison to the likes of Joe Marshall. But thanks for the ego massage
Rob
Ooo, this is convenient. I was thinking the same question the other day on taking the muni out on the Malverns. A lot of tracks up there are rather narrow with raised shelves of grass either side; excellent pedal strike territory. These sections generally coincide with bits with a very large drop off to one side or a hill so steep you would have a very long walk to get back up to the unicycle, if you still had enough unbroken bones to do so. Being somewhat unwilling to launch myself off a cliff I ended up riding very slowly and cautiously, dismounting on bits that without the drop would be excellent fun.
My muni is a 24" with 165mm cranks; if there was a perfect muni for avoiding pedal strikes this isn’t it. Presumably a 26" would be better?
Phil
With a 3" tyre on a 26" uni the total diameter is about 28" and so riding a 29" uni would probably make very little difference to speed.
I compared my 26 with someone’s 28 at crawley last year and found that the total diameter was exactly the same.
Tyre choice makes it as versatile too as I switch between the 26x3" duro and 140 or 170mm cranks and a 26x2.5 hookworm with 125mm cranks so it’s great for fast trails, muni or commuting.
The 26" is a great all round uni and I would recommend it to anyone.
Pete
Yes, a 26x3 is 28" overall diameter. But the big fat downhill tyre is much heavier than a 29er. It’s probably not much different top-speed wise, but less weight to lug up the hills, so journey speed should be higher. That’s what makes me think a 29er would be good for my riding style. But the 26 is a very good all-rounder.
Phil: I’ve noticed a considerable improvement in pedal-strike vulnerability going from 165 to 152mm cranks on my 26. So if that extra half-inch of clearance is noticeable you should get significant benefit from the extra inch you’d get by going from 24 to 26" wheel, possibly at the expense of a bit of tractoring power on climbs.
Rob
I’m always curious about this idea that a 24x3 is the same diameter as a 26, and a 26x3 is the same as a 29. Maybe if the tyres are hugely different in size, but for something like this they won’t be. a skinny tyre on a 26 might be the same as a 24x3, but that isn’t really relevant - the question here is a big fat 24" tyre or a big fat 26" tyre. Even where the 26" tyre is smaller, there is a difference. The 24x2.6 wheel on my muni is noticeably smaller than the 26x2.1 wheels on my bike, so I’m always curious about everyone saying they’re the same.
Apart from anything else the air volume is very significant for muni. If you’re talking about riding that can be done on a 29er, then that’ll be faster, but I can think of lots of riding that while the extra bit of speed is nice, is still very technical and a big tyre is nice to have. Just because you’re rolling not jumping doesn’t mean there aren’t any big rocks.
John
The 24" or 26" for bike wheels is quoted as a nominal outside diameter for the size of tyre considered “normal” for that rim, rather than the method used for car tyres where they quote the rim diameter. A “normal” tyre for a 26" mountain bike wheel is something like 2", so using a 3" downhill tyre adds two inches to the overall diameter. So a 24" rim with a 3" tyre is actually something like 26" diameter, and a 26" rim becomes about 28" diameter. A “normal” 700c tyre is much skinnier, ending up at about 28" diameter the same as a 26x3". A 29er 700c tyre is a bit fatter (700c x 2"/50mm?).
But I agree with you that a big voluminous tyre is good for muni even if not hopping - that’s the main reason I’ve not swapped my 26x3 for a 29er, for fear of pranging the rim on rocks all the time.
Rob
I’ve got a lightly used 26" Profile Yuni I’m considering selling.
Maybe, maybe not. The problem here is, people toss around tire sizes as if they are exact. They’re not. My 26" MUni has almost the exact same tire diameter as my 24" MUni. It very much depends on the tire. So if one doesn’t include the tire width in one’s description or question, it’s hard to answer.
All things otherwise being equal? Bigger is faster, but also heavier. And also a little weaker/prone to tacoing. I like faster, but the rim on my 29" is a little too skinny for the tires I’ve been running on it, so I’ve run into a couple of very inconvenient tire failures far from the car. By contrast, my 24" (with 3" Gazz) is nearly indestructable, and I never have to worry about the same problem. But it’s slower.
A 26" Gazz on a strong rim is really heavy, but probably very strong. Rotational weight is always the least desirable kind, but if speed is more important, go bigger.
People keep coming up with that one. Surely a 26" wheel and tyre is only 8% heavier than a 24" with the same type of tyre. And that’s ignoring the weight of the hub and cranks, which is constant whatever size rim you use - which gives an entire 26" wheelset actually a slightly better weight/speed ratio than a 24" (I know this is over-simplifying things for real-world performance, but I’m just making a point).
You can’t have it both ways:
“It’s only slightly quicker”
but “It’s really heavy”
Unless the 26x3" downhill tyres are a thicker heavier construction than the 24" ones - I suppose that could be true.
Rob
The 26x2.6 and 24x3 Gazz tires are the same weight (manufacturer’s numbers). Presumably this implies that the tires are also close to the same volume. I don’t have the diameter measurements handy at the moment.
Like John says, my 26x2.6" is just under 2" wider in diameter than John’s 24"x3". My 29x2.1" is over 2" wider in diameter than the 26x2.6". Even the 26"x3" is nowhere near as large a diameter as the 29er tyres people are running. All this talk of them being the same is weirdy stuff.
Joe