There are two different IUF Standard 24 inch uni specifications. Track and Hockey specify a maximum diameter of 618 mm while a Standard 24 for basketball is specified as a maximum of 640 mm.
Does anyone know the reason for the difference?
Clearly the basketball spec would allow for a larger tyre on the same rim. Is it the nature of basketball that it is better with a wider tyre,
Or is is just historical accident?
My assumption was always that because basketball involves a reasonable amount of hopping they thought there would rather have a larger volume tyre. Under 618mm basically requires a 24 x1.95 tyre, even 24 x 2.1 is over 618mm.
Therefore basketball used a larger size meaning probably up to 24 x 2.3 or 2.4 will be legal.
I’m not an expert on uni basketball, but I have watched it a lot. 
And played it some, on various surfaces. One reason for a wider tire might be traction. A “soft” tire that’s wide rides a lot better than a soft tire that’s skinny. Some gymnasiums have pretty slippery floors.
If you are playing outside, however, you probably eliminate the traction problem and a skinny tire would probably give you quicker performance (but with a harsher ride).
A skinny wheel might be a little weaker than a wider one, though much of that depends on the details of build and materials. Regular basketball play isn’t supposed to involve contact, but there’s plenty, and falling down, and hopping, etc.
But on the question of wheel diameter, I don’t know why basketball has chosen a different size. It may have to do with a certain tire that was popular with basketball players. I don’t think it can be called a historical accident because this 640mm thing is fairly recent. We came up with an exact size for “24 inch” wheels many years earlier.
The semi-arbitrary 618mm for Track racing is based on having to pick an exact diameter while trying not to eliminate what might be very popular tire sizes. So we went with 24 1/3". This allows some leeway, and also allows for skinny tires on 26" rims. For wheel sizes under 29", racing is all about outside diameter. For 29", also known as 700c, we went to a more modern method and the size is based on a wheel’s BSD, which stands for Bead Set Diameter. That’s a number you can look up, for most rims, and find it online. Tire diameters are much more difficult to get in exact measurement, and accurate numbers are usually not available by tire brand and model.
My guess would involve the recent factor as well. The historical 618mm defined outside diameter size is getting quite difficult to achieve with 24" (507mm BSD) tires anymore. Even tires that may be in spec initially may stretch over time slightly leaving them out of spec. A friend has a Miyata that was the standard. Over time his old tire has expanded to the point that it is now illegal.
Rims have typically widened (especially in the 507 size) over the years. Getting a 24" KH 47mm rim with a tire requires a 1.95 tire to make it work. A couple of years back I remember we were looking to get Track legal tires for NAUCC bound unis. We couldn’t find them easily in stock. BMX went to 2.1 and larger which are typically too big. Even UDC USA was out of stock of 24" legal tires at the time.
Track racing is the hangup. The 26" (559 BSD, but it’s listed as 25" only here) with a 1" tire is essentially now the “Standard”. It’s lighter than pretty much any 24" setup too. As with muni, and most anything but kid’s bikes, the 24" tire size has gone away.
By bumping up the diameter, it allows for a wider tire selection. The bigger tires are going to be heavier than smaller ones, so one will pay a bit of a weight penalty for the increased circumference.
I have two comparable unis the E-Sport Basketball and the E-Sport Race. They are both very lightweight. The Race wheel, the rule beating 559 BSD rim with a 1" tire has a lot less inertia than the 507 x 1.95 on the Basketball. In Track, acceleration is key. The 507 wheel with any tire will lose to the 559 wheel with a 1" tire. The main idea of the rule- to put riders on equal footing, is almost lost due to the 25 x 1" tire (559, which is 26" anywhere else) coming in under the 618mm. Unless, there’s a bad batch of tires which are over spec.
The more recent trend in the IUF (and USA) rules has been to define sizes by the BSD, NOT the diameter. The 29" Standard for Road Racing is 622 BSD. Run what tire you want. The new trend in 29+ tires for mountain bikes will have a larger outside diameter. They will also weigh more and have higher rolling resistance. Cyclocross Unlimited Starts at a 622mm BSD. This allows for 29" and 36" wheels. The newest 27.5+ size (BSD 584) gets close to, and in some cases exceeds the diameter of the 29" size. To run that big of a tire again has it’s drawbacks. By the rules, the 584mm size is listed as a Standard CX uni. A 559 x 5" tire also is a Standard CX uni even though it’s diameter is as big or bigger than a standard 29" x 1.95 or 2.1 tire. Again, there’s a price to pay.
And now, we’ve got 26+ just recently added. It’s cool though, as it’s a great size for Muni.
This is getting interesting, unless you only care about the original question of this thread…
That’s interesting, and a reminder that so much of my hardware is pretty old. Historically, which means back when Miyata was the unicycle of choice back before the company left the US market, their 24" tires were well under 24" diameter; somewhere closer to 23 3/4". Now I’m curious how big my “current” 24" tire is. Currently that tire is waiting for bearings and a new hub, which means new spokes…
The 24" size is fading away. And that’s okay. We should be racing on a common wheel size. 24" was the standard for racing because unicycles used to only come in 20" and 24". But those days are long over! It’s time to switch to a size that makes sense today. I attempted to get this going in 2004 when we added a 700c category (and 2 races) to the Unicon lineup. But it didn’t catch on. This is probably because all the “main” track races were still for 24", so you brought a 24", which made bringing that additional wheel a hardship if you were flying to Unicon (which was in Japan so basically everyone from other countries flew).
I think we need to bite the bullet and completely switch to a new size, so people don’t have to decide which size wheel to bring. Or hold races in both sizes, but only allow riders to enter one or the other.
Which size to use? I still think it should be 700c (or the current equivalent), as it’s the largest “conventional” size, with rims and tires easily available worldwide. Wheel size being the main limiting factor on speed, we should use the biggest one commonly available.
Using BSD instead of tire diameter is a more sensible way to go, because it’s a spec that comes with the rim, and doesn’t change with temperature or tire pressure. It also gives you lots of freedom to choose not only tires, but rims as well. You can go skinny for quicker acceleration, but a smaller size, or fatter for a theoretically higher cruising speed. Specialists might bring multiple wheels, specialized for different events, and that would be okay.
The only thing that worries me about upping the wheel size is crashes, which would be worse. It would increase the incidence of injuries on the track, though Trials and Downhill would probably still be kings in that department anyway…
Sorry about the tangent.
What? Limits in the Unlimited category? No, that’s just a minimum rim size. This allows for a different category that fits most munis, but isn’t dominated by the really big wheels. Otherwise, Unlimited is always unlimited. 
The basketball size is more recent than the racing size. 640mm was chosen entirely arbitrarily; I believe we did it at U Games and then had it adopted by the IUF. It was acknowledging the fact that some tires people use for basketball are larger than the track racing diameter. And there’s no inherent competitive advantage in having a slightly larger wheel diameter in basketball.
The thread is definitely heading right in the direction I hoped.
I asked after some disappointed basketballers found their uni’s didn’t meet the standard wheel spec for races at the Australian National Titles (UniNATS) last weekend.
Not completely it appears. UDC still sells modern 24" munis and when I went on a muni weekend last month a couple of very skilled riders (probably the most skilled there) were riding new ones. They were good trials riders so a small wheel suited their skills.
Track racing tires
So what tires are people using for track racing on their standard 24" unis? The NAUCC is coming to our part of the world next summer. We have 4 24" unis and the smallest road tire we have is a 2.3. One of the unicyclists in the family may want to track race. I’m not going to get a 25 x 1 uni but I would buy another tire.
How wide is the rim?
The one we would probably use would be a Nimbus II. 32mmI believe. We also have a Torker LX that looks to be a similar size, a 42mm Nimbus muni and a 47mm KH that I definitely won’t use because it is bent.
The bmx tires sem to be either 1.75 or 2.1.
A nice light multi purpose 2.0 would be great.
I use and recommend the Maxxis Drop The Hammer (DTH) 24 x 1.75 on a Torker LX 24. This tyre is suited to 32 mm rims but I wouldn’t suggest it for wider rims as it would fall below the rule of thumb of 1.4 to 2.2 tyre/rim ratio.
Great on any hard surface. I ride suburban streets and concrete footpaths in various conditions from new to seriously cracked up and have never found it wanting on those surfaces. Excellent on synthetic tracks. It is too narrow for soft surfaces but then so is a 32 mm rim.
Very good on side slopes. Minimal auto steer. I ride across a lot of sloping driveways.
Very light (400 grams or so) making the uni incredibly responsive.
Sidewalls are very thin so not suited to encounters with sharp rocks.
No tyre is good on all surfaces. Some suggest the Hookworm is a good compromise but I found the 26 x 2.5 Hookworm was terrible on hard surfaces, being highly affected by camber and having poor traction in the wet on smooth concrete. It is also very heavy.
Others have reported the Hookworm is no better as a 24 inch, though I love the Hookworm on my 20.
I had the same issue with my previous Nimbus 24" fitted with a Kenda 24x2.1 NPJ tire made especially for standard size but with time it became illegal and I had to reduce pressure to pass the control which was ridiculous for racing. I had discussion at that time with Roger who was surprised by this expansion, I contacted also Kenda but got no reply from them to my 3 mails …
Now I’m using a Tioga Powerblock 24x1.75 (same as used by a lot of Japanese, including famous world champion Asahi Takada), no more issue, I have a margin of about 4-5mm which is very fine and no problem to issue high pressure
From the IUF Rulebook
“For Cyclocross, Unlimited is defined as any standard unicycle (see definition in chapter 1d) with a rim with a bead seat diameter (BSD) of 622 mm (700c) or larger. Unlimited also includes a unicycle with a rim smaller than a 622 mm BSD only if it has a functioning gearing system which will yield a virtual wheel size greater than a 622 mm BSD.”
We did this to address safety concerns. With the nature of the courses, there is a lot of passing involved in potentially tight situations (with tight corners and spirals, and other obstacles).
Back to the discussion just before this:
In Track racing, a lot of the issue with legal unis could be cleared up by adopting the 640mm diameter used in basketball. Most any 24 falls under this designation with whatever tire is available. At the elite level, Track is extremely competitive and serious. As we’re seeing in this thread though, many people want to use track as a gateway to uni racing. From what I’ve observed at NAUCCs, it is the gateway to racing since the races are short, and it only requires someone to ride a uni on a flat surface. Having starting poles even eliminates the need to freemount.
I’m not saying in any way that Track racing should not be elite either. It’s just a fantastic way for people to experience the thrill that is uni racing.
One last note: the 24" size is still alive in the uni world. Compared to the bike world though, the uni-verse is incredibly small.