# Fitting a 20x2.4 tire to a 20" qu-ax 45 mm rim

Hi,

i have a qu-ax trial/muni with a wide QU-AX 20" DB-45 double walled rim. The width of this rim is 45 mm and the unicycle comes with a 20x2.5 tyre. I now have a 20x2.4 tyre (Eclad Fireball) which is intended for a BMX bicycle. It looks sturdy enough but i was wondering if i should get a 20x2.5 tyre instead.

The difference is small but i noticed that high pressures (the tyre is rated for 100 psi) pulls the tyre off the rim. i now run it at 50 psi without noticing any problems but am worried it will pop off during tight cornering or when jumping of something.

any thoughts on this?

My car parts supplier told me that fitting narrow tyres would be OK, if the tyre width remains wider than the wheel rim.
However, bicycle tyres must be different - Wiki has thrown up the information below.

“Ideally, the tire width should be 1.8 to 2 times the rim width, but a ratio from 1.4 to 2.2 should fit, and even 3 for hooked rims”

Your 2.4 tyre (60mm) on a 45mm rim = 1.333 ratio
The 2.5 tyre (62.5mm) on a 45mm rim = 1.38 ratio

So both of these tyre sizes are beyond the recommended limit according to Wiki.
Make sure you are measuring the rim width from the inside - not the outside of the rim.

I did measure the outside and will measure the inside to be sure. The qu-ax documentation shows a cross section of the rim so if that is accurate it should be around 36 mm or 40 mm not taking the hook into account. This should then yield a ratio of 1.66 or 1.5 which would be ok according to the wiki. Nevertheless the build of the tire will also be important and this tyre has a relatively flexible sidewall. I will just have to try and see how it pans out. The ride is nice and silky smooth compared to a knobby tyre.

You should be OK
I have just checked the spec on my UDC club freestyle 24"
The 36mm rim is fitted with a 1.95 (48.8mm) wide tyre = 1.35 ratio.
I assume that UDC are quoting the outside measurement also, e.g. 48.8/30mm rim = 1.6

PS - I am learning on a 20" Uni with a Maxxis Ringworm BMX tyre.
The tyre will take 110 psi.
But it rides much better at 60-65 psi

At any rate i can’t find a slick or road tyre in the 20x2.5 size range. They are all knobby trial/muni tires like the creepy-crawler. I prefer a smooth tyre because I wanted a unicycle to play unicycle hockey and got the qu-ax for a reasonable price. It seems that the 20x2.4 is on the outside of the range used for BMX.

You probably already know this, but the problem with your new tire has less to do with width than diameter. That tire is for a 406mm rim, but your rim is only 387mm. In the bewildering world of tire sizing, tires for both kinds of rims are labelled 20-inch, but only the 2.5 will fit your wheel properly.

It’s good to know that there is some interchangeability, though. I had been wondering about that. I don’t know how safe your current setup is, though I think 50psi is a lot of pressure, so my guess would be that you should be OK. Maybe try a few long sidehops to make sure your tire won’t come out unexpectedly, but if you like hopping, you’d really be better off with a nice fat 2.5 anyway. I’ve never understood why no one makes slick tires in that size. For hopping, I keep my (slick) tire at around 28psi, but that’s a 406mm wheel. If I had a 387mm like yours, I would be able to use lower pressure, which is supposed to be better for hopping…

Yeah i saw this but just to be sure the unicycles are labeled 387-406 (20 Inch) on the website UDC/Germany http://www.einradladen.com/webshop/387-406mm-20-Inch

and i have the 406mm-20-Inch-Qu-ax-Starter-Trials kind. There is also the 387mm (20 Inch) Qu-ax Trial variant. One has a red isis hub, the other a yellow variant. The 387mm kind comes with the 67-387 (20 x 2.5 Inch) Creepy Crawler Maxxis tire and the 406mm starter variant with a 67-406 (20 x 2.5 Inch) Qu-ax Cross Tire.

The starter variant has a sticker in the rim stating 406x38 which will probably refer to the inside width.

The Eclad fireball tyre just shows the size in inch. (The Stevie Churchill Fireball Tire – Éclat BMX)

Anyway most of the unicycles which are intended for trial use are 387 mm and fit a creepy crawler tyre. So i guess that the qu-ax starter is matched with the 67-406 / 20 x 2.5" qu-ax cross tyre but may also fit other tires intended for the 20-inch labeled rims.

If the éclat pops the rim i will report this on this blog (if i survive the event of course)

I put a Ringworm on my Luxus 20 and found it was very unstable with a tendency to buck to one side or the other. I suspect it is something to do with the tread pattern which has tread blocks alternately crossing over the tyre axis.

I changed to a Hookworm and it felt great. Handled well and ultra quiet. I gave the Ringworm away because I was so confident I would never put it back on a wheel again.

However I am not a fan of the Hookworm for road use on on larger wheels where they have a reputation for being prone to biasing down the road camber.

I thought it was me that was unstable.
At least I can blame the tyre now.

The ringworm is half worn, with very little tread left on it.
I have a Kenda Kutlas 2.25x20 to replace it.
The wider profile and knobbly tread on the Kenda should allow me to ride better on grass and gravel.