Tire conundrum - 19" / 20" replacement issue

I have been comparing the ride on my 19" Athmos with its stock flat-tread knobby tire, and my 24" Nimbus with a Maxxum Hookworm. The Athmos tire (no brand, says Bike Trial 20x2.50 and “FIT RIM HB 396x30” and www.beatles.com.cn) is absolutely awful when riding to the side of the crown of the road. I have to counterbalance to the uphill side like crazy to keep from ending up in the ditch.

The same road on the Nimbus with the Hookworm is a dream. I hardly notice I am riding on an inclined plane. The easy solution is to get a tire with a circular cross section to replace the “beatles” clunker. But here I run into a quandary. The Athmos page claims that the tire is a “Bike Trial 19” x 2.5". This is a specific tire for this rim. A regular 20" tire cannot fit this rim." But it still identifies the tire as a “Nimbus Cyko-Lite 20” X 2.5" " And on the Unicycle.com site I see no other options. So can I get the 20" Hookworm? Will it fit on that rim? And if not that, then what? I would hate to have to rebuild the wheel with another rim and another set of spokes.

Those 19" trials tires are made for hopping and grip. Rolling quality is way down the list of priorities. They have been made smaller diameter so that they can have more volume for extra bounce. They fit on a 19" rim. A 20" tire like the hookworm won’t fit that rim.

Something like this would probably be an improvement since it has a rounder cross section.

I don’t think that any trials tire will be as good as the hookworm for just riding around. If you really want that ride quality, you’ll have to buy a new rim and spokes.


Tire sizes are confusing, possibly the most confusing is the 19" trials size.

Trials tires have a 396mm bead seat diameter (BSD) which is about an inch less than the standard 406mm BSD for other 20" tires. They are often labeled as 20" tires despite going on a smaller rim. The wider tire on the smaller rim means the overall outside diameter is about the same as the typical 20" BMX size. We typically call them 19" just to avoid confusion.

I don’t believe that there are any smooth 19" tires made for just riding around. The wheel size and associated tires were developed for bounce, shock absorption, and grip.

Shaving a trials tire used to be pretty common to end up with a tire like what you want used to be pretty common before wider 20" tires became available. Check out old threads go get ideas on how to do it. The tire won’t last as long but if it feels better it might be worth it for you.


EDIT: That Clean Koala tire looks interesting, it might be worth checking out, but super tacky compounds will pull you more on side slopes and wear out quickly. Cheaper harder tires are typically better for shaving if you want the tire to have any longevity.

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Thank you both for those explanations, things are crystal clear now. The Koala looks a bit bumpy for what I had in mind. So it seems a new wheel is the way to go. Any suggestions as far as rims and Isis hubs? Is anyone interested in the old wheel, complete with tire? You can have it for the cost of shipping from New York City.

Try changing the tyre and adding more air before giving it up :slight_smile:


I forgot about shaving your existing tire. I did that on my first 19" tire after I had worn down the middle of the tread leaving only the knobs on the edges. The cross section was almost square. It was awful to ride unless you only went in straight lines in the exact center of the road. I cut off all the remaining tread with a razor blade. It was a huge improvement. If you have a power tool that would make cutting the rubber easy, I would recommend that (don’t use a razor blade. It would take all damn day on a new tire).

The tire doesn’t have to be completely bald either. I think a lot of people who were shaving their tires back in the day were doing it to save weight, so they were trying to cut off as much rubber as possible. I your case, just rounding off the edges a little to make the cross section more circular would probably make a noticeable difference.

I have a shaved 19" tire on my flatland unicycle. If you don’t do trials, and only flatland/street, there is no point in having the knobs, so a lot of people used to ride “slicks” back in the day. I used a very sharp knife, took 2 hours or so I think.

Now, if you prefer a slick tire, there are plenty of wide 20" tires available, so you can just use a wide 20" rim and still get a tire with a fair amount of volume.