It ain’t what you know that gets you into trouble, it’s what you know that just ain’t so. — Anonymous.
This post is mostly a set of notes to myself on tire sizing and wheelchair tires.
Diameter of what, exactly?
I’m in the process of getting set up to change solid rubber tires on spoked wheelchair wheels. These look like standard bike rims and tires, but the process involves buying a specialized tool for installation, and getting a handle on the peculiarities of wheelchair (as opposed to bike) rims and tires.
I started running across internet chatter on the difficulties in matching tires to rims, in the wheelchair world. And I’m like, how can this be? They mold the tire size right into the rubber tire. Any (say) 26″ bike tire has to fit pretty much any 26″ bike rim. Doesn’t it?
But I was thinking of this logically, like a car tire. As opposed to irrationally, like a bicycle.
To cut to the chase, bike tire sizing is ass-backwards, compared to car tire sizing.
For cars, you start with the metal rim the tire must fit. Tire size (e.g., 15-inch tire) is the inside diameter of the tire, where the tire bead must seat into the metal wheel/rim. The remaining numbers refer to blah-blah-blah (easy enough to look it up). The bottom line is that, for any car with normal street tires, the numbers on your current tire provide all the information you need to know to buy a replacement set that will fit your car exactly. Zero guesswork.
For bicycles, by contrast, you start with the approximate total diameter of the inflated tire. The critical dimension — the size of the metal rim that the tire fits — the “bead seat diameter” — is a matter of industry convention, and is not printed on a conventionally-sized bike tire.
There are some odd conventions. So a typical 26″ bike tire actually sits on metal rims that are around 22″ in diameter. But there are multiple conventional sizes for (e.g.) 20″ bike tires, depending on what type of bike the tire will be used on.
In addition, there’s now an entirely different bike tire size labeling system – ISO — that now labels bike tires in the same fashion as car tires. That is, the ISO tire size lists the size of the rim that the tire fits, just like a car tire. That is, it shows the inner or bead seat diameter of the tire. If a bike tire shows both conventional and ISO sizing, they’ll (maybe) agree on tire width, but the ISO diameter will be significantly smaller than the traditional diameter. That’s because the ISO number is the inside (bead seat) diameter, and the traditional number is approximately the outside diameter.
These two references seem to give the definitive guide to bike tire sizing:
https://www.centurycycles.com/tips/tech-talk-know-your-tire-size-pg1275.htm
https://www.rei.com/learn/expert-advice/bike-tires.html
This post has a nice clear graphic showing what, exactly, is being measured with various bicycle tire sizes.
The upshot is that if somebody hands you a metal wheel for a car, and says, put a tire on that, it’s pretty straightforward. And close to goof-proof. Measure the diameter of the rim, and work from there. But if somebody hands you a metal bike rim, off an unknown bike, and says, put a tire on that? You may actually have to know something about what you’re doing, in order to buy the right tire.
And wheelchairs?
Different sizes yet. So far I have come across exactly one reference that shows actual (bead seat diameter) sizes for any wheelchair tires. Apparently, a standard 24″ x 1~” wheelchair tire has a bead seat diameter of 540 mm. Per this chart.
And, indeed, where a wheelchair tire retailer lists the ISO size, standard 24″ wheelchair tires are indeed listed with 540 mm bead diameter. As in this reference, for solid tires, listing 24 x 1” (23-540 mm), 24 x 1” (25-540 mm), 24 x 1 1/4” (32-540 mm) and 24 x 1 3/8” (37-540 mm).
Near as I can tell, all spoked wheelchair rims take 24″ (ISO 540) tires. For anything other than 24″ wheelchair tires, I can’t even guess.
But 24″ wheelchair rims come in at least two different, mutually-incompatible types. Solid tires that will fit one type of rim will not fit the other, and vice-versa.
Higher-end rims are made for pneumatic tires, but can accept solid tires. These rims have a hole in the rim for a valve stem, and bicycle-style rim with a shallow space in which the bottom of the inflated inner tube would normally sit. Solid tires made for these higher-end wheelchair rims are shaped to fill that shallow empty inner-tube space. When fully stretched, the “bead” of the tire fits inside the metal rim, just like a bike tire.
But cheap wheelchair rims — rims that were never designed to have air-filled tires, and only accept solid tires, used on low-end wheelchairs — take a different tire shape. Those rims are completely unlike conventional bicycle rims. They have vertical sides and a deep channel in which the base of the tire sits. There is no tire bead, and much of the tire sits on top of what would normally be where the bead of a bike tire sits. Even though it’s (nominally) the same tire size, solid tires for those wheels are not compatible with solid tires made for higher-end rims. This, from enableyourlife.com
“If your existing tire is a 24 x 1 inch with a pyramid shape (angled sides with a flat top) … on a (plastic) wheel (plastic) on a basic wheelchair … where there is no hole for an inner tube valve stem on the wheel, this tire WILL NOT FIT.”
Other details.
Removing old solid tires. Near as I can tell, everybody just cuts them off. I’ve seen one YouTube amateur who levered off the old tire using screwdrivers. But it looks like the standard approach is just to cut through the solid tire until you can peel it off the rim.
Heat the tire? That looks well worthwhile. Putting aside the fact that polyurethane is more pliable (stretchier) when warm, a hot solid tire is going to be substantially larger than a cold one. Based on the thermal coefficient of expansion of polyurethane rubber (about 0.0001 in/in/degree F), a 24″ diameter tire (78″ circumference tire) would expand by more than an inch if heated from 50F (garage temperature) to 212F (boiling water). Given how tight the fit is, on these tires, I’ll definitely take that extra inch if I can get it.
Summary
As long as I only deal with 24″ solid polyurethane tires, on spoked metal rims, it appears that tire sizing is standard. Any 24″ tire, marketed as a wheelchair tire, of the right width, ought to fit any 24″ wheelchair rim.
The only variant that I have come across is low-end plastic rims that are 24″ in diameter, but take an oddly-shaped “pyramid” tire. Those should be pretty easy to spot.
It looks like it’s well worth putting together some sort of hot-box setup so I can heat the solid tires before stretching them on the wheel. That can be a simple as a Styrofoam cooler with a space heater inside, and a thermometer to monitor the interior temperature.
I’ve already bought the “heavy-duty” version of what appears to be the standard tool for stretching solid tires onto the rims, and I have a vice I can clamp that tool into.
I’ve already tried doing this without the specialized tool, and it’s just too hard. I’ve also tried mounting regular bike tires, with foam inserts instead of inner tubes. Ditto (Post #1913).
Bike shops will mount regular pneumatic tires. So there’s no need for an amateur to do that. But most bike shops won’t mount solid tires. (Having done that with hand tools, I don’t blame them.)
So that defines my niche. Now it’s just a question of clearing some space and having at it.
