At the Bicycle Exchange, we end up replacing a lot of tires on our bikes. It's a big portion of the expense in refurbishing the hundreds of bikes we donate each year. We replace tires on kid's bikes, mountain bikes, hybrids, cruisers, vintage 3-speeds and road bikes. At times, we also have to replace wheels as well, to get these bikes rolling again.
Don't Trust the Labels
Our tireless volunteers often find themselves on the hunt for replacement tires or wheels, and at times seek help, saying, "I need a 26" tire" or "I need a 24" wheel."
Unfortunately, thanks to a plethora of options and a confusing labeling system, a tire's printed diameter is often insufficient in ensuring a correct fit. Three groups of people often learn this the hard way: 1) Parents of kids who race BMX, road or cyclocross; 2) Mechanics who work on vintage three-speeds; 3) Mechanics at bike nonprofits. Trust me on this one!
If you fit into one or more of these groups, maybe you're already a tire and wheel sizing expert. But confusing labeling creates a frustrating experience for volunteers, and for participants in our current homework bike program, creates extra trips back to our facility for the correct parts.
For example, there are at least five different 26" tire and wheel sizes that are not compatible with each other, we typically have all of them at the shop, and none of them are interchangeable.
For 24" wheels, we see four different diameters. Again, these are not interchangeable. What gives?
ETRTO to the Rescue
Thankfully, there are five letters and three numbers that can eliminate almost all confusion in tire sizing. The European Tyre and Rim Technical Organisation (ETRTO) has long imprinted its own standards, literally and figuratively, on bike tires. The ETRTO diameter is a three-digit number that is in the 300s for 20" wheels and tires and in the 600s for 700c/27"/29" wheels.
Here is a 700c/29er tire, with its ETRTO printed in the form of the 622:
These numbers are typically either printed or molded into the sidewall or tread of a tire.
Many people think knowing the diameter and the type of bike should be enough. "I need a 26" cruiser tire," isn't enough. The wheel could have a 559 ETRTO tire, perhaps a 26" x 1.75" tire, or could use a 571 ETRTO tire, commonly in 26" x 1 3/4" sizes. But as volunteers at SVBE have found out, they're not compatible at all! The decimal vs. fraction should be a red flag, but...
Knowing Decimal vs. Fractional Isn't Enough
Although as the late Sheldon Brown emphasizes, fractional width tires are not compatible with decimal width tires, but knowing your diameter and whether your tire is fractional or decimal surely isn't enough to ensure a good fit. We've seen vintage three-speed bikes come with either 597 ETRTO 26" x 1 3/8" tires, and 590 ETRTO 26" x 1 3/8" tires. We have a few of both hiding in our stock. There are also 584 ETRTO 26" x 1 1/2" tires, and this diameter is also called 650b and 27.5" But they're all 584! And as mentioned above, we have fractional 571 tires for cruisers, and 571 tires for road bikes in metric widths, for wheels that some call "650c," and they're both the same diameter.
Got a 24" wheel road, BMX or cyclocross bike? BMX bikes with a 24" wheel could require an ETRTO 507 tire, or an ETRTO 520 tire.
A cyclocross or road bike could come with a 507 wheel and tire, larger 520 wheel or 540 ETRTO wheel and tire. Even the manufacturers can't agree, and I've found out the hard way when looking for tires for my kid's bikes that bike shops and online retailers don't always specify the ETRTO of a tire they sell.
A correct ETRTO diameter match between rim and tire is paramount, but many volunteers worry about perfectly matching a replacement tire's width with the old tire's width. While a match is nice, especially if you're only replacing one tire, it's not critical. You can safely put a 2" or 50mm wide tire on a narrow road rim, and can put a narrow 1.25" tire on a wider mountain bike rim.
In general, you want your tire to be at least as wide as the (internal) width of your rim. Road rims are now as wide as 25mm (internal). Times have changed from the 13mm (internal) width rims of the 90s! You can read more about tire widths and tire sizing here.
Of course, you need to check the frame, fork and brakes for proper tire clearance.
3 Digits, 100%
So as you shop for your own bike, or need tires in your SVBE volunteering, save yourself some confusion and frustration by knowing your three-digit ETRTO tire and wheel size! It'll help ensure you get a compatible, safe tire.
Don't be surprised if that's the first question we ask you if you are in search of a tire!
Help spread the knowledge, and happy tire changing.