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Say you’ve got a bike that will officially handle a tire up to 700×28 mm in size. Now, in terms of figuring out what sort of tire might fit, you might rightfully think that it’d be a very straightforward process. In other words, you just buy a tire that’s labeled as 28 mm wide or less, mount it up, and you should be good to go, correct? After all, that’s how it works in the automotive world. There, if you’re trying to replace a 205/45-16 tire, you simply limit your search options to tires of that precise size, you choose the one with the characteristics you desire, and you’re good to go.
Unfortunately, that’s not how it works in the bicycle world right now.
That “700×28 mm” tire might actually measure 31 mm across on an Enve 4.5 AR Disc wheel with its 25 mm inner rim width, and therefore that tire won’t fit in your frame. Or on a more old-school rim with a 15 mm inner width, that same tire might instead end up at 26 mm across, leaving precious width and volume on the table. Or then again, maybe that tire doesn’t end up measuring 28 mm-wide on any rim you try because the manufacturer overstated the size on the hot patch.
Correspondingly, reporting tire clearance in such ambiguous terms doesn’t really make sense for frames and forks, either. Yes, your bike will supposedly accept a 700×28 mm tire, but which one? And on what rim? To reduce the potential for problematic — and potentially litigious — combinations, brands are often forced to play it safe, quoting the largest-possible iteration of the stated tire size that the end user is likely to come across. In other words, a 700×28 mm tire will definitely fit, but a 700×30 mm (or bigger) might safely fit as well.
All of these scenarios are real, and all of them reflect problems with how tire sizes are currently reported. 3T has proposed a system to fix all of that, however it’s anything but certain that the bicycle industry will adopt it across the board.
WAM, RAM, and the convoluted world of tire measurement
3T’s system is based on two metrics. WAM, or Width as Measured, is — as the name suggests — the actual measured width of a tire on a specified inner rim width. Meanwhile, RAM — Radius as Measured — refers to the tire’s actual total radius (measured from the center of the hub axle) under the same conditions.
Instead of bearing the somewhat arbitrary “700×28 mm” designation, the tire I described above might instead be listed as “700c, WAM19 = 28 mm / WAM25 = 31 mm”. Packed into that label are several key pieces of information: the tire is designed to fit on a 700c rim, it’ll measure 28 mm across when mounted on a rim with a 19 mm inner width, or 31 mm when used on a rim with a 25 mm inner width.
That sure does seem a lot clearer than just “700×28 mm”, no?
As it turns out, a handful of bicycle and component companies have already started adopting a similar concept for stating maximum allowable tire sizes.
Cane Creek, for example, specifies a maximum tire size for its latest Helm MKII Air 29 mountain bike suspension fork as, “64 mm-wide by 749 mm-tall (29) / 75 mm-wide by 739 mm-tall (27.5+)” — in other words, the exact WAM and RAM metrics that 3T has proposed, albeit without the catchy acronyms.
Somewhat similarly, Fox designates in conventional terms (29×2.5″ etc.) the maximum tire sizes that are to be used with its mountain bike suspension forks. However, in a tacit admission that those conventional tire sizing methods are anything but consistent, the company also specifies in more detail the minimum amount of space between the crown of the tire and the underside of the arch — 8.5 mm — which, in this case, is the much more critical dimension.
The issue of standards
The bicycle industry seems to get progressively worse at maintaining any semblance of universal “standards” — or, rather, it has an annoying habit of haphazardly creating new ones whenever a brand deems it convenient.
But when it comes to tire measurement, one universal standard actually does exist.
ETRTO — the European Tire and Rim Technical Organization — and the corresponding International Organization for Standardization (better known as ISO), have long been the gatekeepers on this topic, establishing very clearly defined guidelines on how bicycle tires are to be measured. Included in the range of key details are inflation pressure and duration, the prescribed inner rim width for a given tire width, and even how to account for mountain bike tread in terms of how it relates to the casing width.
There are also precise guidelines for other pertinent areas, such as dimensions, shapes, and tolerances for various rim and tire types, and even what combinations of rim and tire widths are deemed safe to use (although that’s a topic I’ll save for another day).
It’s because of ETRTO that every clincher bicycle tire not only has the usual hot patch that consumers see on the sidewall, but the more standardized “XX-YYY” numerical designation that is usually molded directly into the tire. For example, “28-622” would indicate a clincher tire that measures 28 mm in width when mounted on an ETRTO-compliant “crochet” rim with a 622 mm bead seat diameter (otherwise known as 700c in conventional terms).
ETRTO’s guidelines are also very clearly defined in terms of tire measurement, so brands can be very clear on how their tires should be measured (and marked).
That’s all well and good, but one major problem with ETRTO’s guidelines for tire measurement is that they’re woefully out of touch with modern times. Specifically, there’s a key chart within the ETRTO document that not only denotes the “design rim width” that should be used with a particular tire size when determining the actual inflated width, but also approved pairings of tire and rim widths — both of which highlight rim widths that most riders would now consider to be unacceptably narrow.
Remember that “700×28 mm” tire I mentioned early on? According to ETRTO, that 28 mm is what you get when the tire is mounted to a 17 mm-wide rim, inflated to maximum pressure (with a tube inside), and left to sit for 24 hours. In reality, though, that tire might be mounted to a rim that’s 4 mm wider, tubeless, and at a pressure half of what’s indicated as the maximum figure. Would that tire still be 28 mm-wide, or would it be wider? Or maybe it’s narrower? Your guess is as good as mine.
One big issue for ETRTO is that it takes a long time (and a lot of cooperation between a lot of different parties) to establish industry-wide standards. As a result, there can be a significant lag between what comes out of those discussions and what’s actually happening in the real world, and it’s precisely that lag that has yielded so much confusion in terms of tire width measurements today. Adding further to the mystery is the fact that ETRTO’s measurement guidelines aren’t easily accessed by the public, nor are they universally published on tire brand websites.
And what about safety? This is a topic I’ll leave for another day, but suffice to say it’s the wild west out there in terms of what widths of tires are considered OK to use on different rim widths. Guess what: ETRTO has guidelines for that, too. However, much like the tire width measurement scenario, those guidelines don’t reflect the current reality, so many brands just choose to ignore them completely in the interest of improving performance (and selling more stuff).
How wide will a tire be in reality? What if I want to run these tires on a hookless rim (and to be clear, “hookless” as per commonly used marketing vernacular is not the same as what ETRTO considers to be “hookless”). What about variations in tire manufacturing?
Feeling confused? Join the club.
Don’t hold your breath
Ultimately, all 3T is trying to do with WAM and RAM is provide consumers with more detail so that they can make an informed buying decision, and with everyone trying to max out on air volume and tire width, every millimetre counts.
But as enticing as WAM and RAM sound in concept, there are a few technical blanks that need to be filled in before it could be considered a bulletproof “standard” that can be followed without a shred of ambiguity like ETRTO. But more importantly, it would also require industry-wide acceptance in order for it to take hold, and the tea leaves aren’t especially promising.
Back in the late 1990s, Bontrager tried to move things along with a dual-numbered system for its mountain bike tires: the first being the tire’s casing width in millimeters, the second the tread width. For example, the company’s 26-inch (this was the 90s, remember) SuperX tire was labeled as having a “54/52mm” width, indicating that the casing was slightly wider than the tread cap. It wasn’t a perfect system, but it at least provided people with an idea of what the tire would look like in use.
WTB’s Global Measurement System is more recent, but provides the same information as Bontrager’s system with the addition of a designated rim width (in fairness to Bontrager, rim widths were far less variable back then). Neither system has expanded substantially outside of the brand that introduced the idea.
“We still use GMS on our website product pages and catalogs, but unfortunately it wasn’t embraced by the industry as a whole,” said WTB’s PR and product marketing manager, Clayton Wangbichler. “Nobody wanted to bite. Nobody wanted to put in the effort. The unfortunate truth is other companies likely didn’t want to use a system that a competitor pitched, even if it was a universal system that didn’t require the company’s name be tied to it. Unfortunate, but now the industry is in a whole different predicament when factoring all possible inner rim widths into the equation.”
Likewise, Luke Musselman of Goodyear Bicycle Tires doesn’t see an imminent move away from ETRTO, either.
“As a tire manufacturer, we will continue to follow the guidance set forth by ETRTO, ISO, etc. that establish standardized practices in an effort to help keep riders safe,” he said. “By doing so, we contribute and support a common language that will improve everyone’s understanding from bicycle company product managers through to the end user. I do believe there is an opportunity to better inform the consumer from the position of bike, wheel (rim), and tire companies of these practices with less of a marketing spin and more factual basis.
“That being said, it is complicated, and a lot of answers will be ‘it depends’ due to the three components involved: frame/fork, rim, and tire. Just as in most things, there is no clear-cut, straightforward or simple answer.”
Many of the industry folks I spoke to at least expressed promise at the WAM and RAM idea, albeit with the caveat that it would need to be further developed before it could be more seriously considered.
“Overall, I believe the proposed 3T system is a step in the right direction,” Wangbichler said. “There is clearly a lot of variance, and therefore confusion, related to how tire volume and width are affected by inner rim width. We need a way to clearly state, ‘This tire will be X millimeters wide when mounted on a rim with X inner rim width.’ The 3T system does so and in a way that is both accurate and clear. [The current ETRTO system is] not as ideal a method of measurement, but the resulting benefit of riders having tire and rim dimensions that get along and are internationally/universally standardized across brands is, in my opinion, far greater than the benefit of a more specific tire sizing method.
“International standardization is the key and should always be the main goal with these systems. With that in mind, it seems that an iteration of 3T’s proposed system (which is inarguably more accurate and helpful) would serve the industry better if it were adopted within the current international standards currently being followed.”
“The system looks good and understandable for the consumer,” added Continental product manager Jan-Niklas Jünger. “If that system can be adopted by all ETRTO members, we might follow. That’s all I can say.”
Ok, so while WAM and RAM have their merits, it seems safe to say that you’re not going to see the system adopted industry-wide in the very near future, although stranger things have certainly happened over the years. Remember that Gerard Vroomen — the current head of 3T — was one of the principals behind Cervelo back when the concept of stack and reach was introduced, and look at where that sits now: it’s the de facto gold standard for providing a baseline for how any bike will fit, though it took a decade or so for it to take hold.
Even if WAM and RAM don’t become more widely used, there is at least a somewhat broad push from a variety of tire brands to provide more useful figures. Specialized’s road tires now include measured casing widths corresponding to several different rim sizes right on the product packaging, and WTB and Goodyear both pepper the product pages on their websites with more detail than we’ve been used to seeing in years past.
Regardless, you can still get around all of this if you have the right information.
While ETRTO doesn’t make its information easy (or cheap) to find, WTB has at least distilled some of the more salient points in a post on its website (which you can find here). At the very least, you can at least see the design rim width for whatever tire size strikes your fancy. From there, you can use a correction factor to get a close approximation of what the tire width will be on the rim you’re interested in using: simply add 0.4 mm of inflated tire width for every additional millimetre of internal rim width (or subtract if you’re using something narrower, which seems highly unlikely).
For example, if you’re wondering whether that 700×28 mm tire will fit in a frame that has 38 mm of space between the chainstays when mounted on a rim with an internal width of 21 mm, you can just add 1.6 mm (0.4 mm x 4) to the baseline figure to get the final result of 29.6 mm. This would leave 4.2 mm of space on either side of the tire, which, depending on your perspective, is just right or leaves room for a bit more.
It’s by no means a perfect solution — and it doesn’t work as well for mountain bike tires as it does for road and gravel ones — but given the current situation, it’s far better than nothing.
Light at the end of the tunnel
I should point out that the bicycle industry doesn’t actually want consumers to be confused about tire width, nor is there some conspiracy to undersize (or oversize) tires. When all is said and done, it seems safe to say that everyone would like more consistency and clear guidance on the topic, and that’s exactly what might be coming in about 12-18 months.
That’s when the latest revision of the ETRTO rim and tire guidelines are scheduled to be finalized and released to the public, and the updated document will be far more representative of current usage habits. Wider design rims will be indicated across the board, for example, as well as far more detail in regards to stuff like tubeless tires and hookless rims.
Barring some other major revolution in tire and rim preferences, this next revision should have more staying power than the one it replaced, too. While the rim and tire world has completely upended itself in recent years, there are at least signs of pending stability now that most of the cycling world seems to have settled down a little bit. Everything seemed to get a lot wider very quickly, but there now doesn’t seem to be a lot of interest in continuing at that same rate of change. Everyone seems reasonably content — at least for now.
“The next revision of ETRTO/ISO will finally address the fact that the reference rims for tire measurements were obsolete,” explained Specialized tire and tube director Wolf Vorm Walde. “When wide tires have to be related to impractical narrow rims, things get weird and we are left with inconceivable size designations. With the adjustment of ETRTO that allows to relate wider tires to wider rims, size designations will be more realistic for the wider tire again.”