Technical FAQ: Why are labeled tire sizes inconsistent?

Lennard Zinn talks to a host of industry experts about why labeled tire sizes can be so inconsistent.

Dear Lennard,
I’d like to get your take (and hopefully that of an industry spokesman) on tire sizing. While it’s widely recognized that the relationship between labeled tire width and measured width has wide variation for a given wheel (much less over differing rim widths), I’ve not seen anything that discussed difference in tire height (rim edge to top of tire). With the trend in increased tire size coupled with frame constraints, identifying tires that will fit my frame is an increasing challenge. Case in point: I have been riding Continental Gatorskin 700×28’s on my Mavic 622x15c wheels, with adequate clearance at the chain stay and the seat stay cross brace. I recently upgraded to a Mavic 622x17c rim and moved to a 700×28 Conti Grand Prix 4000 S II. When mounting wheel on the frame, the chain stay clearance looked ok, but when I rolled the bike away from the stand, the rear wheel was locked-up. Lo and behold, the tire was hard up against the cross brace. Fortunately, I had a new pair of Gatorskins in 700×28, so I swapped tires. The Gatorskins cleared, with an estimated difference to be 4-5mm between the two “heights.”

Long story short, what is the best way to determine tire size suitability when evaluating tires given challenging clearance? I’ve looked at measuring the tire width when laid flat and found tires with smaller advertised widths having a larger overall bead-to bead width when compared to a larger diameter tire (from the same vendor).
— Robert

Dear Robert,
I passed your question on to a number of tire manufacturers, many of whom replied, and their answers are below. While outer diameter is never specified, even the tire width imprinted on the side of the tire is not required to be accurate, since there is little standardization of bicycle-tire dimensions. Specs are largely up to the tire manufacturer, and I know that when I started going to bicycle-tire factories over 20 years ago, I was shocked to learn that a spec of +/- 4mm from the imprinted width for the measured width was not out of the ordinary.
— Lennard

From Continental:
There are a number of variables, which affect not only height and width, but even shape: rim width, tire pressure, even the thickness of the rim tape can play a role. Although the ETRTO (European Tire Rim Technical Organization) endeavors to provide standards around tire and rim specifications, “height” is an element that is not specifically defined. The sizing printed on the tire refers to diameter and width expressed in millimeters i.e. 23-622 represents what we all commonly know as 700x23C. Continental does however call out circumference measurements on our packaging to make calibrating non-GPS enabled cyclometers easier, but does not make reference to actual height.

As to production width tolerance for road tires, we are generally -0/+2mm. This tolerance can vary slightly more when considering larger volume models such as MTB tires. Unbeknownst to most, tires do also ‘grow’ with inflation and use.

— Brett Hahn
North American brand manager, Continental Bicycle Tires

From Specialized:
This is an interesting question indeed and one that is discussed between frame designers and tire team at Specialized all the time. I guess at Specialized we have the advantage that we can have this discussion and design frames, wheels, and tires in conjunction. Cooperative design becomes increasingly important as tire and rim sizes scale much more than in the past. We go as far as designing frames around tires (Tarmac, Roubaix, all 6 Fattie MTN) or vice-versa (S-Works Turbo 22mm front for the Venge, Roubaix Pro 30/32 for the Diverge). Depending on what we want to achieve with the overall bike.

Design guidelines are fixed in the ETRTO and ISO standards. These standards still apply when designing tires. Actual use can vary from the standard — especially rims used. But for consistency reasons, we stick to the standard rim for each size when defining the tire square sections and outer diameter.

The figures that have to be controlled to fit tires are the outer diameter in service and the tire width.

For the given standard rim, the tire height from the bead seat is very close to the tire width. The height is measured from the center of the bead which would be 1.5-2mm below the top of the rim shoulder.

Apart from the mold layout, construction has an impact on tire size, too. Casing cord (60tpi stretches less than 110tpi), casing cut angle (shallow cut allows more stretch), and casing reinforcement layers (cross-woven is tighter and restricts growth under pressure more than unidirectional cord) all have an impact.

The standards are design recommendations. They are not mandatory. This may explain why some brands take liberties on tire sizing.

— Wolf Vorm Walde
Tire product manager, Specialized

From Vittoria:
ETRTO and ISO describe in detail the dimensions for bicycle rims, but neglect bicycle tires. In any case, the size of a race tire must be measured on a 15C rim, inflated at its max. air pressure.

For all reasons listed below and in order to avoid mentioned difficulties, Vittoria allows a tolerance of max. +/- 1.0mm tire width and +/- 5.0mm tire diameter.

The described case relates to clincher tires. Its size (width and diameter) is influenced by several parameters:
1. The distance (inside measurement) between the rim horns determines the effective tire size. There is a trend toward wider rims (and tires). This goes for road and MTB alike. The wider the rim, the wider the tire, but also tire diameter is affected. For more detail, please see the graphic above.
2. The rim diameter slightly influences the tire diameter. The larger the rim diameter, the larger the tire diameter.
3. Clincher tires casings are by design constructed slightly oval. But the relation of tire height to width is strongly related to the tire tread thickness and tire bead position (position in rim horn). The deeper the tire bead position, the smaller the tire diameter. The thicker the tread, the larger the tire diameter.
4. Full-carbon race rims usually have a stronger rim horn to withstand the high inflation pressures. This has a strong influence on the position of the tire bead. See above.
5. The clincher tire production itself has certain manufacturing tolerances of the section height and width.

In the case of tubular tires, the situation is slightly simpler:
1. The tire shape is almost round. Its diameter (tire height) is influenced by the rim diameter. The larger the rim diameter is, the larger the tire diameter. The tire width remains virtually unaffected.
2. Also, the tubular production itself has certain manufacturing tolerance.

— Rene Timmermans
CEO EMEA, Vittoria SpA

From Challenge:
ETRTO only gives rim specifications.

This is the opposite of for cars or motorcycles. When you mount a car tire with more air volume, you usually have to change the rim and decrease the size.

With bikes, it is the opposite. The rim is always the same, and the tire increases in diameter as you increase section.

This is one aspect. The second is how the tire is made. Nylon tires are vulcanized in a mold and usually pre-formed so there are different shapes from mold to mold and manufacturer to manufacturer.

Regarding external size there is a complete study in a book written by Michelin all about this; it says, for instance, that a 27 or a 30 should be mounted on a smaller rim to have the external size always the same. If I am not mistaken, a 650B rim with big fat tires like 47 ends up the same tire diameter as a 700C X 25 or so and explains a new trend coming out now.

Years ago, the spec on tire width relative to printed tire width was about +/- 5mm. The problem is that today we have changed the rim’s internal width; therefore, the indication of the section depends on which rim size the manufacturer used to determine tire width. Again, automotive standards will tell you, but on a bike, you can mount a 25 on from a 17mm rim width to a 25mm.

Lennard, it is a jungle out there!

I did this graphic quickly, so don’t take it as gospel, but it is quite easy to see how the external diameter changes according to section.

On my pickup, I changed tires and mounted a new 22-inch rim; previously I had a 16-inch rim. The sidewall of the tire is reduced by half and has much less volume, resulting in less comfort, more rigidity, and more control due to less bounce of the tire. I did this since I drive 90% of the time on paved road but my friend that likes off-road changed tires and rims and went to 14-inch wheels with four times the tire volume I have. The tire is more compliant to off road terrain. It distorts more and is more absorbent for his use, but both of our tires have exactly the same outer diameter. In both cases, our gear ratio did not change.

With road bicycles, the tire size used to be small, from a 23mm to a 27mm, and that range was taken into consideration by bike builders until aerodynamics came into play and frame and fork clearances became tighter. Still some teams today for Paris-Roubaix and other classics need special bikes. But in the old days, you would mount from 21mm to a 27mm for the classics all on the same bike.

Lots of evolution and new trends like cross, gravel, adventure, so there will be new things coming out. We can already see a big fat tire mounted on a 650B wheel allows you to still use your road bike. The outer diameter is the same as 700C; this technology of varying the rim diameter to keep the tire diameter constant is closer to the automotive system.

In our industry, tech “standards” are written and have to be changed again as soon as they are finished writing them.

— Alex Brauns
CEO, Challenge Tires

From Ritchey:
This issue is complicated, as I’m sure you know. We go back and forth on this internally quite a bit. While I can tell you there is no defined industry spec, I can tell you what our standards are, and how we measure everything.

All of our tires are currently developed based on a 17mm internal width. The +/- width on tires concerns the casing, not necessarily the tread. I would imagine, depending on tread size, this could vary even more. Take, for example, Vittoria’s 25c Randoneur vs Rubino. Similar casing, much different profile because of tread thickness.

— Fergus Liam
U.S. marketing manager, Ritchey