I appreciate the article you wrote regarding tire sizing. Like you, I was occasionally frustrated by tires not measuring their stated width.
Your article was accurate in that identical tires mounted on rims of differing widths will yield different section widths. Keith Bontrager also brought up a good point in that the inflation pressure plays a role in the inflated tire’s width. But unfortunately these are not the only two factors influencing a tire’s inflated dimensions, so let’s take a look at them all. They are:
- Internal Rim Width
- Bead Hook / Rim wall Height
- Inflation Pressure
- Tire Construction
- Manufacturing Tolerances
As you can see from the above list, the problem is quite complex. During my 5 years as the Senior Design Engineer for Maxxis I strove to improve the accuracy in our stated tire section widths. If you’re familiar with Maxxis mountain bike tires, you may know that the ‘older’ tires are undersized. Continental's current mtb tires seem to be undersized as well. So the actions I took included designing the tire on a rim width that would likely be used by a majority of our customers, and holding our production engineers to tighter tolerances. Twice I insisted that the sizing of prototypes I received for testing was not acceptable, and as a result those molds had to be scrapped and new molds ordered.
Regarding time and manufacturing tolerance, below is a graph of the section width of two tires measured over a 24 hour period. You can see that even after 24 hours, the tire is still growing. Most carcass ‘swell’ has already occurred, however. But if the tire was only inflated to 30 psi, for example, it might take a full week for the ‘casing swell’ to stabilize. And manufacturing variation is higher than what you see below. I’ve seen mtb tires that vary over 1.5mm tire to tire.
Regarding ‘Tire Construction’, sometimes tires with different constructions are cured in the same mold. For example, many of the Maxxis “Exo-Protection” and non-Exo tires are cured in the same mold. The casings of the non-Exo tires grow more with inflation pressure, and as a result they generally measure a little wider.
So you can see that the problem is quite complex. There is no standard for rim design (apart from the UST specification), and resistance would likely come from the tire manufacturers for a number of reasons, one of which would be added cost in tire development.
Let me know if you’d like to learn more about anything I’ve touched on, and I’ll be happy to oblige.
Here’s another graph describing the relationship between a bicycle tire’s casing height and width and the rim width for your reference.
W. Bryan Holwell
Carlisle Transportation Products
R&D Engineer - Power Sports