How WTB Determines Tire Width
How is Tire Width Accurately Measured?
ETRTO guidelines state that a tire should be inflated to 65psi and allowed to sit for at least 24 hours before being reduced to 35psi for measurement. This process simulates the amount of stretch a tire casing will experience after about a week or so of riding. If you mount a WTB tire and measure it immediately after inflation, the width of the tire will always measure narrower than the stated width of the tire. ISO standards state that width of a tire family should always be determined using a version of the tire that has a single-ply casing and doesn't feature puncture protection. This standard ensures the measured tire will be the largest version of a tire made from that mold (excepting normal human, material, and machine tolerances).
What is Tire Section Width?
Tire section width is used to most accurately determine the size designation of a tire. Also commonly referred to as “casing width,” section width is expressed as the first two digits in the ETRTO size designation “XX – XXX” marked on the sidewall of every WTB tire. For more information on rim and tire compatibility, visit wtb.com.
Naming Tires at Their Widest Point
WTB quotes tire size based on overall width (widest point) whereas ETRTO designation only works with the casing width. This means that although a tire might be called a Venture 40, the ETRTO designation is actually a 37 because the tread overhangs the casing by 1.5mm on each side, bringing the overall width to 40. We measure the maximum width of every tire to ensure you can make a safe decision when trying to determine whether or not a tire will fit in your frame or fork, especially in terms of chain stay and seat stay clearance.
Diving Deeper. Designing Tires Around Specific Rim Widths
In the past, WTB has designed tire casings around inner rim widths that we felt were most logical for any specific tire section width. This worked well due to being a company of riders who are in tune with how components perform best, but we've always strived to follow international standards that would ensure the most amount of consistency for not only our products, but the products of the bicycle industry as a whole. In 2019, WTB began designing tire casings around specific inner rim widths that are to the ISO standard. The measuring width codes are below. ETRTO is a little behind on rim choices. ETRTO is also working to align their standards with ISO, so the below will be reflected in ETRTO when the release the next standard revision.
Different inner rim widths will provide different measurable section widths using the same tire. Using a rim that is narrower than the inner rim width the tire was designed around will result in a tire section width that measures narrower than the stated width of the tire. Conversely, using a wider rim will result in a measurable tire section with that is wider than the stated width. While width of the tread does change relative to the rim width, the greatest variance is usually seen in the casing of the tire, most notably in the sidewall region. As a general rule of thumb, every addition or subtraction of 2.5mm from the designed rim width will also add or subtract 1mm of tire section width.
What is Inner Rim Width?
Inner rim width signifies the narrowest point between the bead hooks on the walls/ legs that hold a tire on the rim. All WTB rims have a bead hook, but some rims from other manufacturers do not. Regardless of whether a rim is hooked or hookless, the inner rim width is measured at the same location. All WTB rims are named according to their inner rim width (i25 means the inner rim width is 25 mm), but that may not be the case with rims made by other companies. All rims, regardless of manufacturer, should be marked with their ETRTO designation of XX-XXX. The first two digits refer to the inner rim width, the last three digits refer to the rim diameter. In order to ensure a completely sealed tubeless system, always use tubeless tape that is 5 mm wider than the inner rim width of your rim.
Acceptable Width Ranges
While there are plenty of heavy machines speeding up the process, every bicycle tire is essentially “handmade” in the sense that every material is placed and meticulously lined up by hand. No part of the manufacturing process is what you would consider automated and therefore we have to allow for a certain level of both human and machine tolerances in place that account for the variations from tire to tire that can, and will, occur even within the same batch of production.