🔗 The Science behind Ultra-Wide Rims

When we started riding wide, supple tires more than a decade ago, we wondered about rim width. Do extra-wide tires need extra-wide rims? Figuring this out was part of our research into tire and bicycle performance.

Last week, Zipp introduced their new 303 XPLR gravel wheels. Their main selling point are ultra-wide rims. Their outer width of 40 mm should provide a better match with 40- to 45 mm-wide tires that are popular for gravel racing. According to Zipp, one advantages is better airflow between tire and rim. The tire/rim combo is not shaped like a light bulb, but the tire smoothly transitions to the rim. Lower tire pressure is a second benefit, which, according to Zipp, optimizes speed and traction on rough terrain. A concern with the new rims is their inner width of 32 mm. That's significantly wider than the 28 mm maximum allowed by ETRTO / ISO standards for 40-45 mm tires. By-passing the standards isn't a bad thing in itself, but it means that Zipp needs to figure out which tires can be used safely on their new wheels.

This aspect of the new rims reminds us of the controversial crash of Thomas De Gendt in this year's UAE Tour. De Gendt lost control when his 28 mm tire came off his 25 mm-wide (internal) Zipp rim—a combination that's also outside the ETRTO / ISO standards. At the time, there was much speculation whether the rim was too wide to support the tire, and whether that caused the tire to blow off. In the end, it appears that the crash was caused by a broken rim. However, running a narrow tire on a wide rim may have been a contributing factor, because it makes the tire more likely to bottom out (see below).

There is a lot to unpack with the new ultra-wide rims, so this may be a good time to revisit our research into how rims and tires interact.

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