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Is it possible to use mountain bike rims which use the original UST (Universal System Tubeless) standard for using tubeless tyres with tyres that are tubeless ready, but do not conform to the said UST standard?

Namely, I am thinking of getting Mavic wheels which are, naturally, UST (Mavic created the standard along with Michelin and Hutchinson), but since the choice of UST-certified tyres is quite limited, I intend to use other tubeless ready tyres (specifically, by Maxxis). I am already reasonably sure that this is possible, so, assuming I'm correct about this, I am especially interested in how well it performs. Anybody used or seen such combination being used? Is sealant neccessary?

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Yes, works fine, but you do need sealant. UST tires have an additional rubber layer in the tire to achieve airtightness; tubeless-ready tires do not and have porosity that necessitates sealant. The extra layer adds meaningful weight and reduces suppleness, which are several big reasons UST isn't popular.

The UST bead profile is a precisely defined form factor for both tire and rim. This is useful or necessary to achieve an airtight lock without sealant. Tubeless Ready tires and rims use physically and conceptually similar bead profiles that don't conform to any defined standardized form factor, or at least not one defined by a third party. (Each tire and rim company does tend to have their own name and profile design for their own tubeless system, if you want to call those standards). In practice, we've now reached a point where sealant-based Tubeless Ready tires and rims can be mixed and matched more or less freely with good results.

A side note is that some companies, WTB as the major example, make tires that adhere to the UST bead profile only, but are still only TR in the sense that they need sealant. There's arguably some potential advantage there, since in the abstract it would seem like defined standards is a better way of doing things than everyone just loosely copying each other like we mostly have now, but it's largely seen as irrelevant because the not-quite-standardized bead profiles of current tires and rims have gotten to the point of very high reliability and burp resistance anyway.

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