The rim needs to be Crotched. hookless is not allowed. And the rim cannot be too wide. The Bontrager rim is too wide 21mm inner with for a 28mm tire. According the E.T.R.O. standard the maximum is 17mm inner with.

^^^ ahh no... 21 inner is not too wide for a 28 Roval whom is owner by specialized uses inners of basically 21 (20.9 or whatever) I doubt spesh would make a tire that could not work with their Roval wheelsets.. all the popular CL(x)32/50/64 are 21 inner.

gp5000 Tubeless work on it? Seems like the Bontragger tubeless tire is not so great.

I am only stating the oficcial guidelines. Everybody can try for themselve. :)

https://bikerumor.com/...-width-best-results/

See ertro table within this article.

At least this shows some hookless rims are ok at a 28mm tyre.
But does suggest 19mm max internal width for a 28mm tyre for a crochet hooked rim.

Here is the compatibility chart from Specialized

https://specialized.picturepark.com/Go/BifLoWao/D/63465/1


The XXX6 rims aren't on the list but it does show it's compatible with the ENVE 4.5 AR which is a 25mm ID hookless rim. Based on that, I think you should be safe...but I'm not an engineer.

Unfortunately ETRTO standards are painfully outdated and pretty useless now that 21mm and wider internal width rims are becoming the norm

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Matt