I don't believe that there is a maximum width.

I'm sure I will get tossed to the sharks for saying this, but I had a 27mm Zipp Tangente Speed SL tubie on my 900 disc. As long as it clears the frame, you should be fine but ideally you want to keep it within that 105% rule (https://blog.silca.cc/...ure-and-aerodynamics)

I have run 20-25mm Conti's on them for years.

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