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I'm thinking about putting a bit wider tires on my mountain bike, but I'm having problems distinguishing facts from fiction about minimum required inner width rim for a tire. I have two mountain bikes with pretty narrow rims: 17 and 18 mm inner width. There are 2.1" wide tires (with tubes) on them and I had no problems. When I look at the tire manufacturer websites, e.g. https://www.maxxis.com/catalog/tire-468-140-minion-dhf, it seems that my rims are way too narrow for any of their tires. Their recommended inner rim widths start at 25 or 28 mm for a 2.35" tire I would like to get.

On the other hand, the rim manufacturer goes with much narrower requirements, e.g. https://cambriabike.com/products/mavic-xm-719-26-mtb-disc-rim?variant=32343195746378 claims that 19 mm inner width is good for 1.3" to 2.7" wide tires.

Is it safe to mount 2.35" or even 2.5" wide tires on my 18 mm inner width rims?

EDIT

I found this page with tire/rim compatibility chart, which is more lenient, but warns about sub-optimal performance of a wide tire on a narrow rim.

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    Safe? Yea. Perform well? No.
    – MaplePanda
    May 14 at 6:49
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    Note that Mavic is the only manufacturer that's not trying to sell wide rims.
    – ojs
    May 14 at 6:57
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    Schwalbe says it’s almost okay: schwalbe.com/files/schwalbe/userupload/Images/FAQ/…
    – Michael
    May 14 at 12:31
  • (Note that Schwalbe has an interest in selling a wide variety of tyres.)
    – Michael
    May 14 at 12:38
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    Note that wider MTB rims are a relatively recent trend, so legacy 26" rims and older guidelines will tend not to reflect that. Tires are designed with a specific rim width range in mind, so what works for one tire model might not work for another. And some tire series run for a long while, so the same model rim recommendation might vary year by year.
    – Klaster_1
    May 14 at 15:49
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Visualize the extremes, i.e. a rim that was "obviously" too narrow for a 2.1". Say 10mm internal. As you get narrower, the handling gets floppier, particularly in aggressive riding. The lie of the contemporary charts is they make this effect look binary where in fact it is progressive.

Mountain bikes started out with fairly rational rim to tire width pairings. In the 90s it got weird and one could in fact assemble a terrible handling rim and tire combination from mainstream product entries that were all nominally compatible and made for the same applications. For all their oversimplifying, the charts helped pull us back. Cyclists need simple.

There is probably a math-driven answer that deals with pressure, casing construction, rider load, and the forces imparted by different styles, situations, and terrain types. It would probably still be a gradient more than a binary, but the person who could really answer that might be able to find some areas where it gets closer to being a binary, i.e. if there are certain proportions in some situations where if moved beyond things get rapidly worse. It's probably one of those seemingly mundane topics in cycling that would actually take something like a graduate level physics class to crack into.

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    Agreed. Additionally, that wide tyre will frett on the edge of the rim as it rolls. There will be a worn strip around the edge of the sidewall, about 2mm up from the metal of the rim.
    – Criggie
    May 14 at 6:06
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    @Criggie I went to check my tires for signs of wear near the rim. The front one has practically none, but the rear one has a visible wear band about 2 mm wide adjacent to the rim. Thank you for pointing this out. I so dread having to replace the rims myself for the first time. May 14 at 7:40
  • @PaulJurczak Tyres are wear items like brake pads and cables and chain. You would notice this in your periodic checks and plan to replace before they fail.
    – Criggie
    May 14 at 10:25
  • @Nathan Knutson Is it no longer the case that a tire width of something like 1.5x to 2.25x the inner rim diameter can be considered safe and not degrade performance of the specific tire? I realize nowdays, as u mention, there are many other parameters to consider like casing width to tread width, etc. But in general, would the 1.5-2.5 formula be considered fair advice?
    – Jeff
    May 15 at 17:25
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    @Jeff I personally don't like talking about it in terms of a multiplier like that because people will always find ways to apply it that don't work, mostly involving high pressure and/or rough use. Also in my view, performance is usually degraded long before it's unsafe. May 15 at 22:59

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