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  1. #1
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    Whats the smallest tire i can put on a 38mm internal rim?

    Hi, im looking for new tires. im currently riding a 3inch on a 38mm internal and want to go smaller. I was super curious on how small I can go.

  2. #2
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    You can go all the way down to about 1.4x, but I've noticed that people that go down below 2.0x start to have sidewall cut problems and greatly increased treadwear (and no side knobs anymore). Right now you are at exactly 2.0x so maybe you just need a new smaller tire and (smaller rimmed) wheel.
    The masses are...well...you know.

  3. #3
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    Maxxis Minion 2.5 WT
    2016 Santa Cruz Hightower 29er
    2016 Chromag Surface 27.5+
    2013 Transition TransAM 29er
    2017 Motobecane Lurch FS 26 fat

  4. #4
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    Quote Originally Posted by VTSession View Post
    Maxxis Minion 2.5 WT
    which is much closer to a 2.3 in reality...


  5. #5
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    About a 2.87.

  6. #6
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    Quote Originally Posted by richj8990 View Post
    You can go all the way down to about 1.4x, but I've noticed that people that go down below 2.0x start to have sidewall cut problems and greatly increased treadwear (and no side knobs anymore). Right now you are at exactly 2.0x so maybe you just need a new smaller tire and (smaller rimmed) wheel.
    I agree with this. I bought a 30.5i set after talking with trek and asking if their 2.3 se5 would work well with that rim width. They told me their 2.3 tire fits a 22-41i width rim. My sidewall hangs out about 1mm past my side nobs and it bugs me bad. But so far so good this season and I’m running a great psi combo. For me I see that the 2x is the number to go with but everyone’s tire choice and riding styles vary.


    Sent from my iPhone using Tapatalk

  7. #7
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    Quote Originally Posted by .WestCoastHucker. View Post
    which is much closer to a 2.3 in reality...

    I bought a digital caliper this week. Holy crap were my tape measurements off. Many tires are 0.1-0.3 inches more narrow and 0.5-1.0 inches smaller in diameter than listed/predicted. BTW for measuring tire height, I simply took 22 inches for the 26" rim or 23 inches for the 27.5" rim, and added twice the diameter of the measurement I took from the casing thickness from the rim to the top knob. Not all tires measured yet (one bike is still at the LBS) but below is a summary of what was measured vs. what was labelled as width on the tire:

    26 x 1.95 Kenda Aggressive, 18mm rim = 2.0 inches (a correct measurement, yay), 25.0 inches high. Yes, a 26" standard tire is 25" high. No one told me this before lol. No wonder I hate it on dirt. I do like it on pavement.

    26 x 2.40 CST Rock Hawk, 21mm rim = 2.35 inches (pretty much right on); 26.2 inches high. Notice how the skinnier tires measure more accurately on skinnier rims. Read on...

    26 x 2.70 Maxxis DHF-something, on 26mm rim = 2.45 inches wide, 26.4 inches high. On a 35mm rim it should be 2.55 inches wide; 40mm rim should be 2.625 inches wide (within 0.1 of advertised).

    26 x 2.80 WTB High Roller, 25mm rim = 2.50 inches, 26.4 inches high. Major disappointment. I still like the tire but on this rim it is obviously not a plus tire. If this was mounted on an i40 rim, then 40-25 = 15mm / 3.14 circumference = 5mm or 0.2 inches wider if on a 15mm wider rim. So it would go from 2.5 to 2.7 wide. Height on paper on a 40mm rim would go from 26.4 to 26.7. inches high, still lower than a standard 27.5 tire's height. This makes sense because in theory a 26 x 3.0 on a 45mm rim should be 27.0 inches high. The reason I had to use 25mm for the rim was that I needed to stick with 9x100mm up front for the axle. So basically a half-assed 1/2 plus bike that's really not plus. Again, disappointing. Oh well. I think my next bike will be a real + bike with real + rims.

    27.5 x 2.35 Kenda Nevegal, 23mm rim = 2.25 inches, 27.2 inches high. If in theory you are supposed to put a 2.34 or 2.4 on a 30mm rim, then 7mm extra / 3.14 = 0.1 inches more and you get back to 2.35 and around 27.4 inches high. Tread is worn so maybe 0.1 inches higher new.

    27.5 2.50 Maxxis DHF, 23mm rim = 2.375 inches, 27.2 inches high as with the Kenda above. Wider tire, stunted height (tire has 1200 miles on it and knobs 1/2 worn). If mounted as in the above example on a 30mm rim, 0.1 inches wider to 2.475; if on a 35mm rim it would be 0.15 inches wider to 2.525; 2.50 is exactly between the two. So Maxxis, at least in this case, is not 'lying' about its tire widths, people like me are just putting them on skinnier rims than what's recommended.


    What I have learned on a practical level: 27 inches and over is fine for rollover height (for me and my trails). Even 26.4 with the WTB tire is usually fine. 25 inches high on dirt trails is NOT fine. A lot of pedal strikes, hangups, and out of control downhill. Tire size (height) DOES MATTER. But after 27 inches it doesn't seem to matter as much, at least for XC stuff. There are no (or hardly any) 26+ bikes so it's going to be a 27.5+ bike for sure in the future, with real + rims that keep a + tire the correct width.


    Summary (difference in actual & theoretical tire width / 3.14 / 25.4):
    Skinny 17-25mm rims for 1.95 to 2.8 tires, smallest to largest vs. STANDARD tire width to rim recommendations of: 2.0 20mm; 2.2 25mm; 2.4 30mm; 2.6 35mm; 2.8 40mm; 3.0 45mm

    26 x 1.95 18mm rim actual 2.0 inches wide, 25.0 inches high.
    Standard rim 20mm for theoretical 2.0 wide = off actual by only 0.02 inches = correct

    26 x 2.40 21mm rim actual 2.35 inches wide, 26.2 inches high.
    Standard rim 30mm for theoretical 2.40 wide = off actual by only 0.06 inches = pretty correct

    27.5 x 2.35 23mm rim actual 2.25 inches wide, 27.2 inches high
    Standard rim 30mm for theoretical 2.35 wide = 0.09 inches, or 0.1 inch, exactly the difference between 2.25 and 2.35 = correct

    27.5 x 2.50 23mm rim actual 2.375 inches wide, 27.2 inches high
    Standard rim either 30mm or 35mm for theoretical 2.50 wide is either 0.09 inches or 0.15 inches off, so tire on standard rim should be 2.465 or 2.525 = correct

    26 x 2.70 26mm rim actual 2.45 inches wide, 26.4 inches high
    Standard rim either 35mm or 40mm for theoretical 2.70 wide is either 0.11 inches or 0.175 inches off, so tire on standard rim should be 2.56 or 2.625 = THIS ONE IS OFF, Maxxis 'lied' about this particular tire, it's at least 0.75 inches more narrow than advertised even with the correct rim width.

    26 x 2.80 25mm rim actual 2.50 inches wide, 26.4 inches high
    Standard rim 40mm for theoretical 2.80 wide is 0.19 inches off, so tire on standard rim should be 2.69 inches wide, 0.11 inches OFF. WTB 2.8 is actually 2.7.


    All tires under 2.7 if adjusted for correct rim width have accurate numbers; only the 2.7 and 2.8 tires do not. Maybe this starts to be semilog instead of linear after 2.5.
    Last edited by richj8990; 1 Week Ago at 10:35 AM.
    The masses are...well...you know.

  8. #8
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    Quote Originally Posted by evan.c View Post
    Hi, im looking for new tires. im currently riding a 3inch on a 38mm internal and want to go smaller. I was super curious on how small I can go.
    You can try a 2.3 tire with the right design. High volume with a very rounded tread profile. So they won't square off on this rim. Bonty XR2 Team 2.35 can be run on that rim at 14psi or less on the front and 4-5 more on the rear to start. If you get rim hits add a little. If you don't get any bring the pressure down 1 psi at a time.

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