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I'm contemplating replacing the tires on my mountain bike with smoother tires since I am rarely ever on anything other than paved surfaces. I read up on sizing and am aware of the xx-xxx ETRTO/ISO sizing that should be on the sidewall but I don't see it anywhere. Could only find inflation pressure range and some manufacturer's marks. They are Kenda. I could see K-816-015 on the front and K-816-027 on the rear. The tires have the basic size 26x2.1. The rim has RIGIDA 1.75 stamped on it. AFAIK everything on the bike is original (I've had it since new from 1994). Inner rim width is 21-22 mm.

Based on Sheldon Brown's site and also having googled Rigida 1.75 it seems to point my rim/tire being based on 559 mm bead seat diameter.

So, the questions am I right about 559 mm, and (2) for a smooth wall tire I was thinking of 1.75", but could I go to 1.5"?

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  • Googling suggests these are your tyres. amazon.com/Kenda-Aggressive-Bicycle-Blackskin-2-10-Inch/dp/… Seriously knobbly! One commenter notes they're effectively slightly under 2.1" with a measurement of 1.95" width.
    – Criggie ♦
    Commented Oct 24, 2018 at 9:18
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    Late in getting back to this, but yes those are the tires alright. And my measurements concur, they are smaller than stated.
    – user39927
    Commented Nov 22, 2018 at 18:06

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Older tires may not have the ETRTO/ISO size markings. On your tires the '26x2.1' marking denotes rim diameter and tire width.

  • '26 inch' mountain bike tires have 559mm rim diameter.
  • 2.1 inch is the nominal width.

With an 21-22mm inner rim width you could run a 1.5 inch / 38mm tire, but probably not much narrower.

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To quote Sheldon at https://www.sheldonbrown.com/26.html

There are no fewer than 5 different, incompatible "26 inch" sizes which you are likely to encounter. And one of them is even sometimes called a 27-inch size! A so-called "26 inch" wheel/tire could have an ISO rim size of:

  • 559 mm 26" x (decimal)(Mountain)
  • 571 mm 26" x 1 (race) or 26" x 1 3/4 Schwinn or French: 650C
  • 584 mm 26" x 1 1/2 or French: 650B or Mountain:"27.5"
  • 590 mm 26" x 1 3/8 E.A.3 French: 650A
  • 597 mm 26" x 1 1/4 E.A.1 or 26" x 1 3/8 Schwinn

Though there's a 99% chance your tyres are ETRTO 559 size.

As for sizes, don't undersell yourself by aiming for narrow ones. A 2" smooth tyre will be like riding on buttered air compared to knobblies of the same size. A 1.5" would be as small as you would want to go.

Do have a look at How do I know what size tyres can I fit on my rims? for more info. Your current tyre is 2.1" so about 55mm across, suggesting the inner width of your rim is about 25mm. You will want to remove the tyres and measure accurately before deciding.

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    The inner width I mentioned above was measured after removing the tire from the rim.
    – user39927
    Commented Oct 25, 2018 at 12:57
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    Also, good point about the wider tire being more comfortable. As a kid I never rode a "10-speed" bike (what we called 2x5 road bikes back then), so have never ridden on narrow tires. I'll stick to something similar in width then. Thanks.
    – user39927
    Commented Oct 25, 2018 at 13:19
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    Late getting back to this, but since I have the tire off, I decided to measure the rim itself. The rim has the tire off but is otherwise all set up, ie spoked to a hub. What I did was put the rim next to the wall, with the axle perpendicular to the wall. Then I laid a small straight piece of wood across the top of the rim until it touched the wall. I measured up from the floor to the underside of the piece of wood, and read 568 mm. Then I measured from the outer edge of the rim inward to the bead, about 7 mm. So, bead seat diameter is ~554 mm measured, so that confirms 559 tires will fit.
    – user39927
    Commented Nov 22, 2018 at 18:13

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