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I have a road bike tire with the following markings:

50-559

26x1.95


K935-001

I weigh about 200lbs and will probably carry a pannier holding 40lbs. Some research said that my tire should inflate to about 110 PSI. This tire reads that it doesn't support a PSI over 65 (bought the bike used). So I figure I need a new tire.

However, from here it’s been hard to research what I should get. When I search Amazon I don’t find a lot of tubes that describe the width as 28mm. Instead I see a lot of measurements like 26x1.95, which is the dimension my current bike has. I figured maybe the 1.95 represent the width in inches, but when I convert it to millimeters it’s about 44 millimeters, way bigger than what I found recommended.

So I’m just not clear one how to reconcile all these incomplete and contradictory-seeming sources of info. All I really want to know is: When I go on Amazon to get the tubes for my bike, which measurement (or ranges) do I need to search for to be sure it fits and won’t explode under me?

Criggie
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Addem
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  • Have you tried putting 65 PSI in both tyres, then going for a test ride down the street and back? You may not need to do anything special other than add air. – Criggie Dec 18 '20 at 05:38
  • BTW K935-001 is probably a manufacturers code and means nothing. The other two numbers are the useful parts. – Criggie Dec 18 '20 at 05:40
  • @Criggie Yeah, I inflated the tire to its max specification, and when I get on it flattens out to an extent that I would guess is not good. I don't exactly know what's a normal compression so I'm not sure what it should look like. But when I watch YouTube videos of people on bikes, they don't seem to compress that much. – Addem Dec 18 '20 at 14:36

2 Answers2

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50-559 means 50mm wide tire, for a rim that measures "ERTO 559mm" in diameter. This is a very common size.

The "inch" equivalent is a tire for a 26" wheel, with a width of about 2 inches. Any "26 inch" tire that's about 2 inches (or 50mm) wide will fit your bike, such a 26x1.95. The difference between 1.85, 1.95 and 2 widths is not large.

You could potentially get a wider tire, like 60mm / 26x2.4 inch, and it will fit your rim, but you would have to check if the wider tire will rub on your frame or fenders. Wider tires will ride better, but be heavier.

You could also get a narrower tire like 35mm / 26x1.5, and if should fit too, but the current trend is towards wider tires. Narrow tires will ride worse but be lighter.

Tire pressure that you should use is relative and will depend on your weight and the tire size. The number printed on the tire is the maximum tire pressure not to exceed. Usually, you will run less pressure than the max. On my cargo bike I run 26x2.5 tires with a max of 60psi. I actually run 35-40psi.

Tubes will work for a range of tires and you don't need to be exact. Practically any 26 inch tube between 1.5 and 2.5 inches will work for that whole range of tire widths. So you can use a 26x1.95 tube for a 26x2.25 tire, for example. The important thing is the correct wheel size (26") and a width that is reasonably close to rhe tire width.

BetterSense
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    So then what I read about 28mm is bunk and I should forget it? The tire looks very narrow, to my eye. Anyway, so is it correct that a net weight of about 240 lbs needs 110 PSI? And any tube between 1.5 and 2.5 can support that? It’s confusing because my current tire says it can’t. – Addem Dec 18 '20 at 03:18
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    I don't know what you read about 28mm tires. 28mm is a narrower tire used for road bikes. Nothing is wrong with it. But if your bike has 50mm tires now then you should be looking for tires a similar size. As I mentioned above, it is not possible to tell a tire pressure based on your weight. It also depends on your tire size and application. I weigh 225 pounds and I run only 8psi in my fatbike tires. But I used to run 120psi in my 25mm tires. – BetterSense Dec 18 '20 at 03:26
  • Tyre pressure calculation takes into account tyre width AND rider weight. The wider the tyre and the lower the weight, the lower the minimum pressure. And the other way round. – Carel Dec 18 '20 at 14:18
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Some clarifications

  • A tube does not have an air pressure maximum - it is restrained by the tyre and rim (*1)
  • A tyre does have an air pressure maximum
  • A wheel/rim has a weight maximum but not an air pressure maximum (*2)

So your combined system weight is going to be about ~260 pounds (120 kg) by the time you include the bike, the panniers, the rider, and contents of your pockets/bottle.

Most conventional bikes have about a 40:60 split, so 40% of your weight is on the front and the rest is on the rear. This changes when braking or accelerating, or if you've bounced through a pothole. Upshot, your front wheel is holding about 104 pounds (48kg) and the rear is about 156 pounds (71 kg).

At 65 PSI in each tube, your front wheel will have a contact patch of 1 3/5 square inches, and the rear will be 2 2/5 square inches. This is the area of flattened rubber that touches the road to equalise the total mass.

Your 50mm (2 inch) tyre will compress by some amount to make that surface area. So the energy to deform that tyre initially comes from the weight, but to roll forward the energy comes from your pedalling and that's why big soft tyres feel sluggish.

To put that another way, a higher tyre pressure deforms less so saps less energy to roll. (*3)

The max and min width of tyre your rim can take depends on the inner rim width. There's a full Q&A on that subject here: What is the maximum or minimum tire width I can fit on my bicycle

Bike tyre measurements are not simple - there are a crazy number of systems where a bigger number can be smaller than a smaller number.

For your existing 50mm 26" tyres, you need a 50-559 (to use the ETRTO system) which is the same as a 50mm wide for a modern 26" rim. Just make sure you have a third tube as a spare.

Finally - I don't know where the 28mm figure comes from. Its possible you've seen something about modern road bike tyres whose widths have increased from 19mm to 23 to 25 to 28 on pro bikes, but these are for a different rim size and style of bike.

Remember, Amazon is not a bike shop.

They're just a box-mover who wants to sell stuff. If you need practical real useful advise, a Local Bike Shop/Store can be much more helpful. Or ask here on Bicycles.SE, or ask in [chat].

  1. yes a tube will eventually pop if inflated outside a tyre, that's not what I meant
  2. yes a rim/wheel will fail if the air pressure is too high AND the rim is worn
  3. to a point - there's a sweet spot in pressure, where too much has no grip. We want the goldilocks pressure for each wheel and conditions.
Criggie
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    I'm ~100 kg and was more, and my MTB runs 52-559 tyres with 60 PSI perfectly well. I drop the pressure a bit when I rode in a dirt-race too, for added grip. – Criggie Dec 18 '20 at 05:39
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    Kudos for the A---on line! – Carel Dec 18 '20 at 14:14
  • Ok, I guess I was primarily confused by the tire having the pressure limitation, because the tire just seems like a rubber layer over the tube ... wouldn't think that would pop before the tube of sealed compressed air. But ok, if I just replace that tire with a slightly wider tire then I can use the same tube and inflate it more. – Addem Dec 18 '20 at 14:46
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    The tube is just there as a mostly-airtight membrane. It has almost no pressure-bearing ability. The tire casing (and rim walls) is what actually bears pressure. There's something called hoop stress: basically, for a given width, higher pressure requires a sturdier tire, and for a given pressure, a wider tire needs to be sturdier. – Adam Rice Dec 18 '20 at 15:32
  • @AdamRice I see, that makes a lot more sense. – Addem Dec 18 '20 at 16:59
  • @Addem surprisingly its the other way around. As you go to wider tyres, the required air pressure is lower (though the rolling resistance goes up) Yes you can use the same tube in a wider tyre and add more air, but it will need less overall pressure. – Criggie Dec 18 '20 at 22:08