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I have 57-559 (57 mm width, 559 mm diameter) / 26"x2.125" tires. I weigh about 95 kg / 210 lbs. The tire's sidewall shows a single pressure value of 300 kPa / 43 psi.

However, this calculator shows that given a fixed tire size, different rider + bike weights would have different optimal tire pressures.

Is there a way to calculate such an optimal pressure?

  • This is a really good question, but unfortunately really hard to answer. The way a tyre responds to a given air pressure and users weight is not uniform, and will depend on the construction of the tyres carcass. The optimal pressure can even change depending on the width of the rim the tyre is installed on. – Andy P Jun 4 at 13:43
  • In addition, the optimal pressure is also dictated by the terrain being ridden. Taking your example of a 26x2.1" tyres, the correct pressure for dry/hard/smooth surfaces may be 35-40psi, for a wet/rooty/rough surface then 25-30psi may be better – Andy P Jun 4 at 13:46
  • "The tire's sidewall recommends inflating to 300 kPa / 43 psi." Are you sure? The quoted value is always, in my experience, the maximum safe pressure for the tyre (some tyres also specify a minimum pressure). – David Richerby Jun 5 at 17:30
  • @DavidRicherby No, I'm not sure. There is only one value, and it doesn't specify a min. or max. value, so it was assumed to be a recommended value. // Based on your other comment below, I'll now consider this to be the maximum safe pressure. – plu Jun 6 at 0:47
  • @plu Yeah, that'll be a max pressure. – David Richerby Jun 6 at 9:02
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You can have an "optimum" tire pressure - unfortunately, when riding on road you want higher pressure, when riding on low grip surfaces you want lower pressure (more tire in contact with the ground), when riding on very uneven terrain you want again higher pressure to prevent flats, when jumping you want higher pressures, and so on and so on.
The desired tire pressure depends on where do you want to ride, and to a reasonable extent on what you "feel" - which might be different than what a different person of your same size/weight feels.
What you can do is find a "recommended" tire pressure and work from there. In time tire pressure drops, at some time you will find the pressure too low - measure it and remember it as your "minimal" tire pressure. Try with higher pressure than recommended and "feel the ride" - MTB tires with an extreme thread pattern at high pressure rumble very much on asphalt, a lower pressure "mellows" the ride (vibration into the hands).

  • If all I care about is the tire's durability (to minimize the chances of flats), and not comfort nor ability to grip on surfaces, would a higher pressure be better overall? – plu Jun 5 at 0:54
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    @plu all tyres wear out over time - by disregarding safety to maximise tyre life then you're doing yourself a disservice. If durability was the only factor, then we'd be riding steel tyres. – Criggie Jun 5 at 1:01
  • @Criggie I should probably rephrase that question. Given that I've already got large tires (57 mm width), and am confident that the tire area for adequately gripping surfaces is already very safe, and I'm riding at cruising / slow speeds mainly on city paved roads, where I'm not likely to slip/fall, then would a higher tire pressure be better overall? – plu Jun 5 at 1:35
  • @plu When I run 54mm on road I inflate to 5psi under the maximum. It's not the most comfortable ride but the easiest as less rolling resistant. If I'm doing a mud / grass track or off road I run maybe half the maximum PSI. It may not work for everyone but it's what works for me – Dan K Jun 5 at 9:10
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I looked into it and found this PDF: 2004, Berto: "All About Tire Inflation", where the optimal tire pressures were found experimentally from a Tire Drop test, where

Tire Drop is the distance that the tire sags under the weight of the rider and the bike. For a given tire size and load, the optimum inflation pressure for comfort and rolling resistance produces a Tire Drop of about 15% of W (the Section Width) or about 20% of H (the height from the ground to the rim).

For my case, that document shows the minimum tire pressure somewhere around 25 psi / 172 kPa for a 26"x2.125" tire. enter image description here

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    Great work - but it should separate front and rear because the weight difference is roughly 60% rear and 40% front. – Criggie Jun 5 at 1:02
  • @Criggie That would mean that tire in the rear should be higher pressure than the tire in the front. My tire sidewall just shows one number for pressure; does that mean that number is just a general guideline that is equal to the average pressure between the front and rear tires? – plu Jun 5 at 1:45
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    @plu The pressure indicated on the tyre's sidewall is the maximum safe pressure (some also mark the minimum). – David Richerby Jun 5 at 17:29
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    What do they consider “optimal”? – Michael Jun 6 at 12:40
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    @Criggie: Good point, which is addressed by this passage in the PDF: Front tire and rear tire loads aren’t the same. ... weight is ... about 45% on the front wheel and 55% on the rear .. for a typical sport tourist ... you should subtract about 10% from the front ... and add about 10% to the rear ... Alternatively ... one size wider ... on the rear... Including this would of course improve the answer. – PJTraill Jun 10 at 20:23
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There is no such thing as optimal tyre pressure.

Tyre pressure is always a compromise between comfort, rolling resistance, grip and likelihood of puncturing. This depends not only on tyre dimensions and rider/bike weight but also on the terrain you're riding on and the weather.

  • Exactly, couldn't have put it much better. – Dan K Jun 5 at 9:11
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Most tires are labeled by the manufacturer with maximum load - inflation numbers. These depend on the design, materials and construction of the tire as much or more so than its dimensions. While a manufacturer may be able to provide optimum inflation info I don't see any way to develop a calculator that would do more than give you a wild guess.

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Even if by "optimal" we mean something specific and measurable like "least rolling resistance", there is no such pressure. Minimum rolling resistance depends on the surface. On a smooth surface, like a varnished wooden velodrome track, high pressure is best: the higher the better, with diminishing returns. On a bumpy surface, there is an optimal pressure with negative returns if it is changed either way. (And note, by the way, that an off-road tire with "knobs" constitutes a bumpy surface, even if ridden on a smooth surface.) The pressure depends on how big the bumps are, and the load on the wheel as well as suspension details. Practically speaking, it can only be experimentally determined; and for that to be valid, we actually need a way to measure the rolling resistance of that specific equipment on that specific surface.

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