Tyre pressure road bike

[Mr.Gib]

Honestly I can only just tell the difference between tubeless and tubed road tires, therefore those people have over-active imaginations IMO.

Just going for rides with various relatively similar tires and tubes will make noticing a difference unlikely for many. However, in a methodical comparison study some people will notice differences. I have nerve damage in my feet from frostbite, I get severe pain from road buzz as one might experience from rough chipseal. I tested many combinations of 28mm tire, tube/tubeless, and pressure in preparation for some serious endurance riding on bad surfaces. Best for my feet was 60 - 65psi Rene Hearse EL with latex, followed by Vittoria Corsa with latex. All others were noticebly inferior in terms of managing the pain. So yeah, some people can tell. And it wasn't just the pain factor, differences in frequency and magnitude of vibration were noticeable. Difference rubber compounds transmit and dampen different frequencies I assume. Makes it possible to "tune" out what you don't like. Fact remain though, there are some of us who feel nothing no matter what we ride. Mrs. Gib is a classic example, stops regularely because she thinks she has a flat which she never has. We have a new rule - the tire isn't flat until she hears the rim hit the tarmac.

For the OP - it's not complicated. Your pressures are too high. Drop it ten psi and if it's Ok than drop it another 5 psi each time you go out. When it feels too squishy than go back up 5psi. I just looked at the Zipp calculator and it's dead-on my preferred pressures using my method for both my 28 and 25mm tires (mostly Vittoria tubed and tubeless).

[bobones]

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[bobones]

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[ryanw]

You're wrong, but I can't be bothered to argue with you. A tyre width does not equate to system volume which is what's needed here. You're stuck in a 2D world and need to understand volume is measured in 3D.

All you need to do (and what everyone with any sense should do), is use the SRAM calculator with the actual tyre width.

[bilwit]

At around 63kgs these days I run my 25mm (measured) at 80/85psi, which is about 5psi less than the Silca calculator tells me. For tubed clinchers you want to be weary of pinch flats when going especially low, I ended up getting flats when I experimented with 25mm clinchers @ 60/65psi. Heavier riders need more pressure in there ofc.

[MikeD]

You're wrong, but I can't be bothered to argue with you. A tyre width does not equate to system volume which is what's needed here. You're stuck in a 2D world and need to understand volume is measured in 3D.

All you need to do (and what everyone with any sense should do), is use the SRAM calculator with the actual tyre width.

Rim width has an effect on mounted tire width. See the following diagram.

[frostorama]

I think the comments regarding the risk of pinch flats with tubed clinchers are incorrect. I am 75kg and ride on Roval Alpinist CLX wheels with a 21mm internal width. With Turbo Cottons in 26mm and latex inner tubes I run 65/70psi front/rear. I have ridden many thousands of miles on UK roads without a single pinch flat.

This combination rolls really well, is comfortable and feels secure on fast descents.

[Hexsense]

Rim width has an effect on mounted tire width. See the following diagram.

It's not about that. It's about same tire width but way different volume.
say 25c tire on 23mm internal width has 28mm measured tire width
vs
28c tire on 16mm internal width has 28mm measured tire width

The first option has way more air volume than the second despite having the same measured tire width. BUT... The second option is taller than the first one (source: see height versus width of 25c tire on 15mm id rim vs 23c tire on 19.5mm rim in this figure where one is wider but another is taller).
So... should they really use the same tire pressure just because it measure the same width?... despite different air volume and height?

[Mr.Gib]

The second option is taller than the first one (source: see height versus width of 25c tire on 15mm id rim vs 23c tire on 19.5mm rim in this figure where one is wider but another is taller).

I have such a mental block on this concept. Doesn't a flexible material just inflate to a constant radius? I would assume the inside of a bike tire will always form some part of a circle - move the beads further apart and the constant radius gets bigger? So wider and taller? Never could wrap my head around the idea of some part of the tire resisting pressure more than another so that it will form a shape with variable radii, unless of course the material is variable in its flexibility and thickness, and then anything is possible I guess.

[ryanw]

Think of it as this:

As the distance between the beads of the tyre increase, so does its volume, to a point.

This is also my argument for total system volume, opposed to just tyre volume.

The greater the mounting plane (rim width), the greater the internal volume, the lower your pressure needs to be, relative to the same tyre on a narrower (read as lower volume) wheel.

[Hexsense]

The second option is taller than the first one (source: see height versus width of 25c tire on 15mm id rim vs 23c tire on 19.5mm rim in this figure where one is wider but another is taller).

I have such a mental block on this concept. Doesn't a flexible material just inflate to a constant radius? I would assume the inside of a bike tire will always form some part of a circle - move the beads further apart and the constant radius gets bigger? So wider and taller? Never could wrap my head around the idea of some part of the tire resisting pressure more than another so that it will form a shape with variable radii, unless of course the material is variable in its flexibility and thickness, and then anything is possible I guess.

We are dealing with two types of wall. Stiff and fixed rim wall and flexible tire wall.
Tire will try their best to inflate to the shape that has the most volume (to reduce its pressure), given the restriction of rim wall. Without any restriction, it's a circle shape. But with rim wall in play, it isn't a circle but still whatever shape happen to provide the most volume from the available casing.
With little volume between rim bead, and a bigger casing tire, it's best shape is to form light bulb. Extend outward from the restrictive rim first then form circle-ish shape.
With wide base between rim bead, and shortened casing (ie wider rim, narrower tire). There isn't enough casing to extend outward first, then still form a tall tire shape so the tire form more round/square shape right away which provide more volume in this case.

Excuse my poor and exagerated drawing. Also note the center of air volume (green dot).

rim width tire width.png (3.05 KiB) Viewed 940 times

[bilwit]

I think the comments regarding the risk of pinch flats with tubed clinchers are incorrect. I am 75kg and ride on Roval Alpinist CLX wheels with a 21mm internal width. With Turbo Cottons in 26mm and latex inner tubes I run 65/70psi front/rear. I have ridden many thousands of miles on UK roads without a single pinch flat.

This combination rolls really well, is comfortable and feels secure on fast descents.

a 26mm on a 21mm internal rim is like what, 28-29mm measured? the bigger you get the lower pressure you can get away with, it's not 1:1 with a 23mm on a 18mm interal rim which is the setup I was referring to @ 60/65

[Mr.Gib]

The second option is taller than the first one (source: see height versus width of 25c tire on 15mm id rim vs 23c tire on 19.5mm rim in this figure where one is wider but another is taller).

I have such a mental block on this concept. Doesn't a flexible material just inflate to a constant radius? I would assume the inside of a bike tire will always form some part of a circle - move the beads further apart and the constant radius gets bigger? So wider and taller? Never could wrap my head around the idea of some part of the tire resisting pressure more than another so that it will form a shape with variable radii, unless of course the material is variable in its flexibility and thickness, and then anything is possible I guess.

We are dealing with two types of wall. Stiff and fixed rim wall and flexible tire wall.
Tire will try their best to inflate to the shape that has the most volume (to reduce its pressure), given the restriction of rim wall. Without any restriction, it's a circle shape. But with rim wall in play, it isn't a circle but still whatever shape happen to provide the most volume from the available casing.
With little volume between rim bead, and a bigger casing tire, it's best shape is to form light bulb. Extend outward from the restrictive rim first then form circle-ish shape.
With wide base between rim bead, and shortened casing (ie wider rim, narrower tire). There isn't enough casing to extend outward first, then still form a tall tire shape so the tire form more round/square shape right away which provide more volume in this case.

Excuse my poor and exagerated drawing. Also note the center of air volume (green dot).
rim width tire width.png

Thank you.

[Weber]

Thank you all for your comments.

I am going to lower the pressure starting at 80 psi and seeing to go little by little to what is recommended.

And think about to get my next road wheelset on 21mm inner width to go in this new direction of low pressures

[tjvirden]

The second option is taller than the first one (source: see height versus width of 25c tire on 15mm id rim vs 23c tire on 19.5mm rim in this figure where one is wider but another is taller).

I have such a mental block on this concept. Doesn't a flexible material just inflate to a constant radius? I would assume the inside of a bike tire will always form some part of a circle - move the beads further apart and the constant radius gets bigger? So wider and taller? Never could wrap my head around the idea of some part of the tire resisting pressure more than another so that it will form a shape with variable radii, unless of course the material is variable in its flexibility and thickness, and then anything is possible I guess.

Yes, it does, almost perfectly. Your assumption is correct, for tires of the construction that are available to buy. The tread of course has variable thickness so the outer surface is not circular, but the casing (the fibres embedded in elastomer matrix) experiences high stress at normal pressures so the tread has virtually no impact on the casing shape, which is circular (constant radius) from the points of rim contact.

To be blunt: diagrams that "show" non-circular casings are purely imaginary for the sorts of tires used on road bikes. Even heavy-treaded gravel/MTB tires at low pressure have very little deviation from this. Except, of course, at the contact patch - the tire conforms to a hard surface and there's a mix of tire/surface deformation on soft surfaces. Even then, the casing will have near-constant radius between the rim and the contact patch.

In addition: there's a lot of confusion over the relevance of "volume".
For a start; which volume? If once specifies the volume of pressurized air contained by the tube/tire or just tire (for tubeless), then it's of secondary (if not tertiary) importance in almost all circumstances. As a thought experiment - compare two wheel/tire systems, identical except that one has a deeper rim well. The volume of air is larger in that one (simple geometry). What are the effects? Assuming that we're talking about "normal" sized tires - let's say an actual 23mm tire on a 700c rim - then about the only practical effects are rim brakes heating the pressurized air; the greater amount of air takes a bit longer to heat up (still a very small change), and potentially much better sealing for tubeless tires - probably a significant, worthwhile advantage.
As for what you would feel on rough/bumpy tarmac (that almost-mythical ride quality) between the two differing volumes ? Nothing. The tire doesn't know the difference because there's virtually no change in air pressure as the casing deforms on normal surface irregularities (roughness/bumps). This is why the pneumatic tire works so well!!!!

Volume in terms of actual tire size is obviously completely different - it really matters, a lot. I'm also of the view that modest changes in shape* (actual tire width combined with rim internal width) can be detected by sensitive riders - as can 5psi on a given setup at "road" pressures of say 75psi. As a rule of thumb for myself - I can't be sure of a 5% change in pressure, but I can usually detect 10%.

*this is a change in the cross-section radius