Tyre pressure road bike

[tjvirden]

Hello there,

I always used 100 psi on my wheels, when I used 23mm and 25mm as wel.

But I read on the internet that when using 25mm tires one should lower it and even when the rim is wider like 21mm inner I could lower it even more being someone like me weighing 72 kilos it could reach use 70psi.

Do you guys ride lowering the PSIs on your road bikes that much?

Is there any note of how much they use for example the worldtour?

For example this is the psi chart from LB https://www.lightbicycle.com/support/us ... &content=4

its insane 75psi.

Could you share your experience with me?

There's a substantial element of personal preference. Plenty of good advice and some highly dubious explanations in the thread.

For me, at about 72Kg (82Kg all-up) when using tires measuring actual 28mm, I generally aim for 68psi front/75psi rear. Occasionally I still use a tire combination of 23mm front / 24.5mm rear and on good pavement I put 90psi front/100 psi rear.

Use the calculators and people's suggestions as a basis to experiment for yourself - there's not one correct answer for tire pressure!

[MikeD]

Hello there,

I always used 100 psi on my wheels, when I used 23mm and 25mm as wel.

But I read on the internet that when using 25mm tires one should lower it and even when the rim is wider like 21mm inner I could lower it even more being someone like me weighing 72 kilos it could reach use 70psi.

Do you guys ride lowering the PSIs on your road bikes that much?

Is there any note of how much they use for example the worldtour?

For example this is the psi chart from LB https://www.lightbicycle.com/support/us ... &content=4

its insane 75psi.

Could you share your experience with me?

There's a substantial element of personal preference. Plenty of good advice and some highly dubious explanations in the thread.

For me, at about 72Kg (82Kg all-up) when using tires measuring actual 28mm, I generally aim for 68psi front/75psi rear. Occasionally I still use a tire combination of 23mm front / 24.5mm rear and on good pavement I put 90psi front/100 psi rear.

Use the calculators and people's suggestions as a basis to experiment for yourself - there's not one correct answer for tire pressure!

That's right. The tire pressure calculators just get you in the ball park. Adjust from there. Also, none of these calculators are sophisticated enough to use the actual volume of the tire and rim cavity, so that discussion is largely irrelevant. Then there's casing stiffness too. I like the Silca calculator because it considers the road surface. It works best for my gravel bike when considering whether I'm riding on smooth pavement vs dirt.

[Hexsense]

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.

I'll say, my drawing is bad. But your statement is kinda right and wrong at the same time. Tire form almost circular shape with nearly constant arc but how close it looks VS a circle depend on casing width VS rim width ratio.

Hypothetically, say you have 50mm wide rim, and only 63mm wide tire casing (about casing width of 23c tire). You can't do outward arc then inward arc to form a near complete circle. There is not enough tire casing for that. (try to draw it if you will). Tire will start upward arc first then inward arc almost right away creating a very short y axis height thus wide but short shape. It's like we slice a very small part of a circle and put it on top of straight rim wall.

In another extreme put 54mm mounted width tire (2.25" mtb) tire on 15mm id rim. This tire likely have >160mm casing width. It's plenty to form nearly a complete circle on top of the narrow base rim. So the tire likely arc outward aggressively first once it clear rim base to form a near complete circle. Look at this combo, it looks light a light bulb with narrow rim at the base of circular-ish tire on top.

First option is wide because of the rim, but with short casing thus low height. Second option is both wide and tall. Real world tire aren't this extremely different as this hypothetical combo but principle still there. They form consistent arc, but doesn't have to represent a complete circle, can be just smaller part of it in the case of wide rim narrow tire.

[Hexsense]

Actually, since it was my fault for being lazy and draw freehand.
I should be the one that responsible by re-draw it more precisely.

Here below 5 combo of varying rim width and tire casing width that mount to the same measured width and created consistent arc.
But they significantly vary in height. They are circular-ish shape (varying cut of a circle). The last one is not a realistic combo at all but other 4 can be seen on road or mtb trail.

Hope it is now clear how can some combo taller than the others while being narrower.

PS. illustration ignore the effect of bead hook that force the tire casing to bend sharply inward before it clear the rim. I assume the rim to be straight side wall and tire casing to be infinitely flexible.

[Mr.Gib]

Thank you again. It's like you have mentioned before: it's all about where the center of the circle is - a location that descends deeper into the rim the wider that rim gets.

[Hexsense]

^wider the rim vs the tire width ratio.
If the tire casing width scale up accordingly with the rim width then that center point doesn't go down toward the rim. They'll be just scale up image (which then measure wider and taller).

[OnTheRivet]

My data point: 64kg rider using
25c tire on 23mm internal width rim on the front,
28c tire on 25mm internal width rim on the rear,
I use 58psi on both wheels. Yes, 25c tire using 58psi. That's not a mistake. It's not really squishy either, wide rim really works.

Same. 64.2kg as of this morning
28c tires 55psi

[tjvirden]

Actually, since it was my fault for being lazy and draw freehand.
I should be the one that responsible by re-draw it more precisely.

Here below 5 combo of varying rim width and tire casing width that mount to the same measured width and created consistent arc.
But they significantly vary in height. They are circular-ish shape (varying cut of a circle). The last one is not a realistic combo at all but other 4 can be seen on road or mtb trail.

Hope it is now clear how can some combo taller than the others while being narrower.
tireshape.png
PS. illustration ignore the effect of bead hook that force the tire casing to bend sharply inward before it clear the rim. I assume the rim to be straight side wall and tire casing to be infinitely flexible.

^wider the rim vs the tire width ratio.
If the tire casing width scale up accordingly with the rim width then that center point doesn't go down toward the rim. They'll be just scale up image (which then measure wider and taller).

I can certainly agree with all of this and the illustration is a good one. In particular the illustration shows, to me, why ETRTO have traditionally been cautious with minimum tire widths for tubed tires - it's easy to qualify that as the tire width reduces on a particular rim, the bead will not be held in place as firmly.

As an aside, I believe that - decades back - Michelin experimented with radial construction but abandoned it as the tire did not track consistently. A radial construction does allow all sorts of different shapes, in contrast to the bias-ply and cotton-casing tires that must be essentially circular.

My own experience is that for all-round road use, a [tire:rim inner width] ratio of about 1.55 is an excellent compromise between all the different factors, excluding aerodynamics. I find 1.2 is too little and 2.0 is too much. I think there are topics about that here somewhere; not much is new.

[tjvirden]

My data point: 64kg rider using
25c tire on 23mm internal width rim on the front,
28c tire on 25mm internal width rim on the rear,
I use 58psi on both wheels. Yes, 25c tire using 58psi. That's not a mistake. It's not really squishy either, wide rim really works.

If you could ever do a caliper measurement of the actual tire width that would be interesting - I'd guess nearly 28mm/30mm f/r?
I can quite imagine that 58psi works well. Lateral squish is a noticeable disadvantage the instant one is out of the saddle and those tire/rim ratios must mean there's almost none!

[Hexsense]

It is 27.9mm/30.3mm so you get it pretty close.