Benefits of wide internal rim width

[PeytonM]

In my mind the progressiveness of the system spring rate changes with increased internal width (keeping the same tire).

Wider internal width increases the volume of air in the wheel tire system, hence reducing the psi on the wider wheel to maintain tire skin tension.

Now imagine hitting a bump that compressors the tire, the volume of the small system as a percentage will experience a greater change and stiffen up faster. The larger system experiences a lower percentage of volume change ideally absorbing the compression.

These changes are still very minor compared with simply changing to a larger size tire. More likely to be felt as reduced road buzz.

[warthog101]

The wider rim also allows a larger tyre without the aerodynamic losses imposed by the lightbulb profile that the same tyre would present on a narrower rim.

[Nereth]

In my mind the progressiveness of the system spring rate changes with increased internal width (keeping the same tire).

Wider internal width increases the volume of air in the wheel tire system, hence reducing the psi on the wider wheel to maintain tire skin tension.

Now imagine hitting a bump that compressors the tire, the volume of the small system as a percentage will experience a greater change and stiffen up faster. The larger system experiences a lower percentage of volume change ideally absorbing the compression.

These changes are still very minor compared with simply changing to a larger size tire. More likely to be felt as reduced road buzz.

The force-displacement curve of a pneumatic bicycle tyre is driven almost entirely by geometry (i.e. the change in size of the contact patch as you vertically deflect the wheel), NOT by the change in volume (and thus pressure). We looked at that analytically recently in the narrow vs wide tyre thread ( viewtopic.php?f=3&t=175949&start=30#p1880270 ), and, experimentally, you can verify by chucking a pressure gauge on your tyre and pressing down on it. It barely moves all the way to rim bottom-out (at which point you maybe gain some single digit % of pressure).

Not to mention, if you want a more comfortable tyre, you would probably WANT stiffness to be superlinear with deflection (which, mind you, a bike tyre naturally is for aforementioned geometric reasons), so you could run low stiffness at normal ride height, but then it would ramp up real high before a rim strike would occur, protecting you.

[warthog101]

I don't think it needs to be too complex.
Every single tyre pressure guide will recommend a reduced pressure as tyre width increases.

https://www.sram.com/en/quarq/campaigns ... -explained

Finding the right tire pressure involves a large number of related variables such as tire size, rim width, bike and rider weight and of course ride style and conditions.

Perhaps the most important variable to consider is bike and rider weight. Ultimately the tire is a spring with air pressure determining how soft (or hard) that spring is. The more weight the spring is supporting the more air pressure will be needed to keep the spring from being in a constant state of compression and unable to absorb obstacles.

The total volume of air that the tire can hold is directly related to the width of the tire. This is important, as you of course remember from high school, due to Boyle’s law which basically says as volume increases pressure decreases and vice versa.

With current trends heading towards wider rims and tires, riders need to reduce pressure accordingly. The trick is knowing exactly how much to reduce by so having an accurate way to measure tire pressure is extremely important since in some cases just a few psi makes a significant difference.

Bottom line, the more you, your bike and your gear weighs the more air pressure you’ll want in your tires. However, the wider your rim and tire is the less pressure you’ll want.

[Nereth]

I don't disagree with that. Just saying that if optimum pressure at width a is X, then that doesn't mean optimum pressure at width b is y such that skin tension is the same. That's all. It's a bit harder than that to get to the optimum pressure. But yes, as a general rule, width go up, then pressure go down.

Also, and I know this isn't a reflection on you, that extract by SRAM - the reference to boyle's law: Completely irrelevant drivel with respect to the tyre pressure choices it is discussing. Written by some marketing bloke who did 20 seconds of research, or an engineer with no intuitive understanding of the principles he was taught. Perfect example of why companies statements can't be taken at face value. It's often like that, this one is just more clear than most cases.

[warthog101]

Yes Boyles law didn't seem too valid since it was applicable to a fixed amount of gas vs volume.
I did agree with the basics of the rest of the article as it applied to size and volume and the pressure required. Volume up - pressure down.

[bm0p700f]

The main issues is rims keep getting wider but unless your using a wider tyre you dont need a wider rim. 23mm and 25mm internal width seem far more common for gravel bikes now.
25mm internal width used to be 2.2" tyre territiry. som gravel bikes use tyres that wide many dont. Many are in the 40mm to 45mm range still. that 20mm to 23mm internal width in relaity.

The other thing we never seem to know is the internal width the tyre manufacturers tests there rims on. thats probably the ideal width that should be used. Since that always an unknown we always end up guessing.

then there those people who wasnt to use a 32mm tyre on a 23mm wide rim. sure it possible it might even be "safe" but why would you? Rims really have got more than wide enough yet they keep getting wider.

[Jaisen]

The main issues is rims keep getting wider but unless your using a wider tyre you dont need a wider rim. 23mm and 25mm internal width seem far more common for gravel bikes now.
25mm internal width used to be 2.2" tyre territiry. som gravel bikes use tyres that wide many dont. Many are in the 40mm to 45mm range still. that 20mm to 23mm internal width in relaity.

The other thing we never seem to know is the internal width the tyre manufacturers tests there rims on. thats probably the ideal width that should be used. Since that always an unknown we always end up guessing.

then there those people who wasnt to use a 32mm tyre on a 23mm wide rim. sure it possible it might even be "safe" but why would you? Rims really have got more than wide enough yet they keep getting wider.

To answer your question, in addition to all the points already made on page one clearly stating the advantages of the wider rims, the reason I also ride 30mm tires on 25mm internal width rims is for the additional robustness of the wheel. I have Specialized Terra CLX II wheels, they have same weight, spokes, and hubs as the lightweight climbing wheels, the Alpinist CLX II, but because the rims are designed around gravel and all road use they are prusmably stronger as well.

I can't see a reason to choose anything narrower like 21mm internal on the Alpinist line. If I wasn't already maxed out on clearance on my frame I would slap on 32mm tires to boot.

[youngs_modulus]

That extract by SRAM - the reference to boyle's law: Completely irrelevant drivel with respect to the tyre pressure choices it is discussing. Written by some marketing bloke who did 20 seconds of research, or an engineer with no intuitive understanding of the principles he was taught. Perfect example of why companies statements can't be taken at face value. It's often like that, this one is just more clear than most cases.

Ooof--I fully agree. That SRAM blurb about Boyle's law doesn't have any actual meaning in a syntactic sense, let alone in an engineering sense. I hope that was written by an intern.