# Tire Air VOLUME



## rjcobra (Mar 18, 2004)

Is there a chart somewhere for looking up the air volume of a given size tire, on a given size rim?

e.g. how would a 27.5 x 2.8 tire on an i35 rim compare in volume to a 29 x 2.6 on an i30 rim?


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## phlegm (Jul 13, 2006)

I can't find a calculator, but this is somewhat relevant, albeit with smaller widths:

__
https://flic.kr/p/4525757653
...and the actual formula for the torus is included there too, so you could actually do the calcs on those 2 specific tires.

However if your implied question is about traction and/or rolling resistance, this is helpful too:
Tire Pressure and Width | RIDING FEELS GOOD


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## Harold (Dec 23, 2003)

rjcobra said:


> Is there a chart somewhere for looking up the air volume of a given size tire, on a given size rim?
> 
> e.g. how would a 27.5 x 2.8 tire on an i35 rim compare in volume to a 29 x 2.6 on an i30 rim?


You couldn't do it exactly the way you want. Tire dimensions often vary a LOT from the dimensions stated by the manufacturer.

So yeah, you'd need the equation for the volume of a torus and then you'd actually need to measure the dimensions of the inflated tire in order to calculate its volume. Which would still be a little off, since you'd be measuring outer dimensions of the tire, and not the inner dimensions.


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## noapathy (Jun 24, 2008)

^^^Not to mention tires can vary in both height and width (as well as casing/sidewall stiffness).

What's this for, exactly? If you have specific tires in mind I'm sure there's at least one person who's tried most reasonable tire/rim combos. Poke around in the Wheels & Tires section as it may have already been discussed at length.


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## rjcobra (Mar 18, 2004)

noapathy said:


> ^^^Not to mention tires can vary in both height and width (as well as casing/sidewall stiffness).
> 
> What's this for, exactly? If you have specific tires in mind I'm sure there's at least one person who's tried most reasonable tire/rim combos. Poke around in the Wheels & Tires section as it may have already been discussed at length.


Just wondering if my optimal tire pressure would turn out to be similar with those two tire/wheel combinations. If the volume is the same, the optimal pressure should be similar, correct?


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## phlegm (Jul 13, 2006)

rjcobra said:


> Just wondering if my optimal tire pressure would turn out to be similar with those two tire/wheel combinations. If the volume is the same, the optimal pressure should be similar, correct?


So, from the article I gave you:
_
"The subject of tire pressure cannot be addressed without also acknowledging the relevance of tire width, since the two go hand in hand. As such, I must make the next statement abundantly clear, in its own paragraph, and in bold type:

Narrow tires DO NOT roll faster than fatter tires. Narrow tires, in fact, roll SLOWER than fatter tires."_

So, even with identical pressures, you'll still have a difference between your two examples given the width. So, in other words, to make the 2 tires perform similarly, you would need to reduce the pressure a bit more on the narrower one.


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## Harold (Dec 23, 2003)

rjcobra said:


> Just wondering if my optimal tire pressure would turn out to be similar with those two tire/wheel combinations. If the volume is the same, the optimal pressure should be similar, correct?


Tire volume isn't the only factor, though. The tire itself and the structure it provides affects the end tire pressure, too.

For the same size of tire (considering the same brand and tread pattern, at least), I DID have to adjust my tire pressure when moving from one with a stiffer sidewall/casing construction to another with a more pliable casing. The more pliable one needed a touch more pressure to provide the same feel. Stiffer casing needed less pressure.

When you're looking at different tire models AND different sizes, you've got some confounding factors thrown in there that will make the answer a much more complicated one if you try to predict it too much.

Now, you're getting into some interesting territory with your question, though. Hopefully at some point this year, I'll be in the market for a new bike and I'd like something that could fit either 27.5+ tires or 29x2.6. Considering how difficult it is to find anything set up as 29x2.6, it's not like I can just go to a local shop with a rental or show up at a demo event and ride something with that wheel/tire size to compare. Deciding which wheel/tire size to set the frame up with will prove to be challenging.

I've ridden a handful of 27.5+ bikes and I like how they handle, so I know I'd be happy with that setup. But would 29x2.6 suit this build better (keeping in mind I've got a 26x3.8" FS fatbike, so I've already got a fat/squishy bike)? That's something I just am not sure about.

When selecting an optimal pressure, I find it much easier to just choose the optimal pressure based on how it feels on the ride. I'll check the pressure later, once I've found what I like for the given trail/conditions. After adjusting for yet other trails and conditions, I can figure out a range of pressures I like, and the conditions that push me towards either extreme. That way, I have a way of predicting what my pressure should be before I start my ride to minimize the amount of adjusting I need to do during a ride later on.


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## rjcobra (Mar 18, 2004)

So with those 2 wheel/tire combinations, I'd have to run higher pressures in the 29ers?


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## MSU Alum (Aug 8, 2009)

rjcobra said:


> Is there a chart somewhere for looking up the air volume of a given size tire, on a given size rim?
> 
> e.g. how would a 27.5 x 2.8 tire on an i35 rim compare in volume to a 29 x 2.6 on an i30 rim?


If you know the bead-to-bead measurement of the tire (for example, 160mm) and you know the internal dimension of the rim (for example, 35mm) then you know the approximate diameter of the circle it forms with the tire on the wheel, inflated.

If you know the distance around the wheel, you can approximate the height of a column of 195mm circumference (in the case of the 35 id rim ) or 190mm circumference (in the case of the 30mm id rim).
It's easiest to use b2b to get relative volumes between 2 27.5" tires, or 2 29" tires, but you could get approximate values using this method.
I doubt it would be very useful, though.
Trial and error, using different pressures in different tires and timing the ride is the best method to determine optimum pressure for your riding.


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## noapathy (Jun 24, 2008)

rjcobra said:


> So with those 2 wheel/tire combinations, I'd have to run higher pressures in the 29ers?


Maybe? Depends how/where you ride and we don't even know if this is on the same bike, same tires, etc. I'd re-read Harold's/MSU's advice. Basically, just take a pump along with you and feel it out. It'll be faster and more fun.


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## Lone Rager (Dec 13, 2013)

Basically, mtb tire carcasses are circular, so volume tracks with actual carcass width, which is why historically with was spec'ed and talked about. Recently people started talking about volume as if it has additional separate meaning from width, but it doesn't, at least for bike tires. 

Belted tires, like on cars and some motorcycle tires can have non-circular cross sections. I know of no mtb tires that have structural belts, though there may be some out there.


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## sturge (Feb 22, 2009)

I go with a ball park pressure then tweak based on feel for ambient temperature and trail selection of the day. Maybe you don't live in a variable climate but tires stiffen when cold and get softer when warm so at a given pressure they are not going to feel the same. I ride anywhere from 10 deg F to 95 deg F.


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## Harold (Dec 23, 2003)

rjcobra said:


> So with those 2 wheel/tire combinations, I'd have to run higher pressures in the 29ers?


I did not say that. And phlegm said the opposite of this.

You're honestly going to have to figure out the appropriate pressure yourself when you get the bike. You can try to get an estimate by looking at what other similarly-weighted people set their pressures at. You should be able to get plenty of data points for the 27.5x2.8 tires since there are plenty of those out there. You'll have fewer data points for 29x2.6" tires, but you'll get some.


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## Le Duke (Mar 23, 2009)

Simple solution.

Fill them both with water. Measure the volume of each. 

Sent from my XT1585 using Tapatalk


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## rjcobra (Mar 18, 2004)

Harold said:


> I did not say that. And phlegm said the opposite of this.
> 
> You're honestly going to have to figure out the appropriate pressure yourself when you get the bike. You can try to get an estimate by looking at what other similarly-weighted people set their pressures at. You should be able to get plenty of data points for the 27.5x2.8 tires since there are plenty of those out there. You'll have fewer data points for 29x2.6" tires, but you'll get some.


Yeah it seems like lots of guys are running the 29 x 2.6 tires at about the same pressures as the 27.5 +, even at my weight. That doesn't sound quite right to me, but I will try it for myself


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## phlegm (Jul 13, 2006)

Le Duke said:


> Simple solution.
> 
> Fill them both with water. Measure the volume of each.
> 
> Sent from my XT1585 using Tapatalk


Those would ride like crap.


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## eb1888 (Jan 27, 2012)

And tread profile also comes into play. A more rounded profile tire can take advantage of a wider rim and lower pressure to form a bigger footprint. This gets figured in with the sidewall flexibility, thickness for cut resistance and tpi construction, usually 120 or 60 tpi.
A more rounded profile tire often has more volume to go with the tread profile design. Plus knob height and size contribute to how fast the tire rolls. Smaller knobs for hardpack and bigger for loamy terrain for example.


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## noapathy (Jun 24, 2008)

phlegm said:


> Those would ride like crap.


I think he meant for them to then be ridden underwater. Wouldn't wanna just float to the surface, right?


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