
Tire volume question: 29er 2.5in width vs 27.5+ 2.6in/2.8in width
Didn't see anything on this topic after a brief search. Apologies if this has been asked before.
The short story, is that I've been considering getting a larger tired hardtail (but not quite 29+). So far, my model of choice is the NukeProof Scout race, which comes in 275+ and 29er models. The 27.5 plus model comes with 2.6in wide tires, but can fit up to 2.8in, while the 29'er model comes with 2.35in tires, but can fit 2.5in wide tires.
I hear a lot of talk about the 27.5 plus tires being nearly the same diameter of a "standard" 29er. I know that a 2.5in wide 29'er isn't "plus", but would also have larger volume, and diameter than a more typical 2.3  2.4in wide tire. I also understand the rollover differences between larger diameter tires/smaller diameter tires.
What I've not heard anything about, is the relative volumes of the tires. Anyone have any idea which has more volume, and which would smooth out the trail more?
Thanks for the help.

it's simple, hahaha
Tires have a shape approximating a torus (like a donut). Torus volume increases linearly with diameter (i.e., tire or donut height) but increases with the square of the torus radius (half thickness of the donut tube).
Thus the difference in volume of similar width tires mounted on 27.5 vs. 29er rims will be about 5%. Increasing the width of the tire from say 2.5 to 2.8" will increase the volume by about 25%. For your case, a 27.5x 2.8" tire will have roughly 20% more volume than an 29x2.5 tire. The 27.5x2.8 tire will thus have more cushion due to the larger volume. The 27.5x2.8 tire also will have a slightly larger contact patch giving more traction. While the 29x2.5 tire will have a slightly larger diameter giving a bit lower approach angle, the the 27.5x2.8 tire may have similar or even better rollover due to extra compliance/cushiness afforded by running lower air pressure.
There's a lot more also going on. Tire performance depends on the types of sidewalls and tread patterns available in the different tire sizes. In addition, smaller wheels give less trail and may quicken steering, while wider tire/rim combinations may slow steering. Smaller wheels lower the bottom bracket which adds stability but reduces pedal clearance. In the end, you'll have to decide by trial and error what works best for you riding on your trails.

Volume doesn't matter. The rim is held off the ground by tension in the tire sidewalls. Tension is due to hoop stress which goes as the width of the tire and pressure. At a given pressure, a wider tire will provide a harsher ride than a narrower one. You gotta run a wider tire at a much lower pressure to get acceptable ride quality. 26, 27.5 and 29" tires of the same width and pressure will have the same ride quality due to tire compliance. Of course the larger diameter will have a roll over advantage.

Originally Posted by levity
Tires have a shape approximating a torus (like a donut). Torus volume increases linearly with diameter (i.e., tire or donut height) but increases with the square of the torus radius (half thickness of the donut tube).
Thus the difference in volume of similar width tires mounted on 27.5 vs. 29er rims will be about 5%. Increasing the width of the tire from say 2.5 to 2.8" will increase the volume by about 25%. For your case, a 27.5x 2.8" tire will have roughly 20% more volume than an 29x2.5 tire. The 27.5x2.8 tire will thus have more cushion due to the larger volume. The 27.5x2.8 tire also will have a slightly larger contact patch giving more traction. While the 29x2.5 tire will have a slightly larger diameter giving a bit lower approach angle, the the 27.5x2.8 tire may have similar or even better rollover due to extra compliance/cushiness afforded by running lower air pressure.
There's a lot more also going on. Tire performance depends on the types of sidewalls and tread patterns available in the different tire sizes. In addition, smaller wheels give less trail and may quicken steering, while wider tire/rim combinations may slow steering. Smaller wheels lower the bottom bracket which adds stability but reduces pedal clearance. In the end, you'll have to decide by trial and error what works best for you riding on your trails.
Awesome response sir .
Took me a minute to get my head around what you're saying. And I've got a few questions to make sure I'm understanding it correctly.
You're saying that Torus volume increases linearly with the diameter, but also the square of the radius.
So some maths to see if I'm close.
2.8in wide 27.5 tire: 27.5 * (2.8/2)^2 = 53.9
2.5in wide 29 tire: 29 * (2.5/2)^2 = 45.3
(53.9  45.3)/45.3 = 0.19.
So, that's where you're getting your ~20% increase in volume.
Of course, thats assuming that the diameter of the tires is the same as the listed rim size, which, I'm assuming isn't correct (I have no idea the average diameter of these wheel sizes, as I don't own either, as I'm still rocking a 26'er atm). But, its probably close enough to get me in the ballpark.
I also did the math just now, and see that a 2.5in wide 29'er is almost identical in volume to a 2.6in wide 27.5 tire (2% smaller). Neat.
Then after that, its hard to determine if the additional air volume is better than the increased rollover of a 29er. Guess that will only be visible with some seat time.
Anyone have any anecdotal thoughts on which rides better?
Originally Posted by Lone Rager
Volume doesn't matter. The rim is held off the ground by tension in the tire sidewalls. Tension is due to hoop stress which goes as the width of the tire and pressure. At a given pressure, a wider tire will provide a harsher ride than a narrower one. You gotta run a wider tire at a much lower pressure to get acceptable ride quality. 26, 27.5 and 29" tires of the same width and pressure will have the same ride quality due to tire compliance. Of course the larger diameter will have a roll over advantage.
Trying to make sure I understand you right.
I understand lower pressure == cushier ride concept. I thought that the difference was due to volume though.
Ie, I thought it was that it takes a certain amount of air volume to support the weight of the bike/rider (or car/truck/etc). I thought that this was why road bike tires have a billion (ok, like 120) PSI, why semitrucks have so many tires, and why the crazy sno trucks have enormous huge tires, at near zero pressure.
That is to say, that a wider tire at the same pressure as a narrower one, has more volume of air, and thus would ride harsher. So to get the softer ride you'd expect, you have to reduce it.
So, I think that still means that calculating tire volume can give "some" indication of if a tire will be more compliant on the trail. Is that more or less correct?
Last edited by ocnLogan; 11092018 at 09:07 PM.

Originally Posted by ocnLogan
...
So, I think that still means that calculating tire volume can give "some" indication of if a tire will be more compliant on the trail. Is that more or less correct?
Yes, the key is that by having a larger tire volume you can run lower pressures without worrying about rim damage. This gives a more compliant ride. Volume can be achieved by increasing tire width (increases with radius squared), rim width (increases apx linearly as it adds to tire circumference), or rim diameter (increases linearly). But it's complicated. Sometimes you may benefit from a more compliant tire (such as not being deflected and rolling smoother), other times you may want it to be stiffer to hold the knobs and provide bite into the trail surface.

Originally Posted by ocnLogan
Awesome response sir .
Took me a minute to get my head around what you're saying. And I've got a few questions to make sure I'm understanding it correctly.
You're saying that Torus volume increases linearly with the diameter, but also the square of the radius.
So some maths to see if I'm close.
2.8in wide 27.5 tire: 27.5 * (2.8/2)^2 = 53.9
2.5in wide 29 tire: 29 * (2.5/2)^2 = 45.3
(53.9  45.3)/45.3 = 0.19.
So, that's where you're getting your ~20% increase in volume.
Of course, thats assuming that the diameter of the tires is the same as the listed rim size, which, I'm assuming isn't correct (I have no idea the average diameter of these wheel sizes, as I don't own either, as I'm still rocking a 26'er atm). But, its probably close enough to get me in the ballpark.
I also did the math just now, and see that a 2.5in wide 29'er is almost identical in volume to a 2.6in wide 27.5 tire (2% smaller). Neat.
Then after that, its hard to determine if the additional air volume is better than the increased rollover of a 29er. Guess that will only be visible with some seat time.
Anyone have any anecdotal thoughts on which rides better?
Trying to make sure I understand you right.
I understand lower pressure == cushier ride concept. I thought that the difference was due to volume though.
Ie, I thought it was that i t takes a certain amount of air volume to support the weight of the bike/rider (or car/truck/etc). I thought that this was why road bike tires have a billion (ok, like 120) PSI, why semitrucks have so many tires, and why the crazy sno trucks have enormous huge tires, at near zero pressure.
That is to say, that a wider tire at the same pressure as a narrower one, has more volume of air, and thus would ride harsher. So to get the softer ride you'd expect, you have to reduce it.
So, I think that still means that calculating tire volume can give "some" indication of if a tire will be more compliant on the trail. Is that more or less correct?
Well, what LR is saying is that ride quality is not solely due to the volume of air enclosed in the tire. The compliance/stiffness of the enclosure, that is, the tire, matters also. And the stiffness of the tire sidewall is related to its inner diameter or width.
However, while I am sure that the compliance of the tire sidewall/casing makes a difference, I'm not sure hoop stress is the quantity of interest. That is simply the stress state of the tire material(s), it does not take into geometry and material properties of the tire. A more highly stressed tire casing is not inherently more or less flexible than a less stressed one.
I am relatively certain that tire casing properties can swallow up gains from air volume rather quickly.
Nonetheless, the empirical evidence is that wider tires are stiffer and less compliant at a given pressure than narrower tires. It's just that I am not convinced that calculating cylinder stress at different pressures (which does, in fact yield a higher cylinder/hoop stress at a larger tire width/diameter) yields the exact explanation.
Long story short, it's complicated and I don't think you are going to math out an answer. There are competing considerations and I think the calculation is as likely to mislead you as steer you properly.

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