[FishMan473]

I’ve been thinking about tire width lately: why do tires that are just slightly bigger ride noticeably smoother? Why to fat bike tires that are, realistically, just a bit bigger than other fat bike tires seem to “float” so much better.

Then I started to think about tire volume as a function of the cross section being a circle (not quite but close), then half the tire width becomes the radius. Square that and multiply by pi and you get the volume. So volume goes up way faster than width.

As an example, if you take a (seasonally appropriate) 100mm wide fat bike tire and compare it to a 110mm wide fat bike tire, the width difference hardly seems significant. However if you do the math, that’s about a 20% increase in volume. Which is, well, a lot.

Thoughts?


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[ocnLogan]

I was thinking about this a bit last year. I started a thread asking about it, and got some good information. Basically what you just said.

https://forums.mtbr.com/26-27-5-29-...es/tire-volume-question-29er-2-5in-width-vs-27-5-2-6in-2-8in-width-1091918.html

I used the same formula that you did. Which, written out more like an equation looks like this:

Tire diameter * (tire width/2)^2 == volume of the tire.

As you stated, width makes a lot more difference than the diameter does. Here is the example I did in the thread.

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. Meaning, the plus tire is ~20% larger than the 29'er in this example.

And you'd then assume that the plus tire would be more "floaty" than the 29'er. But, of course, this is all assuming everything else is equal between the tires (casing, etc). As any of those things other variables out there could change the ride.

Oh, and the other thing I was thinking that this could potentially be useful for, is extrapolating tire pressures when you change tire sizes. As basically, you only need the tire to contain a certain amount of air volume to hold up your body weight. So a larger tire, that air doesn't need to be pressurized as much to ride the same way.

Or put a different way (again, assuming casings the same/etc), you need the certain volume of air in the tires to hold your weight. You can squish it into a 23mm wide road tire at 120psi, or you can let it fill up a fatbike tire at a mere 5psi. And, also why the reverse (5psi in a 23mm road tire, and 120 psi in a fatbike tire) wouldn't work at all. One would be totally flat, and the other would be like riding a solid tire.

I mean, maybe that's not all that useful to know... but kind of interesting to think about I guess.

 

[jeremy3220]

I'm not totally on board with the idea that larger volume = smoother ride as an absolute. I think Cushcore is a good example of this. It reduces air volume and smooths out the ride. Some of this is due to being able to run lower pressures and some of it due to more material damping. The issue is when you increase volume but can't lower pressures enough to benefit. Higher volume tires often have relatively lighter construction.

The last three rides on my hardtail were on 3 different rear tires; a 2.6 Rekon, a 2.6 XR4, and finally a 2.4 Rekon (all with the 90g Rimpact insert). I replaced the 2.6 Rekon (ran at 25 psi)with a 2.6 XR4 that I was previously running up front. The XR4 is a higher volume tire than the Rekon and on my first ride I realized immediately I had to run lower pressure because it felt harsh. Dropping the pressure a couple psi helped but it still felt like the rear was getting bucked off of every root and rock; plus the tire felt vague and bouncy (almost like it was out of phase with the bike and terrain). I hated the XR4 on the rear (it's ok on the front) so I put the 2.4 Rekon on. The 2.4 Rekon at 25 psi felt both better damped and supportive (kinda like going to a heavier duty casing without the weight penalty). Part of that is due to the insert offering more support on the smaller 2.4 tire. With no inserts, I found 2.6's slightly smoother riding but more vague and bouncy and I also destroyed a 2.6 XR4 on the front of my bike running it with no inserts. At the pressures I have to run in 2.6's there's no ride quality advantage.

 

[FishMan473]

How wide are your rims?


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[jeremy3220]

How wide are your rims?

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30mm. I'm sure the 2.6 XR4 would work a bit better on i35 rims but not that much better.

 

[tweeder82o]

you're on the right track regarding volume, but it can be independent of width. we don't see it as much in the mtb world, but we do in the car world.

think of a mid-sized or compact crossover, they usually have some what narrow width tires but come with tall side walls. which still ups the volume and cushness of the ride.

conversely, a sports car can have super wide tires but thin sidewalls, which improves traction but gives your a harsher ride (with better handling)

other factors like rim width, rubber material, air pressure can impact the tires performance too.

but yea, 2.4 to 2.8 might seem like only .4 of width but the volume difference is bigger which affect your ride more than the mere .4 width would suggest (that and sidewall height is different too)

 

[jeremy3220]

you're on the right track regarding volume, but it can be independent of width. we don't see it as much in the mtb world, but we do in the car world.

think of a mid-sized or compact crossover, they usually have some what narrow width tires but come with tall side walls. which still ups the volume and cushness of the ride.

conversely, a sports car can have super wide tires but thin sidewalls, which improves traction but gives your a harsher ride (with better handling)

other factors like rim width, rubber material, air pressure can impact the tires performance too.

but yea, 2.4 to 2.8 might seem like only .4 of width but the volume difference is bigger which affect your ride more than the mere .4 width would suggest (that and sidewall height is different too)

Your car example consistent of more than a volume change. A sports car tire (UHP) uses different construction. A typical crossover uses a single ply carcass, where a high performance tire typically uses a two ply carcass and likely a different carcass (cord dia, topping gauge, etc). In addition you'll have a different bead package, sidewall gauge, jointless belt material, belt cord epi and dia, etc. The construction type and sidewall height are bigger factors than volume. Not to mention sports cars ride differently for a multitude of reasons other than tires.

 

[ocnLogan]

In theory, tire width and volume aren't the same, that is totally true. Its just in mountain bike tires, the width/height of the tire scales more or less with the width. So currently the discussion is more or less one and the same.

If bike tires had the automotive tire concept of width being selected separately from diameter (aspect ratios), then we'd be in a different situation.

I think the main thing, is that the discussion only really makes sense if all other factors remain constant. Meaning, the tire construction, rim width, rubber compound/etc can all change the volume for sure. But if those things are all held constant, then the formula is fairly representative of "real life".

At least based on my current understanding anyway.

 

[jeremy3220]

In theory, tire width and volume aren't the same, that is totally true. Its just in mountain bike tires, the width/height of the tire scales more or less with the width. So currently the discussion is more or less one and the same.

There are construction differences between sizes and tires of the same size though. 2.6" and plus tires generally use a lighter design philosophy. The 2.4 and 2.6 Rekon in dual compound are within 1 gram of each other. So the 2.6 isn't just a scaled up 2.4 and this is largely true for most mtb tires. So 2.5" tires usually feel more similar to the 2.3" version than the 2.6.

 

[ocnLogan]

There are construction differences between sizes and tires of the same size though. 2.6" and plus tires generally use a lighter design philosophy. The 2.4 and 2.6 Rekon in dual compound are within 1 gram of each other. So the 2.6 isn't just a scaled up 2.4 and this is largely true for most mtb tires. So 2.5" tires usually feel more similar to the 2.3" version than the 2.6.

Did you mean to say that there are construction differences between sizes of the same model of tire? Ie, the 2.4 rekon, and 2.6in width rekons aren't the same, just scaled versions of each other?

Agreed. Although, in the context of this discussion, I think that the volume does scale that way, even if not weight.

That said, even if the volume does change that way, the suppleness/weight of the casing can totally change the ride characteristics/pressure required, so its still related to the feel of the ride. Thats one of the reasons its easy to say it works in a certain way with the forumulas... but in the real world its not really a perfect way to predict stuff.

I was curious, and just looked up my front tire (WTB Vigilante 29x2.6). The 2.5in version in the same casing is ~110 grams lighter, which might mean its the same construction. Hard to know exactly.

Anyway, still an interesting thing to think about imo.

 

[jeremy3220]

Different model tires of the same size can have more or less volume than each other.

 

[FishMan473]

30mm. I'm sure the 2.6 XR4 would work a bit better on i35 rims but not that much better.

i30 or 30mm external? Either way, that's too narrow for a 2.6" tire. So support is your issue. I've got a i35 rim for my 2.6" XR4's and it's great, but honestly I'm eyeing upgrading my wheels with even wider carbon rims. I suspect i40 would be even better.

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[jeremy3220]

i30 or 30mm external? Either way, that's too narrow for a 2.6" tire. So support is your issue. I've got a i35 rim for my 2.6" XR4's and it's great, but honestly I'm eyeing upgrading my wheels with even wider carbon rims. I suspect i40 would be even better.

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i30. The general consensus is that works well for 2.6

The 2.6 doesn't have better traction than the 2.4 so I'm definitely not changing rims.

 

[Lone Rager]

Width is what matters and has always been the standard of comparison. The behavior of a tire is due to the hoop stress in its carcass, and that is proportional to width. It's only of late that people talk about tire volume, but that's not what directly matters. Of course you can't have a wider tire without having more volume.

 

[jeremy3220]

Width is what matters and has always been the standard of comparison. The behavior of a tire is due to the hoop stress in its carcass, and that is proportional to width. It's only of late that people talk about tire volume, but that's not what directly matters. Of course you can't have a wider tire without having more volume.

I mean you could but it would be pretty heavy.

 

[Fuse6F]

Psi is pressure over area.
Lower psi is smoother/softer/more supple however you wish to characterize it.
Lower psi offers more mechanical grip.
But you need more contact patch area to carry the same weight at a lower pressure. Which is why your road bike tire has more psi (its got a very small contact patch).

Tires are part of the suspension and they must have the energy they absorb damped out or you will not like the ride. So there is this magical chase to get best traction, smoothest ride, lightest weight, best tire damping, cut resistance, etc etc.

It all changes based on things like surface, speed, skill level, suspension performance, rider weight, etc.

There will never be a one best tire.... only a one best tire for you.

Im heavy (265) and like my grippy compliant beginner to intermediate level purgatory 3.0” grid casing tires. The grid casing is way better (for me) than the original control casing versions. My I45 rims also an improvement over my i38 ones (same tire).

But this tire is too heavy for lighter riders (1100gm) and isnt a pro level tire so its out of favor and we see 2.5” being the sweet spot. For the regular sized people.

Everyone agrees, a fat bike is way more workout than a regular bike.

Its all part of the game.

A tire has a fixed spring rate which is observed during a ride this rate is different from one volume/design size to the next. Your going to like one over the other based on this performance difference as perceived by you.

 

[mikesee]

Everyone agrees, a fat bike is way more workout than a regular bike.

Only if used improperly.

Take that fatbike to a soft surface -- like quasi packed snow -- set tire pressures appropriately, and then leave any/every other bike style wallowing at the trailhead, completely unable to ride, regardless of effort.

 

[mikesee]

i30 or 30mm external? Either way, that's too narrow for a 2.6" tire. So support is your issue. I've got a i35 rim for my 2.6" XR4's and it's great, but honestly I'm eyeing upgrading my wheels with even wider carbon rims. I suspect i40 would be even better.

Experience dictates preference.

If you say 40mm rims work best for you with 2.6" tires, who am I to argue?

I'll just add that the wider-is-better pendulum is finally starting to swing back. People are asking for more reasonable rim widths to pair with their tires. Not everyone, but most people buying wheels today are using 30mm rims with 2.6" tires, 35mm rims for 2.8" tires, and 40mm rims for 3" tires.

Outliers (350# dudes, people pulling trailers, survivalist types that always carry their machete and 3 (minimum) types of firestarter, brainwashed Jones zealots) will always push beyond those widths. They are a small but mightily vocal fraction of the riding/wheel buying public.

 

[jeremy3220]

Psi is pressure over area.
Lower psi is smoother/softer/more supple however you wish to characterize it.
Lower psi offers more mechanical grip.

I don't think it's that simple. The air pressure is the internal spring rate of the tire, where the air pressure and tire structure make up the total spring rate (which really would be expressed as vectors rather than it being uniform across the surface). This is why one tire feels stiffer than another at the same or even lower psi. Additionally not only the spring rate but the damping rate affects the smoothness of the ride. This is why higher volume tires tend to feel smoother at lower speeds but bouncier at higher speeds. In my experience the same thing can happen when I lower the pressure too much in certain setups and the tire gets bouncy/harsh like it's packing down in rough sections.

Also, lower pressure offers more grip by increasing the contact patch, until it's low enough to decrease the contact patch by allowing the tire to roll.

 

[Fuse6F]

Only if used improperly.

Take that fatbike to a soft surface -- like quasi packed snow -- set tire pressures appropriately, and then leave any/every other bike style wallowing at the trailhead, completely unable to ride, regardless of effort.

You meant to say only if the standard bike is used improperly.