Why do Fat-tires have higher rolling resistance?

[thunderstorm80]

Let's compare two tires - one 28x2inch wide and the second 26x4inch wide. (so both would have more or less same circumference and so we can take the circumference out of the discussion)
Let's also assume that both tires are made from the same compounds and have the same thread pattern for road. The only difference is the size and width.
Assuming the 2inch runs at 60PSI, the 4inch would need 15PSI to have the same tire deformation = same rolling resistance.
With those pressures, it will still be said that the fatter tire robs more energy.
Why is it so?
I ask without talking about the aero-drag. I know the fatter tire cause more drag, but it's negligible at slow speeds (like 20kph)
I also know that the rotational mass is higher, but I ask about steady state conditions.

Thanks for any help about this

[Chalo]

Rolling resistance comes from multiple sources. Rubber hysteresis is the one you have identified. Scrub within the contact patch is another source of rolling resistance, and fatbike tires have lots of it. When you take a toroidal tire and make it conform to a flat surface, different locations within the contact patch must move around to reconcile the differences. You can hear it happening with a fatbike tire, most especially when riding along a curve. In your example, the fatbike tire's contact patch has about four times the area of the normal bike tire, which means four times as much rubber surface scrubbing though larger displacements.

A third source of resistance that isn't exactly rolling resistance, but is inseparable from it, is the kinetic energy lost by transferring shocks and vibrations to the rider. That's where overinflating your tires can slow you down even as it reduces the other kinds of rolling resistance. Soft squishy fatbike tires are very good at reducing this form of kinetic energy loss. But when the surface isn't very bumpy, that's not much help.

[StuRat]

Yeah what Chalo said.

Everything has its niche. Fatbike tyres are WAY more efficient in their realm. Road tyres in theirs.

Maybe the best way to summarise it is this. The rotational mass that is harder to get going, proves its purpose at the next bump. Its harder to slow down too.

Steady state?? Are we talking scientific lab tests, or reality? Steady state, you want a track bike, on the velodrome, wearing lycr...

[thunderstorm80]

Steady state?? Are we talking scientific lab tests, or reality? Steady state, you want a track bike, on the velodrome, wearing lycr...

I said that so we can neglect the rotational mass differences: I wanted to focus on the differences when you ride at constant speed.
When you also talk about accelerations, than of course the heavier fat-tires will be at disadvantage.

Would you say the fat-tires are better for electric bike touring when you mostly go on roads?
I assume that when carrying not only the regular camping equipment but also battery+motor+charger+accessories and you reach 170kg or so, the fat-tires should handle better and give more cushion and less vibrations from bumps than standard 2inch tires. Is it true? Of course if the road is perfect it's "over kill", but when you hit the first surprise bump, you would be happy you chose the fat-tires (and you saved yourself from bent spokes, big shocks to your cargo, etc...). That's what my intuition says, but maybe I am wrong?

[thunderstorm80]

Scrub within the contact patch is another source of rolling resistance, and fatbike tires have lots of it. When you take a toroidal tire and make it conform to a flat surface, different locations within the contact patch must move around to reconcile the differences. You can hear it happening with a fatbike tire, most especially when riding along a curve. In your example, the fatbike tire's contact patch has about four times the area of the normal bike tire, which means four times as much rubber surface scrubbing though larger displacements.

I don't see what's the difference between this and hysteresis. Or, are you talking about friction here? (because you always have a little displacement between a powering tire and the surface it "pushes" backwards. You have the opposite displacement on the dragged tire)
And if it's so, the contact surface is 4 times larger, but the pressure is also 4 times lower. Doesn't that makes every small contact area to experience only 1/4 of the force, and so the total friction force/scrubbing should be the same?
If I didn't understand it right, please explain in more detail.

A third source of resistance that isn't exactly rolling resistance, but is inseparable from it, is the kinetic energy lost by transferring shocks and vibrations to the rider. That's where overinflating your tires can slow you down even as it reduces the other kinds of rolling resistance. Soft squishy fatbike tires are very good at reducing this form of kinetic energy loss. But when the surface isn't very bumpy, that's not much help.

So what you say is that the cushioned tire acts as a spring and so it can temporary absorb and release back some of the energy which was attempted to be lost by the bump, while if it's over-inflated than most of that energy loss goes to the rider and the frame which dissipate it (but unpleasantly) ?

[neptronix]

More rubber touching the road = more friction, of course.

The 26 x 4 tire is going to have a wide contact patch and this is most useful for getting traction when there is little.. when hitting small irregularities, comfort will be better; so this is great on off road if you're rolling over rocks and branches.. but potholes and stuff on the road are usually not small, so actually performance will be a bit worse on the road vs the 28 x 2 tire.

The 28 x 2 tire is going to have a narrower contact patch but just as tall overall diameter, although it will compress less under load, so effectively with the rider's weight on it, it has a slightly larger diameter which is a bit better when you hit a wide pothole or whatever.

The ideal tire for taking bumps on the road would be a 32 x 1.5 tire because of the diameter alone.

But the best way to handle bumps is to have proper suspension. Even a 20 inch wheel can be buttery smooth with the right suspension.

Fat bike wheels.. instead of suspension, are a very energy lossy way to improve ride quality. It's a step backwards.. versus say, a 29 x 2 inch wheel with suspension on both axles.

[E-HP]

Assuming the 2inch runs at 60PSI, the 4inch would need 15PSI to have the same tire deformation = same rolling resistance.

I think this assumption may need clarification, since, I believe, this is the primary contributor to energy lost between the two. If "same tire deformation" means that the same amount of material/rubber is being deformed for both tires, then the assumption seems good. But since the fat tire has so much more rubber to deform, you would need to pump it up even more than the skinny tire for the amount of rubber to be deformed to be equal.

[Chalo]

Scrub within the contact patch is another source of rolling resistance, and fatbike tires have lots of it. When you take a toroidal tire and make it conform to a flat surface, different locations within the contact patch must move around to reconcile the differences.

I don't see what's the difference between this and hysteresis. Or, are you talking about friction here?

I'm talking about the difference between surface friction within the contact patch, and rubber's characteristic of heating up when you stretch it.

And if it's so, the contact surface is 4 times larger, but the pressure is also 4 times lower. Doesn't that makes every small contact area to experience only 1/4 of the force, and so the total friction force/scrubbing should be the same?

Rubber is used for tires because it doesn't act like what you learned in school. It keys to asperities in the surface, which is why a bigger tire gets more traction even if the weight on it is the same. For the same reason, a bigger contact patch exhibits more drag even if the weight on it is the same. Air inflation helps rubber do more of the same thing, increasing the size and leveling out the pressure within the contact area for more grip-- even though classical friction theory suggests contact area doesn't matter.

A third source of resistance that isn't exactly rolling resistance, but is inseparable from it, is the kinetic energy lost by transferring shocks and vibrations to the rider. That's where overinflating your tires can slow you down even as it reduces the other kinds of rolling resistance.

So what you say is that the cushioned tire acts as a spring and so it can temporary absorb and release back some of the energy which was attempted to be lost by the bump, while if it's over-inflated than most of that energy loss goes to the rider and the frame which dissipate it (but unpleasantly) ?

That's exactly right.

The late Jobst Brandt demonstrated experimentally that between tires of equal construction, the fatter tire will have less rolling resistance than the narrower tire when inflated to the same pressure. That point is key. It more or less equalizes the contact patch area so the differences in rubber distortion and hysteresis can be demonstrated. But using the same pressure also means that the fatter tire will yield a harder ride. Thus in the real world, there will be more energy lost to bumping and vibrating the rider and other payload.

[StuRat]

Steady state?? Are we talking scientific lab tests, or reality? Steady state, you want a track bike, on the velodrome, wearing lycr...

I said that so we can neglect the rotational mass differences: I wanted to focus on the differences when you ride at constant speed.
When you also talk about accelerations, than of course the heavier fat-tires will be at disadvantage.

Would you say the fat-tires are better for electric bike touring when you mostly go on roads?
I assume that when carrying not only the regular camping equipment but also battery+motor+charger+accessories and you reach 170kg or so, the fat-tires should handle better and give more cushion and less vibrations from bumps than standard 2inch tires. Is it true? Of course if the road is perfect it's "over kill", but when you hit the first surprise bump, you would be happy you chose the fat-tires (and you saved yourself from bent spokes, big shocks to your cargo, etc...). That's what my intuition says, but maybe I am wrong?

Yep exactly. E-touring and fatties are good. (not sure I'd go to 4 inch, but 3 inch yeah)
Firstly you can't have a suspension frame because they're not designed for carrying all your shit. Just having a sus fork (hardtail) is pointless.
As you said, fatties give squish to the whole bike including spokes and rims, which can ruin a trip if you break them.
Any loss in acceleration is made up for in lack of deceleration when you hit bumps, you just roll over them like they weren't there, and if you're out exploring for a week or more it saves a lot of bodily stresses.
If you go off road, which you should, roads are boring and scary, the traction difference is huge. Not just steering, the back wheel can be hard to weight on a touring frame because the chainstays are a few inches longer, high pressure skinny tyres make it much worse.
Then there's pea gravel, anything under two-inch there and you'll wish for your couch.
If you get back on a nice smooth road, just pump up the psi, best of both worlds... maybe not, everything is a compromise, the best choice really depends on the environment.

Tubeless is must though for lower pressures, and wide rims help at low pressures too.

[Hillhater]

I dont think fat tires were ever intended for road use...other than for show !..
They are intended for loose surfaces..sand , gravel, snow, mud, etc. offroad,..where a narrow tire just stops you dead.
Comparing performance on road surfaces seems pointless,.....
......its like comparing a car road tire, to an off road Truck AT tire.

[thunderstorm80]

Wow... must say I learned a lot from this thread!
I still think, and correct me if I am wrong, that fatty tires are the only reliable option for "full" suspension (albeit without the damper) when you tour with a heavy loaded bike - both front panniers and rear. Full suspension bikes just can't handle it.
The price to pay is higher rolling resistance.
I also understand that if you want to minimize those rubber losses on the fat-tires, you must inflate them to "rock hard" pressures such as 20PSI, where they would be more stiff and create more vibrations and shocks than 2inch tires at anything below 80PSI. (80/20 = 4 = 2^2 according to the diamater difference)

Still I think, that for long-distance touring in countries with really bad roads and surprisingly deep potholes which can ruin your trip if you would have a typical 1.75inch tire at 80PSI, the Fat-tire at 10PSI will save your bike, your trip, and maybe your life.
For such benefits, and the ability to lower the pressure even further for accessing off-roads, I think I am ready to pay that extra rolling power-loss. (especially since this is an E-bike tour)
This is where the Surly Blackfloyds (26x 3 . 8inch) fit perfectly, although Surly originally designed them for "When Fat-Bikes want to run". (Which is kinda silly considering the much higher rolling resistance).

There is one more question which came up now - Is there a higher wear and tear when you run a fat-tire at 10PSI vs 20? This could be significant on a long touring journey. I tend to believe, that as long you don't go crazy low on the pressure so the tire is almost coming out of the rim at curves, it is exactly like obeying the minimum pressure limits of "Standard" 2 inch tires.
Am I correct?
And if there is much more wear and tear on the lower allowable pressure limits, is the ratio of wearing can be compared to a 2inch tire? For example, if running 2inch tire at it's lowest allowed 40PSI (for the sake of the example) makes the tire's life shorter by a factor of half vs at 80PSI, I can assume the same ratio for the 4 inch tire when you compare 10PSI vs 20PSI?

[Chalo]

Tire shock attenuation is too complex for easy analysis. Suffice it to say that in the same way a 1.9" tire at 65 psi can be an overall harder ride, but still better when bad stuff happens compared to say, a 1" tire at 100 psi; a big ole fat 3-4" tire when "overinflated" to 20-35 psi can be uncompliant but still much more forgiving when encountering the worst of surface flaws.

At work today, I built up the first of the new batch of custom 64mm wide pedicab rims. For test fitting, I mounted a couple of 26x3.5" tires inflated to 36 psi, and a 29x2.5" tire inflated to 65 psi (their maximum ratings). Even though both pressures were excessive, they were still more streetworthy that way than smaller tires inflated to the same pressures.

[thunderstorm80]

Tire shock attenuation is too complex for easy analysis. Suffice it to say that in the same way a 1.9" tire at 65 psi can be an overall harder ride, but still better when bad stuff happens compared to say, a 1" tire at 100 psi; a big ole fat 3-4" tire when "overinflated" to 20-35 psi can be uncompliant but still much more forgiving when encountering the worst of surface flaws.

At work today, I built up the first of the new batch of custom 64mm wide pedicab rims. For test fitting, I mounted a couple of 26x3.5" tires inflated to 36 psi, and a 29x2.5" tire inflated to 65 psi (their maximum ratings). Even though both pressures were excessive, they were still more streetworthy that way than smaller tires inflated to the same pressures.

Assuming lower pressure (but a reasonable one) achieves better shock attenuation, what about the tire's mileage?
Is running a tire like that for thousands of km's will shorten it's life-span significantly vs at the max allowed pressure?

[Chalo]

Assuming lower pressure (but a reasonable one) achieves better shock attenuation, what about the tire's mileage?
Is running a tire like that for thousands of km's will shorten it's life-span significantly vs at the max allowed pressure?

My impression from replacing thousands of tires is that both over- and underinflation accelerate tire wear. 15-20% static sag of the tire height is about right and results in decent tread life.

[flat tire]

Cuz there's more tire to deform.