gomango

I see loads of folks riding fat tire bikes on our city streets in the summer and I think it looks daffy.

However, I run 26 x 3.8s on my Blackborow up at our cabin on fire roads.

Rest assured, there is no ",fad" about their ability to ride in all conditions.

I can reach places on my fishing expeditions that I had to carry a pack into previously.

It's like owning a pack mule that I don't have to feed. Imho

shoota 

That's exactly what I was thinking.

himespau 

I agree with all of these points.

T-Mar

Agreed, they're great in soft conditions where you need more flotation, such as sand, mud and snow.

Bikerider007

I believe I only have 28's on a fixed gear. I like the nimble feeling of a skinnier tire. And it doesn't feel like I have blocks weighing down my wheels. On a rando or cruiser I get it, but I have not hit that wide tire point as of yet.

Revracer

I have used this site as a resource for rolling resistance:
https://www.bicyclerollingresistance.com/

They have a great comparison between 23, 25 and 28mm in my favorite tire, Conti-GP4000.
https://www.bicyclerollingresistance...0s-ii-23-25-28

However, it is interesting to note that what is gained by going from a 23 to a 28 is soon lost when lowering the pressure from 100 to 80 or less. Lower pressure requires more effort / watts.

I also live in an area of rolling hills. If I am not climbing I am descending to some degree and I am also a believer in rotating mass / acceleration as a past drag racer. Weight added to the outside of a wheel when accelerating has a multiplicative effect on power needed versus weight anywhere else. When climbing the effort is akin to accelerating each pedal stroke. When descending, we could argue that the added weight adds the inertia (hence faster descents), but the differential does not equal out. Most recently I converted a commuter bike from 700x22 tubular to 700x32 Gravel King tires. Ride was so much smother, but the additional 100-200g per wheel slowed my 30 mile commute by roughly 1-2 mph. I will continue to do the fat tire thing on that bike, add fenders an make it a comfortable ride it anywhere bike, but that bike will not be taken when I want to be fast.

Kontact 

And that's exactly the problem - once you set an appropriate tire pressure for rider weight vs tire size, the advantage of large tires disappears and we are back to "feel".

More inertia isn't going to make you descend faster. Inertia causes you to speed up slower and keep speed longer and handle slower. Increased weight increases acceleration downhill, just as it fights going up hill.

Salamandrine 

I like 700x28 most of the time. Wide enough for me. A good compromise for mostly paved roads but a bit of gravel, which is pretty typical for me. Sorry but weight matters. Where I live, the rides mostly consist of up the hill, down the hill, repeat ad infinitum. Decades of experience have led me to this size as a good choice for long somewhat fast rides over mixed terrain. Sport touring.

If I have time for the occasional after work ride, I will usually take my Clem with it's 50mm tires, because when it's getting dark I can roll right through gravel, pot holes, dirt tracks, etc, without having to worry about my lines or crashing. Slow-ish but fun. Actually that bike is less slow than you'd think really.

HTupolev

Most people are still not giving up their 28s. Actually, the performance road world is still mostly on ~25s, at least if we're talking nominal widths.
Actual inflated widths can get a bit more interesting, since current aero rims have very high inner widths that widen tires quite a bit. But this ironicallly introduces some problems; a rim profile designed to pair with a 25mm tire works great when you throw a 25mm tubular on the tubular version of the rim, but a "25mm" GP4000SII that inflates to 29mm on the same rim can ruin much of the aero benefit.

Not quite. In BRR's comparison, the 28mm tire still had less hysteresis loss at 80PSI than the 23mm tire did at 100PSI. But this might just be tire-to-tire variation or a quirk of Continental's manufacturing (it's strange that the 28mm tire scales so differently from the 25mm tire than the 23 does from the 25).

But another way of looking at it is that the wider tire can be pumped higher relative-to-width before suspension effects become a problem. For instance, a 28mm tire at 90PSI will probably still ride as smoothly as a similarly-constructed 23mm tire at 100PSI, and will typically have less rolling resistance coming from hysteresis.
Alternately, you can pump the fatter tire lower, and it can roll just as well as a skinnier tire on smooth surfaces but better as roughness increases.

This benefit of course gets counteracted by the increases to mass and profile.

The effect is double, since the mass at the rim needs to move as fast as the vehicle both forward and in circles.

No it isn't. If you move some weight from the frame to the rim, the bike will take more energy to accelerate, but it also decelerates more slowly. The microaccelerations just get smaller.

If there's a climbing performance impact from heavy rims that goes beyond the simple increase in weight, it's going to be mostly in the realm of physiology/handling.

You increased the mass of the bike+rider system by one or two tenths of a percent, and the inertia by double that. There's no way that this would cause anything on the order of a 5-10% loss in overall speed by itself.

I'd look into the rolling resistance of the new tires and the aerodynamics of your two wheel setups.

At low speeds, before air drag and terminology considerations become very dominant, adding weight to the rims of a vehicle will actually cause its downhill acceleration from gravity to be slower; the increase to inertia is double the increase to gravitational force.

rhm

I'm not convinced there any point in telling you what I'm not convinced of.

Kontact 

Excuse me, I meant that increased weight will yield more total acceleration yielding a higher top speed downhill. "More acceleration" could also be read as steeper, but I meant total velocity change. Which I used the word "weight" not "mass".

nlerner

So many variables: road surface, individual perception, tire construction and materials, ideology, psychology. The comparisons often end up to be apples to oranges to monkey paws.

Fwiw, my favorite tire--700c Gran Bois Cypres--measures ~30mm in width, depending on the rim.

rm -rf

Yes, I agree with all your points.

The bicyclerollingresistance chart for the GP4000S tests. I added the purple horizontal line. The 28 has the same rolling resistance at 77 psi as the 25 at 100 psi, and the 23 at 106 psi. (And those are the rear tire pressures I've used on these three sizes. Front tires at 15% less pressure.)


Other energy losses
Jan Heine promotes large tires, 38mm, 44mm, etc. Part of his argument is that they save rider energy that goes into jiggling and bouncing the rider+bike. I think it's likely to be significant on rough surfaces. The rider's energy loss doesn't get tested with the bike lab rollers.

I ride smooth tread 38mm sometimes, and they are fantastic on rough chipseal and broken pavement. The handling seems a little more stable / sedate / controlled than the same bike with 28mm. And the tires are heavier, 200 grams more for each tire. But they don't feel slow at the cruising speeds I use when riding these -- any aero penalty is very low, and they aren't slow at all due to rolling resistance on smooth roads.

For reasonable pavement, I'll usually want the 28s. But the 38s are fun, I can ride over just about anything.

Kontact 

When I actually plotted ideal air pressure vs tire size and applied it to that graph, it wasn't a horizontal line but sloped a little bit to where 28s had more rolling resistance than 23s.

It is also important to remember that the real difference between tire sizes when it comes to performance isn't width, but volume. The difference between a 28 and 25 in volume is much greater than volume difference between 23s and 25s.

bikemig 

Bike Forums would be dead tomorrow if more people followed your advice,

bikemig 

I like 32c tires but the bike path I ride on regularly has its share of rough patches and I'm not, ahem, as fast as I used to be.

beicster 

I like wider tires because I weigh about 240 and I definitely prioritize comfort over speed. I say, do like me and ride what makes you happy. There is bound to be someone who looks at your setup with the kind of facial expression that says, "They obviously have no idea what they are doing."

HTupolev

That's pretty arbitrary. "Ideal air pressure" doesn't really exist as a general thing, and takes significant effort to pinpoint even in highly specific cases.

Width strikes me as a considerably more direct description of the effects. Loads of relevant things scale pretty close to proportionally to it. Vertical travel before bottoming out, deformation for a given amount of vertical deflection (i.e. a 1" tire deflected by .5" is deformed about the same as a 2" tire deflected by 1"), casing tension for a given air pressure, weight of the casing and tread given otherwise-similar construction, the A in CdA...

Wide tire salespeople regularly emphasize how rapidly volume scales with width, but I never see them explain why this is important. I can't really think of anything except that it's a more dramatic-sounding sales pitch.

Kontact 

The answer to both questions is likely the 15% compression number that comes up a lot in these discussions. Regardless of whether the number is 15 or 40, the idea is that a round cross-section pneumatic tire should have a certain preload to both have a reasonable initial contact patch and not bottom out. Larger tires should have more total compression, but not a greater percentage of tire height than a smaller volume tire since they work the same way, just at different scales.

Ideal tire pressure charts are based on this concept, and show how to achieve whatever optimal preload given volume and weight variations.


You can use tire width to refer to the differences between tires, but the thing that does the work is volume, and volume scales geometrically to tire radius. This is the same idea as a chart that show wind resistance not being linear to the wind speed number - it is also geometric.



So when discussing the rolling resistance comparison between a 25c and a 32c, it makes sense to set their preload the same (15% compression, or whatever) and then see how much resistance they have. And when you have to same preload compression on the two tires, you'll find that the 32c tire has a cushier ride because the amplitude of compression that you get is numerically larger for the same percentage of tire compression, which translates into being over to roll over a bigger stone with less change in tire pressure - so you feel it less.

The reason you might not want to ride a 32 vs a 25 is that the vast majority of stones and bumps on your road route may be of a size that the 25c tire absorb within the easy working amplitude they have at a 15% compression pressure. If you don't often encounter larger bumps, you're hauling around a lot of extra rubber for no real reason.


This is similar to why passenger cars don't have the long travel suspensions of off road racing vehicles. They don't require that amount of amplitude to the suspension system, so they use stiffer springs with less travel.

due ruote 

HTupolev

Berto's recommendation was created as a starting rule of thumb, not really an answer to either question.

We usually talk about wind resistance as quadratic* because the force scales very quadratically with relative wind speed in the circumstances we're dealing with, and because force is a pretty standard starting point when talking about resistances in kinematics.

With tire behavior? You say that volume "does the work", and then demonstrated this by explaining tire compressions in terms of volume and PSI. But you can also explain compressions in terms of the tire's height and how far it gets compressed as an air spring, and this is if anything closer in line with how people tend to talk about suspension systems (i.e. fork travel and spring rate).

*"Geometric" is generally used when looking at behaviors that are exponential, not polynomial.

jimmuller 

Rumor has it that some time between maybe 1635 and 1640 AD an Italian gentleman name Galileo demonstrated that this wasn't true when gravity was the driving force. About 50 years later a cantankerous Englishman named Newton proposed several "laws" of motion and gravity which explained why Galileo's experiment did what it did. Four and a half centuries of theoretical and experimental physics have not found fault with their work on macroscopic scales. (Galileo died in 1642, Newton was born in 1642.)

The effect of mass on the rim of a wheel is to increase by a factor of two the inertial effect of that extra mass, but to increase the pull of gravity by only the extra mass. Hence it actually makes the acceleration less compared to the equal total bike+wheel mass with the mass not on the wheel. Braking is so efficient that the extra angular momentum of the rim is meaningless. However acceleration under one's own leg power is reduced by the extra mass on the rim, when compared to a constant torque on the pedals of course.

Come on, let's get our physics straight.

As for tire pressure and width and resistance, one thing that gets me is the ever-present disclaimer "when the pressure is equal". One wouldn't normally run the same pressure in a skinny tire as in a wider tire. Pump it up higher and the contact patch is smaller so the tread squirms less, especially during turns. And I have yet to see a tire testing machine with a surface that looks like the road surfaces I ride. One thing mentioned that is true is a narrower tire has a lower air volume. The real effect of this is that as a suspension component the tire has a higher spring rate. This would hold true no matter how much pressure you put into it. I haven't worked out exactly what all these effects do for real road speed. What I do know is I like a certain feel in the tires, and skinny does it. When the tire weight is higher I can feel it.

jlaw

Overall, I love my 38mm Panaracer Gravel Kings and I haven't had a problem keeping-up with my friends on their skinny tires. I am definitely more comfortable running a bigger tire at 50 psi +-. Less road buzz and fewer situations where I have to actively avoid road debris,cracks, etc. - less fatigue at the end of a ride. Never going back!

ascherer 

Kontact 

Sure let's get our physics straight: You're neglecting wind resistance and terminal velocity. In air, heavier/denser objects accelerate faster and to higher velocities downhill than lighter objects.

Glad to clear that up.