rpenmanparker 

But you avoid explaining why a larger tire needs less air pressure. Focus on the contact patch. It is the only place that outside force is being applied...except for the atmospheric air pressure. But that is discounted by the tire pressure gauge.

rpenmanparker 

ikr

garciawork

I thought it was mainly due to the fact that road surfaces aren't perfect, and there was something about a lower pressure, larger contact patch absorbing the bump taking less energy than having to "bounce" over it. I recall reading that a 29x2.2 or somewhere in that range v a 27.5x3 (+) on a downhill roll test resulted in the + tire rolling faster, and they also chalked it up to the tire deforming a little more rather than bouncing over.

But, I am not an engineer, nor a fast rider, so... yeah. I like 25's or 28's, tubeless, between 85-95 psi for comfort.

2manybikes

More inside surface area of the tire, increases the force trying to push the tire off the rim. The bigger tires list a lower pressure on the tire side wall to keep the tire on the rim.

Not a dumb joke this time.

69chevy

Now you have me wondering what you are actually asking, because you lost me with this statement.

wphamilton

I've assumed that the air (and tire) does act as a linear spring. I know that the tire's construction, including the direction of the threads, will introduce some non-linearity but surely that's small enough to be nit-picky?

Playing Devils Advocate, if you adjust your tire - however skinny or fat - to drop 15% at your weight (W) and it acts like a linear spring, will it not drop 30% at a weight of 2W? And so on ... at some force, for either tire, it mashes to the rim 95-100%. Which implies that the same force, the same pothole at the same speed, has the same likelihood of a pinch flat for either the skinny or fat tire. If it's all about pressure and the magnitude of force, that is.

OK that depends on "what" the tire drop is 15% of so let's back up. Is the 15% rule relative to the tire bead height? Inside rim to bottom of tire? Something else?

rpenmanparker 

Hey, I liked it.

But your explanation would relate to why you would have to inflate a bigger tire to lower pressure, not why you would be able to. Most of the recommendations for larger tires are couched as what you can do, not what you have to do.

HTupolev

The wider tire is capable of providing more contact patch before it bottoms out. It has more rubber surface area available to press on the ground before anything bad happens. If you compress two tires so that their rims are just barely not pinching the tube/tire, and one is wider than the other, the wider tire will have way more rubber touching the ground.

rpenmanparker 

Yes, if the fat tire is inflated to a lower pressure, then it has the same (similar?) chance of pinching. That's one of the advantages of the larger tire, you can lower the pressure without INCREASING the chance of pinching. Not that you reduce the chance of pinching, but that you can ride at a lower pressure and not increase the chance of pinching as would happen if you lowered the pressure in the narrower tire.

rpenmanparker 

And will therefore at the same pressure take a greater stress to compress that far because the larger patch area times the pressure will give a larger force resisting the compression. Exactly. Unless you lower the pressure. Then you will get a more similar stress required to get the same compression in the two size tires. That is what I was saying from the start.

wphamilton

Well literally, not having a greater likelihood of flats may be the "reason you can lower pressure" on wider tires. Rather than the size of the contact patch. The contact size by itself does not increase friction, raise or lower rolling resistance, or affect pinch flats. It does keep some of the tire on the ground when rolling over small obstacles, and might grab some dry pavement faster when sliding over a slick spot.

I'm still obsessing over the 15%. I think it is measured from the bead, but there is a fixed volume of air in the rim also so 15% drop for a small tire is more drop compared to the total air volume than is 15% drop for a big tire. So the big tire following this rule really would take bigger impacts without flatting.

rpenmanparker 

As I see it what the contact patch does is support the bike. The larger the patch, the lower the pressure needed in the tires to give the required force to prevent the tire from fully collapsing. Tell me how you know when your tire pressure is too low or you have a leak. Your contact patch gets really big, right? Right up to the point that the instantaneous pressure in the tire X the patch area gives the necessary force to prevent further tire drop.

Forget about 15%. It could be any number. Tire experts general agree that a certain fixed % drop gives a good compromise of comfort and rim-tube protection. I see it as the distance from the apex of the tire bead to the top of the rim which is what you are trying to avoid hitting. That is why the air between the brake tracks is unimportant. You can only drop the tire to where the tops of the brake tracks hit the ground or the squished tire keeping the brake tracks off the ground. So the constant %age is the fraction of the distance the tire can be allowed to drop. Trial and error suggest that 15% is a good number.

I'm pretty sure that the air volume is not what protects the tire. Virtually all impacts are big enough to bottom out the tire. But are they forceful enough? Air in the brake track space has nothing to do with it. Only the pressure in the tire and the size of the contact patch. It isn't about volume, it is about pressure. And despite the implicatons of the ideal gas law, those are not really the same here.

WhyFi

To the people that are telling Robert that he's overthinking it - welcome, you must be new here.

rpenmanparker 

As if.

wphamilton

To the extent that compressing the air is involved, it would depend on ALL of the volume that constrains the air. But, we know that tire pressure does not increase (or very little) when we load the axles. Therefore the air is not being compressed by load, and all that is moot. Maybe it's different when you hit something producing a lot of impulse into the tire (rather than on top of a flat surface), but I think that would have to be extraordinary.

Or I could be wrong and that mostly applies to just static loads.

I see where you're headed though. For tires at a given pressure, the size of the contact patch is directly proportional to the force against it. An area two inches wide could bottom out on a small tire that didn't have two inches width between the rims, but not on a bigger tire that has more room for a bigger contact area. Right?

zymphad

My understanding it depends on the surface. In lab situation, smoothest surface roller, high pressure, thin tire is fastest.

But in real world, with tarmac, 25-28 is best. Vibrations reduce rolling, and speed. Wider tire with more give can absorb the vibrations and keep the tires on the surface, therefore lower rolling resistance. And then as the road gets rougher, pave etc 28-32 is desired.

I haven't ridden 28, so no idea how sluggish it is.

Psimet2001 

Same force. That's it.

Force that holds the clincher bead on the rim as well as the supporting force of the rider and rig.

Moving on.

Doge

Sorry - not reading all of this.

The wider contact patch is more round, the narrower is more elliptical. There is more case deformation on the narrow at the same PSI. That is the trick - same PSI. Although in theory the patch area is the same it isn't because of case deformation.
When you are running the wheel at ride speed there is delay in the deflection at least due to case/tread hysteresis. Just so no-one thinks I'm neglecting the centripetal force of the rotating mass of the tire/rubber/air in tube, it is very small. Many riders can feel that.

But when I tested I normalized all that. So I ran a 25 @110psi and I ran a 23 @ 130 (I did the math then, but forgot the specifics.). Anyway, this is another area where I think on a smooth road the thinner higher PSI will do as well or better - and weigh less.
Many have been duped into thinking as wider at the same PSI is lower Crr they are faster. Just pump the thinner up more. Get to bumps, it is different.

rpenmanparker 

Totally agree. Fortunately those two forces are comparable. Tires are firmly enough mounted to tolerate the pressures with which they ride best.

2manybikes

It seems like there is a confusion between the two. As for me.......back to the oatmeal raisin cookies.

rpenmanparker 

My all time fave.

rpenmanparker 

Right, compression at the contact patch is so small relative to the tire volume as to be de minimis.

rumrunn6

because that much volume filled to a higher pressure would be a volume that the tire could not physically hold. like a SCUBA tank but those are metal. the bigger volume exponentially increased way beyond the physical limits of the design. improve the design and maybe you got a shot at it

rpenmanparker 

You are doing it too, conflating what you have to do with what you want to do. People don't lower the pressure in their larger tires so that they won't blow off. They lower the pressure to have a comfy ride. They don't choose larger tires while worrying about them blowing off. They choose larger tires so they can run them at lower pressure. I'm not asking you why they have to lower the pressure but why they CAN lower the pressure. And BTW it is common for larger tires to have more robust heads so that the higher pressures could be used. There is no danger of 25 and 28 mm tire blowing off at normal pressures.

rumrunn6

oh ok. don't know why a bigger tire CAN run at a lower pressure. gimme 10 yrs & I'll get back to ya

but I don't think my new 45mm riddlers (whose max is 55 lbs) could be run at 120 psi