rpenmanparker 

Would an engineer care to explain what higher air volume in wider tire has to do with being able to run lower pressure? I will tell you that while thinking about this recently, I came to the conclusion it has nothing to do with contained air volume. Assuming you want a uniform 15% tire depression ("drop") when the bike is loaded, two things happen when the tire is depressed. First the pressure increases inside the tire just s tiny bit. But the contact patch area increases significantly as the tire is squashed. It seems to me the reason you can lower pressure on a wider tire is that the contact patch with the ground is bigger on wider tires. The force at the contact patch has to support the bike. So a lower pressure times a larger area will give the necessary force (F=P X A).

Think about this as well: a lower profile tire would contain less air but have a larger contact patch. You should be able inflate it to lower pressure than a narrower, taller tire with a higher air volume.

Comments? I really want to know if I am making an obvious mistake.

f4rrest

Size of contact patch.

I think you answered it yourself.

But I'm not convinced of the benefit. Larger tires at lower pressure feel sluggish.

dabac

Yep, that's it.


I've thought about that WRT homogenous tires.


How far that technology can be taken remains to be seen.
But it seems fairly obvious that "no one" has spent any particular effort on making a line of performance oriented homogenous tires.


You can get tires made for different air pressure equivalents, but this doesn't seem to be put in relation to tire width and profile.
Meaning for the same rubber spec, a wider and/or bigger radius tire will most probably ride harder than a narrower, smaller radius tire.



The trouble IRL though is that you often encounter fairly sharp lumps and bumps.
These kinda destroy the nice pressure/surface area equation.
You need a bit of height to deal with that.


And for road riding, there are the aero concerns.


A bit of handling or comfort is probably easily sacrificed for a rim/tire combo that forms a slightly more aerodynamic shape.


And of course the fact that bikes lean to corner.

dabac

For the same design, a wider tire will be heavier. Ignoring pressure, heavier tires always feel less responsive.


Then there's the question of how much lower. Pressure low enough to allow significant wallowing and deformation isn't nice.


Don't forget the influence of tire tread and sidewall design.
My race-spec MTB tires for dry conditions ride nicer on half the pressure and twice the width than the 1.1" slicks on the commuter.
And maybe gives 1/4 of the mileage unless I shred the sidewall first.

gregf83 

Why? Presumably a larger tire can have a larger drop without being concerned with bottoming out.

The formula you quoted is correct and tells you the contact patch area is independent of tire width.

You can't have a narrow tall tire without very stiff sidewalls. Narrow tires naturally have a low profile and require a higher pressure to keep from bottoming out on the rim. If a tire is 'taller' it will also be wider.

HTupolev

The Frank Berto tire pressure rule of thumb.

gregf83 

That doesn't really answer the question. Not sure it's germane to this discussion though.

Sy Reene

I presume the 15% rule of thumb is the arbitrary point where deflating more than this, has a negative impact on speed for given amount of work, while inflating more than this doesn't yield any gain but leads to more discomfort. I wonder about similar -- especially in the tubeless vs tubed discussions.. "I can run much lower pressures in my tubeless without flatting..." But why would you?

WhyFi

The "why?" really depends on context. What do you mean by "much lower" and what's the reference point?

In any event, the two biggest factors would be comfort and keeping your tires on the ground instead of bouncing. If we're talking off-pavement, then there's also the grip factor - a larger contact patch on loose surfaces is a good thing.

69chevy

I am an engineer. You are overthinking it.


Lower pressure can be run with wider clincher rims, because the wider rim minimizes the "pinch" on the tube. Just widening the tire doesn't help here.


Pressure is a measurement of weight acting on an area.


If you increase the area (wider tire) then less "weight" (force of air pushing tire out) is necessary to support the weight of the rider pushing down. As a bonus, when encountering a bump that compresses the tire (and air inside), the lower pressure air will allow more deformation of the tire.


So in a nutshell, the wider tire can support the same weight at a lower pressure, yet can act as a better "suspension" over bumps. But... if the rim bed isn't wider, it will be even more prone to pinching than a narrower tire on the same wheel.

gregf83 

Yes, I couldn't find any justification for the 15% number other than a rule of thumb supposedly intended to 'optimize' tire pressure. Using the 15% number, Berto generated some graphs showing recommended tire pressure for a given weight and tire width.

The reasons for lower pressure are comfort and traction. The latter not relevant for dry road conditions. Mountain bikers will often run with the lowest pressures possible for better traction. That's why tubeless was embraced in the MTB community while roadies just argue about it.

wphamilton

Does it? I thought the pressure decreases a tiny bit under load, because the tire gets out of round (constrained volume increases). But that's not important to the question IMO.

The answer is, a wide contact point as opposed to a narrow one has less vertical depression for a given force. In other words, the rim is further from the ground when we hit the pothole, so it takes a bigger hit to get the pinch flat. "Narrow" being a misnomer IMO because it's really the length in the radial plane that's relevant. It just happens to also be narrower for bike tires. Just draw a horizontal line through the tire between the ground and rim. When that line is longer, the rim is lower.

If the area of the contact patch is axle load/tire pressure, then lower pressure increases the area and increases the length of the contact patch. But that's counter-balanced by the difference in shape of the patch, due to the wider tire. Which means at some pressure the wider tire is worse for pinch flats - we already knew that, but keeping in mind the "why" helps me conceptualize it abstractly.

rpenmanparker 

But besides anything having to do with the tube-rim interface, also wider tires compress less at a give inflation pressure and risk pinching the tube less for a given applied force. I'm asking why. I had previously thought it had to do with P1V1=P2V2 somehow. But, as my original post stated, now I think it is mainly due to the larger contact patch of the wider tire. Even if the pressure stays the same inside the tire when it is slightly compressed at the contact patch, the wider patch gives more push back force at the same pressure. The tire doesn't have to drop very much to get to the patch size that will resist the applied force of the bump or curb or pothole or whatever.

rpenmanparker 

I don't buy that "further away from the rim" explanation, because it is all relative. The curb or pothole doesn't have a limited "stroke". It can't just penetrate a fixed amount and then stop so that a taller tire would be more likely to isolate the rim edge from the road hazard. That isn't how it works. The hazard compresses the tire until the force pushing it is balanced by the force pushing back. That could be 1 mm or 1 m. The force pushing back is the internal pressure times the contact area of the road hazard on the tire surface. Greater contact area means greater force at same pressure. Even with a sharp pothole edge, the wider tire would have a wider contact area and balance the force with less penetration than a narrower tire.

But here is what I am objecting to. There is a rule of thumb that says no matter the tire size, it needs to contain the same number of air molecules to be "properly" inflated for a given rider and bike weight. So larger contained volume means lower pressure needed or P1V1=P2V2. I understand the gas law, but don't understand what it has to do with a tire being properly inflated for comfort and resistance to pinch flats. If you can explain why the number of air molecules needs to stay constant, I will stop arguing for the contact patch explanation.

69chevy

Again, you are overthinking it.


The tire with more internal surface area will compress less at the same pressure (with a riders weight on it) because the pressure (pounds per square inch) in the tire is acting on more square inches in the larger (wider) tire.

wphamilton

OK I'm not sure that I 100% buy the patch length argument either, but consider the situation where you have the same impact force, the same pressure and therefore the same contact patch area. One contact patch is wider and shorter than the other one. There is some force of impact where the narrower one is hitting the rim while the shorter one still has some air between the ground and rim.

It sounds like a rule of thumb to me, maybe derived from the rule of thumb advising 15% (or some other number) depression of the loaded tire. I don't know of any justification for either other than empirical, and I confess that I ignore the rule.

2manybikes

On many tires it will say "maximum inflation (I'll just say 100 for this example) - 100 P.S.I.) followed by (again just picking a number for an example) 120 P.S.I. with hook bead rim.

This is to prevent the tire from blowing off the rim. The hooked bead is more secure.

A wider, bigger tire has more surface area on the inside that is being pushed out by the air. Again, just using a number for as example, if you take off a tire, cut it in half. And lay it flat the bigger tire might have, say 50 square inches of surface to push against, the smaller tire might have, say 40 square inches. This increased surface area lowers how much pressure you can put in the tire, before it blows off the rim.

Many of the benefits already posted are true benefits. Lower rolling resistance due to different shaped contact patch, less likely to pinch fat at low pressure, etc. etc. No disagreement with them. Of course everything else being equal (it almost never is) the bigger tire is heavier.

But the lower maximum pressure listed on the tire is to prevent the tire from blowing off, due to the increased interior surface area of the larger tire.

rpenmanparker 

That is the same as the constant air molecules argument, but it has no theoretical basis. You are saying that the total force on both tire carcasses has to be the same. Why? We are not talking about uniform compression of the tire such as in a hyperbaric chamber. We are talking about compression at a small contact patch.

woodcraft

Six strokes of floor pump into road tire= rideable.

Six strokes into mtn bike tire= not rideable.

Not a very useful rule, IMO.

2manybikes

Exactly. Also...

The increased surface area of the bigger tire will also increase the force trying to push the tire off the bead. I think the lower maximum pressure listed on the tire, compared to smaller tires, creates lots of confusion.

gsa103

In general, the contact patch governs traction, static loading, and rolling resistance.
Air pressure behaves like a non-linear spring, the more the tire compresses, the more the pressure rises. Lower pressure is a weaker spring, high pressure is a firm spring.
A taller tire effectively has a longer travel, and a tire with more volume will behave more like a linear spring.

In response to large bumps, the wide tire at lower pressure is going to be more supple and absorb more shock.

boostieboy

Pressure and volume are inversely proportional to one another. It's simple physics.

rpenmanparker 

Only if you keep the number of air molecules (and the temperature) the same. Why should you do that?

rpenmanparker 

Actually air pressure acts like a linear spring. Consider a piston in a closed cylinder. Force and air pressure are proportional to displacement of the piston.

2manybikes

Every time I start to count the air molecules I lose track and have to start over.