High tyre pressure and puncture resistance

[RooEUC]

I have seen several comments around the internet (mainly on mountain bike forums) that high tyre pressure reduces puncture risk.  I'm wondering how true this is.  I keep my EUC and MiniPro pumped up to the max for this reason but I would really enjoy a softer ride when going off road (which is almost every day) and I'm considering going down about 10-15PSI lower.  Getting a puncture is a bad scenario that I want to avoid as much as possible because carrying such a weight home on foot 15km is a no go.  I'm thinking about the physics of how higher pressure might resist punctures and the only reason I can see it offering a difference is in the fact that there's less surface area touching the ground and so slightly less chance of hitting a sharp object.  This difference is very marginal though.  As far as the pressure itself goes, until the sharp object has pierced right through to the air filled cavity, the pressure makes no difference.  The thick layer rubber offers no more resistance when pumped up to 50 PSI than when at 40 PSI.  In fact, high pressure tubeless tyres will be more likely to blow out with a bigger hole than low pressure tyres when punctured.  So what's the story with this claim of high psi resisting punctures?  Maybe I'm wrong in my ideas here. 

[LanghamP]

If you ride through sharp debris then nothing will save your tire. It's going down. However, low pressure allows "snakebites" whereby the pressure is too low to prevent the wheel from bottoming out onto the road through the tire, hence the twin punctures known as snakebites.

[Mono]

On 9/22/2017 at 8:08 PM, RooMiniPro said:

I'm thinking about the physics of how higher pressure might resist punctures and the only reason I can see it offering a difference is in the fact that there's less surface area touching the ground and so slightly less chance of hitting a sharp object.  This difference is very marginal though.  As far as the pressure itself goes, until the sharp object has pierced right through to the air filled cavity, the pressure makes no difference. 

I think surface area is (only) part of the reason. The other part seems to become evident if you try to stick a nail in the tire yourself (or think of doing it). If the tire is soft, the nail will deform the tire and "get stuck" in the dip such that pushing it further will puncture the tire. If the tire is hard, the nail has a greater chance to slip off to the left or right before enough deformation has taken place. It's hard to quantify the difference though, which should be relevant for the decision. I have stopped to maximize tire pressure to avoid punctures in favor comfort. The trade off has become to avoid snakebites.

[Smoother]

48 minutes ago, Mono said:

I think surface area is (only) part of the reason. The other part seems to become evident if you try to stick a nail in the tire yourself (or think of doing it). If the tire is soft, the nail will deform the tire and "get stuck" in the dip such that pushing it further will puncture the tire. If the tire is hard, the nail has a greater chance to slip off to the left or right before enough deformation has taken place. It's hard to quantify the difference though, which should be relevant for the decision. I have stopped to maximize tire pressure to avoid punctures in favor comfort. The trade off has become to avoid snakebites.

I'm not saying you're wrong because I don't know the truth, but I would have thought, for some sharp objects, lower pressure is better.  Imagine this: you roll over a sharp flint like rock.  A sharp edge is upward.  On a hard tire, the tire resists deforming and therefore the full force of the sharp edge is pressed hard against the tire.  Now imagine the same rock, and a soft tire.  As the full weight of the rider rolls over the rock, it deforms, folding around the rock, spreading the load over more of the rock's surface, maybe even folding all the way to the ground, giving way to the sharp edge rather than forcing it into a "rock and a hard place" confrontation, literally.  After all, no one ever says "its like being stuck between a rock and a soft place".  Doesn't make sense does it!

In Bruce Lee's martial art (jeet Kune Do, "Art of the Intercepting Fist") He described giving way to the energy of your opponent rather than resisting it. I see a hard tire as resisting, and a soft tire as conforming.  Look at this clip (only about 35 seconds)

 

Of course a tire is not water, and a sharp rock is not a tea pot But you get the idea.  However i'm happy to be proved wrong, as this is just my theory, and theories are not facts until proven.

[LanghamP]

2 hours ago, Smoother said:

I'm not saying you're wrong because I don't know the truth, but I would have thought, for some sharp objects, lower pressure is better.  Imagine this: you roll over a sharp flint like rock.  A sharp edge is upward.  On a hard tire, the tire resists deforming and therefore the full force of the sharp edge is pressed hard against the tire.  Now imagine the same rock, and a soft tire.  As the full weight of the rider rolls over the rock, it deforms, folding around the rock, spreading the load over more of the rock's surface, maybe even folding all the way to the ground, giving way to the sharp edge rather than forcing it into a "rock and a hard place" confrontation, literally.  After all, no one ever says "its like being stuck between a rock and a soft place".  Doesn't make sense does it!

In Bruce Lee's martial art (jeet Kune Do, "Art of the Intercepting Fist") He described giving way to the energy of your opponent rather than resisting it. I see a hard tire as resisting, and a soft tire as conforming.  Look at this clip (only about 35 seconds)

 

Of course a tire is not water, and a sharp rock is not a tea pot But you get the idea.  However i'm happy to be proved wrong, as this is just my theory, and theories are not facts until proven.

An under inflated tire gets pinch flats while an over inflated tire gets puncture flats. Think balloon; what pops easier with less force? 

[Smoother]

7 hours ago, LanghamP said:

An under inflated tire gets pinch flats while an over inflated tire gets puncture flats. Think balloon; what pops easier with less force? 

So a MEDIUM  inflated tire is the best compromise then.  Which I suspect is where most of us ride.  When my tires are pumped up high, my wheels feel very darty, and wobbly, like i'm riding on a knife edged.  Not fun.  I learned like this, and rode hundreds of miles at 60+psi.  Since then I have learned the Cadillac feeling of a medium pressure, somewhere in the 40's; smoother and much less twitchy.  Smoother! Get it?  ah, never mind.

[Toshio Uemura]

14 hours ago, Smoother said:

I'm not saying you're wrong because I don't know the truth, but I would have thought, for some sharp objects, lower pressure is better.  Imagine this: you roll over a sharp flint like rock.  A sharp edge is upward.  On a hard tire, the tire resists deforming and therefore the full force of the sharp edge is pressed hard against the tire.  Now imagine the same rock, and a soft tire.  As the full weight of the rider rolls over the rock, it deforms, folding around the rock, spreading the load over more of the rock's surface, maybe even folding all the way to the ground, giving way to the sharp edge rather than forcing it into a "rock and a hard place" confrontation, literally.  After all, no one ever says "its like being stuck between a rock and a soft place".  Doesn't make sense does it!

In Bruce Lee's martial art (jeet Kune Do, "Art of the Intercepting Fist") He described giving way to the energy of your opponent rather than resisting it. I see a hard tire as resisting, and a soft tire as conforming.  Look at this clip (only about 35 seconds)

 

Of course a tire is not water, and a sharp rock is not a tea pot But you get the idea.  However i'm happy to be proved wrong, as this is just my theory, and theories are not facts until proven.

 

Talking about tyre pressure and puncture:

It's not going to be directly proportional. Leaving aside the quality of the tyre and what it will be rated for, the shape of the graph is likely to be a U shape:

at the lower pressures the tube will be susceptible to puncture because it cannot repel sharps adequately, in addition really low pressures might let you trap the tube between the road and the rim.

as the pressure increases, the tube and tyre will combine to actively repel sharps: the external object will need more and more pressure and exertion to break through, so becomes less and less likely to be able to do so

at some point, though, the tube's pressure becomes so high, that the risk of a blowout increases: flaws in the tyre, the tube, especially around the valve, are more likely to be exploited, also that collisions e.g. with potholes, will be harder to accommodate.
It's not going to be as simple as that, but might work as a general rule.

But:
Whatever the pressure-puncture relationship actually is, it is totally dwarfed by the choice of tire material/construction. The tire pressure choice is for handling characteristics, comfort and pinch-flat resistance. If you want to significantly increase puncture resistance, you do it by choosing a different tire, not by changing the pressure.