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Tire Pressure & relationship to punctures
I've noticed that when I keep my tires inflated to 80 or 90 psi I almost
never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the highway shoulders and city streets. Chuck |
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#2
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Tire Pressure & relationship to punctures
caaron wrote:
I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the highway shoulders and city streets. What are the tires supposed to be inflated to? |
#3
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Tire Pressure & relationship to punctures
Chuck Aaron writes:
I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c Michelins and I ride mostly on the highway shoulders and city streets. There is no logical explanation for what you observe. The firmer the backing, the easier penetrating a tire casing becomes. Just visualize trying to stab your tire with a jack knife with 20psi inflation compared to 120psi. Penetrating the tread requires a certain force for a given object and what you report does not follow. I believe two things could be at work. One is that when riding with more inflation pressure, you are riding faster and more attentive, especially because riding over rough spots gives a rougher ride, which should reduce flats. The other is that in an effort to make sure your low pressure hypothesis is correct, you are inadvertently making are avoiding debris in the road more than you were. I don't think you have a random sample of sufficient size to be sure of cause and effect. There are seasonal changes in puncture causes such as how many beer bottles get tossed from cars and how much puncture vine is along roadsides. You didn't say what was causing your flats so I can only guess guess. Often enough riders will report here that the new brand of tire they are using greatly reduced the incidence of flats even when there is no significant difference between old and new tire other than a puncture resisting name lime Armadillo or the like. Flats are cause by beer in winter and cigarettes in summer: http://www.sheldonbrown.com/brandt/thorns.html Jobst Brandt |
#4
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Tire Pressure & relationship to punctures
caaron wrote:
I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the highway shoulders and city streets. You're being more careful when the tires are softer. Problem solved; off you go... BS |
#5
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Tire Pressure & relationship to punctures
Chuck Aaron wrote: I've noticed that when I keep my tires inflated to 80
or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (clip) ^^^^^^^^^^^^ Jobst Brandt wrote: There is no logical explanation for what you observe. The firmer the backing, the easier penetrating a tire casing becomes. (clip) ^^^^^^^^^^^^^ Jobst, it looks to me like you may have misread what Chuck wrote. He gets more flats when the tires are harder. That is consistent with: "The firmer the backing, the easier penetrating a tire casing becomes." It looks to me as though there must be a steep break in the curve between 90 psi and 110 psi, according to Chuck's experience. Is this compatible with what you know from other sources? |
#6
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Tire Pressure & relationship to punctures
On Thu, 18 Aug 2005 19:00:25 -0400, "caaron"
wrote: I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the highway shoulders and city streets. Chuck Dear Chuck, Theoretically . . . Higher pressure makes a tire easier for a sharp object to penetrate. A softer tire will roll over and "tent" over a blunt enough piece of debris, while a harder tire will concentrate pressure and impale itself on the blunt debris. It's somewhat like lying on a bed of nails. If a 200-lb person lies down on a single nail, the pressure is 200-lbs/nail-point and drives the nail right through the flesh. If a 200-lb person lies down on 200 nails, the pressure is only one pound/nail-point, and the circus audience is disappointed. Similarly, a small piece of debris must push up hard enough against the tire to pierce it. To do so, the debris must be tall enough and sharp enough. Even a needle will not puncture your inner tube unless it's tall enough to reach through the tread, the casing, and the tube. On the other hand, a blunter piece of debris has to be tall enough to raise enough of the tire to concentrate enough force to shove the debris through the tread--otherwise, the tire just "tents" over the debris, which is why blunt pebbles don't go through the tire at normal pressures. (Squeeze the tire and blunt pebble hard enough in a vice and the stone will make a nasty hole in the tire.) At least two other factors confuse things. First, higher pressure reduces the number of impact flats. The greater the pressure, the greater the impact that the tire can absorb before the tube is pinched between the obstacle and the rim. Second, higher pressure changes the shape of the contact patch. At first, it would seem that the smaller contact patch of a higher-pressure tire would reduce the area swept by the tire and the amount of debris available to damage it. But higher pressures in the range that you mention often change the contact patch from a long, thin, oval to a shorter (and wider) oval: low high pressure pressure /XX\ XXXXXX \XX/ The ASCII diagram above is exaggerated, but illustrates the idea. The higher pressure tire has less area, but sweeps a wider path and therefore rolls over more debris. The high-pressure tire sweeps a debris path 2X wide, compared to the 1x of the lower pressure tire, and applies more pressure to the point of the debris that it encounters. So yes, higher pressure will encourage more punctures (rather than pinch flats) where the debris is only modestly sharp--great pressure will fall on more pointy objects. (With needle-sharp goatheads, pressure doesn't make much difference. It's quite common to see the filthy little things sticking out of the curved side of a tire.) Carl Fogel |
#7
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Tire Pressure & relationship to punctures
But higher pressures in the range that you mention often
change the contact patch from a long, thin, oval to a shorter (and wider) oval: low high pressure pressure /XX\ XXXXXX \XX/ The ASCII diagram above is exaggerated, but illustrates the idea. The higher pressure tire has less area, but sweeps a wider path and therefore rolls over more debris. The high-pressure tire sweeps a debris path 2X wide, compared to the 1x of the lower pressure tire ... Why do you think such an effect occurs? That would imply that increasing the pressure would result in a more obvious bulge of the sidewall in the vicinity of the contact patch. But the actual effect is the opposite; at 120 psi there is much less sidewall deformation at the bottom of the tire than there is with 80 psi. Assuming the same tire and loading, the contact patch at 80 psi will be both longer and wider than the one at 120 psi. |
#8
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Tire Pressure & relationship to punctures
On Thu, 18 Aug 2005 19:00:25 -0400, "caaron"
wrote: I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the highway shoulders and city streets. Perhaps the tubes are stretched thinner at higher pressures? Tubes that are sized the same on the box seem to have different thickness of rubber and different cross section size. Less rubber between the road and air space and more flats. |
#9
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Tire Pressure & relationship to punctures
Leo Lichtman writes:
Chuck Aaron wrote: I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently... Jobst Brandt wrote: There is no logical explanation for what you observe. The firmer the backing, the easier penetrating a tire casing becomes... Jobst, it looks to me like you may have misread what Chuck wrote. He gets more flats when the tires are harder. That is consistent with: "The firmer the backing, the easier penetrating a tire casing becomes." Oh, then that makes it all much clearer. But then again I want to know what sort of flats these are to understand whether the assessment is appropriate. Thorns would not be materially affected by this, they penetrating equally well at a range of pressures due to their high concentration of pressure, a sharp point. It looks to me as though there must be a steep break in the curve between 90 psi and 110 psi, according to Chuck's experience. Is this compatible with what you know from other sources? I can't substantiate that from my experience because my incidence of flats is too random and has insufficient frequency. Jobst Brandt |
#10
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Tire Pressure & relationship to punctures
Paul Kopit writes:
I've noticed that when I keep my tires inflated to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I keep them at 110-120psi I get flats very frequently (maybe one flat per week). Can anyone explain why that is, and if there is any disadvantage to keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the highway shoulders and city streets. Perhaps the tubes are stretched thinner at higher pressures? Tubes that are sized the same on the box seem to have different thickness of rubber and different cross section size. Less rubber between the road and air space and more flats. Tube rubber is essentially incompressible and the inside volume of a tire casing does not change with pressure. Therefore, inflation pressure cannot alter inner tube shape in a tire. Jobst Brandt |
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