To the physicists here, ? re: tire pressure
07-24-14, 01:54 PM
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And totally missed answering the question- "My basic question is: why do larger tires, (internal volume?), take less tire pressure?"
Contact patch (the part of the tire in contact with the ground)- a 1" square contact patch at 80 psi will support 80 pounds. a 1/2 " contact patch needs 160 psi to support the same weight. Skinnier tires have a smaller contact patch when properly inflated, so they need higher pressure.
(cut me some slack- I had physics, but back when the school computers used punch cards.)
Contact patch (the part of the tire in contact with the ground)- a 1" square contact patch at 80 psi will support 80 pounds. a 1/2 " contact patch needs 160 psi to support the same weight. Skinnier tires have a smaller contact patch when properly inflated, so they need higher pressure.
(cut me some slack- I had physics, but back when the school computers used punch cards.)
07-24-14, 03:43 PM
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You don't have to be a physicist for this. PSI is simply the amount of pressure/force being exerted per square inch of tube/tire. While the actual amount of air you're putting in a larger tire may be more in terms of total volume, the fact that it's going into a larger area means the force being exerted on a square inch is actually less. That translates into a more comfortable ride. As tire size increases, the PSI is always going to decrease, even though the total volume of air is more.
But comfort on a bike also depends on your weight and other factors. Lots of folks can run standard 23 mm tires without putting 120 PSI in them and not pinch flat whereas heavier folks can't, which is why some look to 25 mm tires, tubeless and other things for a bit more comfort and protection.
But comfort on a bike also depends on your weight and other factors. Lots of folks can run standard 23 mm tires without putting 120 PSI in them and not pinch flat whereas heavier folks can't, which is why some look to 25 mm tires, tubeless and other things for a bit more comfort and protection.
07-24-14, 04:32 PM
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This question starts with a false assumption, namely that "harder" tires (more width x more pressure) roll easier. They do if the road is glass smooth, but rolling resistance is only a single factor among many.
Traction is probably the next bigger factor, and increased pressure reduces traction significantly. Comfort is another, and so is the tire's weight and wind resistance. The goal is to find the best balance of all of these. Research shows that rolling resistance drops rapidly as tires move from low to high pressure, but as the pressure rises, the rate of improvement diminishes, while the other factors begin to worsen measurably.
Since this is a matter of balancing a bunch of variables, there's no single best number for everybody. Riders, especially light riders, on smooth roads will favor high pressures and narrower lighter tires. Heavier riders, or those on lousier roads, do better with more width (raising the rim higher off the road) and lower pressures.
You might look at the auto industry. While there's variation in a band, thire pressures tend to be held at 24-32psi, and the tire width increased in proportion to the weight of the vehicle. You could drive an SUV on the little wheels made for a Miata, if you jacked the pressure high enough, but the handling and traction would go to hell.
BTW- while the principles and math are easy, optimizing and designing accordingly are harder. Use any pressure coming from a chart as a starting place, and experiment on either side, looking for either the lowest pressure before you feel an uptick in rolling resistance, or the highest pressure before you feel skittish handling or "road fatigue" from bumps.
If you've selected the right width tire, both methods will yield a similar pressure.
Traction is probably the next bigger factor, and increased pressure reduces traction significantly. Comfort is another, and so is the tire's weight and wind resistance. The goal is to find the best balance of all of these. Research shows that rolling resistance drops rapidly as tires move from low to high pressure, but as the pressure rises, the rate of improvement diminishes, while the other factors begin to worsen measurably.
Since this is a matter of balancing a bunch of variables, there's no single best number for everybody. Riders, especially light riders, on smooth roads will favor high pressures and narrower lighter tires. Heavier riders, or those on lousier roads, do better with more width (raising the rim higher off the road) and lower pressures.
You might look at the auto industry. While there's variation in a band, thire pressures tend to be held at 24-32psi, and the tire width increased in proportion to the weight of the vehicle. You could drive an SUV on the little wheels made for a Miata, if you jacked the pressure high enough, but the handling and traction would go to hell.
BTW- while the principles and math are easy, optimizing and designing accordingly are harder. Use any pressure coming from a chart as a starting place, and experiment on either side, looking for either the lowest pressure before you feel an uptick in rolling resistance, or the highest pressure before you feel skittish handling or "road fatigue" from bumps.
If you've selected the right width tire, both methods will yield a similar pressure.
07-24-14, 06:48 PM
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Because at the same pressure, the 38mm tire will be putting an outward pressure on the rim that is ~69% greater than the pressure imposed by the 23mm tire.
Source: Understanding the Influence of Pressure and Radial Loads on Stress and Displacement Response of a Rotating Body: The Automobile Wheel
Download the PDF, look at page 3, section 2.2, formula 5.
Reference Figure 4 to see what the variables mean.
Source: Understanding the Influence of Pressure and Radial Loads on Stress and Displacement Response of a Rotating Body: The Automobile Wheel
Download the PDF, look at page 3, section 2.2, formula 5.
Reference Figure 4 to see what the variables mean.
07-24-14, 07:26 PM
#31
don't try this at home.
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The theory is that a tire with some flex absorbs small bumps, instead of bouncing the bike upward. The rider's body soaks up the vertical movement and that takes energy away from forward motion.
The thin, flexible tires can flex over the bumps without wasting as much energy as stiffer, cut-resistant tires would.
~~~~~~~~~~~~~~~~~~
Continental tested their tires at different pressures. I don't know how they tested--is it a metal roller or something more like a real road? It does say "50 kg"
From another thread:
Continental's rolling resistance chart is interesting. The 25c rolling resistance is almost 20% worse (.49) at 80 psi than at 116 psi (.41) What units are they using in the chart? It appears to be Crr x 50 kg?
But: the 25c rolling resistance at 87 psi is equivalent to their 23c at about 112 psi. So larger tires can run at lower pressure and still be efficient.
~~~~~~~~~~~~~~~~~~~~~~
I slightly overinflate my 25c compared to what I was doing with my 23c. They seem very fast and very comfortable on rough roads.
I'm 170 pounds, and like 85psi front and 100-105 psi rear on Continental GP4000 25c. I should try 80 and 95-100 to see if affects handling. I was running 95 front and 105-110 rear on 23c.
The thin, flexible tires can flex over the bumps without wasting as much energy as stiffer, cut-resistant tires would.
~~~~~~~~~~~~~~~~~~
Continental tested their tires at different pressures. I don't know how they tested--is it a metal roller or something more like a real road? It does say "50 kg"
From another thread:
Continental's rolling resistance chart is interesting. The 25c rolling resistance is almost 20% worse (.49) at 80 psi than at 116 psi (.41) What units are they using in the chart? It appears to be Crr x 50 kg?
But: the 25c rolling resistance at 87 psi is equivalent to their 23c at about 112 psi. So larger tires can run at lower pressure and still be efficient.
~~~~~~~~~~~~~~~~~~~~~~
I slightly overinflate my 25c compared to what I was doing with my 23c. They seem very fast and very comfortable on rough roads.
I'm 170 pounds, and like 85psi front and 100-105 psi rear on Continental GP4000 25c. I should try 80 and 95-100 to see if affects handling. I was running 95 front and 105-110 rear on 23c.
Last edited by rm -rf; 07-24-14 at 07:41 PM.
07-24-14, 07:35 PM
#32
don't try this at home.
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The Pinkbike guys tested a carbon rim to over 200 psi before it split. I like how the tester is cringing away from the wheel as he keeps pumping and it starts making noises!
And that's a sturdy pump!
The carbon rim video.
And that's a sturdy pump!
The carbon rim video.
07-24-14, 07:45 PM
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You can turn the question around: why do skinny tires require more pressure? The main answer is, to prevent pinch flats. Suppose you drop off a curb. You don't want the rim to hit the ground. The fat tire has two advantages: 1) because it is fatter, it can put more rubber on the road as the rim moves toward the road. So that give more area over which the pressure can act; 2) because it is fatter, it has more distance over which to act before the rim hits. So that would be one criterion for sufficient pressure: enough to avoid a pinch flat when dropping off e.g. a 9 inch height. A skinny tire just requires more pressure.
Another aspect of the puzzle: fat tires cannot handle as much pressure as skinny tires can. There are two problems. The pressure can pull the tire apart, or the pressure can pull the rim apart. One important additional variable in all this is the rim width. A fat tire on a narrow rim will pull the rim sides apart more. The same tire on a wider rim will have the tire exiting the rim at a less horizontal direction and so exert less spreading force on the rim.
One way to think about the effect of pressure on the tire itself is to think about a clothes line. If the clothes line is not exactly taut but still has very little slack, so it runs almost straight between its supports, then when you hang a wet towel in the middle, the tension in the line will get quite high. If the line isn't very strong, it could snap. If the line has quite a bit of slack, then the tension won't be so high.
At the limit, where the line is very long, the tension will be just half the weight of the wet towel. On each side of the towel the line just pulls half the weight of the towel upwards. But if the line started quite straight... actually the tension can be extremely high. Each side still has to pull up with half the weight, but because the line is almost horizontal only a small part of the tension is upward. Most of the tension will be horizontal, the tension pulling away from the towel in either direction.
A wide tire doesn't curve sharply. That shallow curve still has to push in to hold the air pressure, but the tension mostly pulls away, stretching the tire surface. With a narrow tire that sharper curve means that more of the casing tension goes to pushing against the air pressure.
Here is my nifty tire pressure table: Interdependent Science: Bicycle Tire Pressure
Another aspect of the puzzle: fat tires cannot handle as much pressure as skinny tires can. There are two problems. The pressure can pull the tire apart, or the pressure can pull the rim apart. One important additional variable in all this is the rim width. A fat tire on a narrow rim will pull the rim sides apart more. The same tire on a wider rim will have the tire exiting the rim at a less horizontal direction and so exert less spreading force on the rim.
One way to think about the effect of pressure on the tire itself is to think about a clothes line. If the clothes line is not exactly taut but still has very little slack, so it runs almost straight between its supports, then when you hang a wet towel in the middle, the tension in the line will get quite high. If the line isn't very strong, it could snap. If the line has quite a bit of slack, then the tension won't be so high.
At the limit, where the line is very long, the tension will be just half the weight of the wet towel. On each side of the towel the line just pulls half the weight of the towel upwards. But if the line started quite straight... actually the tension can be extremely high. Each side still has to pull up with half the weight, but because the line is almost horizontal only a small part of the tension is upward. Most of the tension will be horizontal, the tension pulling away from the towel in either direction.
A wide tire doesn't curve sharply. That shallow curve still has to push in to hold the air pressure, but the tension mostly pulls away, stretching the tire surface. With a narrow tire that sharper curve means that more of the casing tension goes to pushing against the air pressure.
Here is my nifty tire pressure table: Interdependent Science: Bicycle Tire Pressure
07-24-14, 08:30 PM
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It's actually still a good answer, thanks. Those tires are constructed specifically to carry heavy loads and are designed to handle the pressure that can accommodate those loads. That's not the situation for tires in general however. Again, in general, as the size increases, the PSI needed decreases.
07-24-14, 09:07 PM
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This answers your question, although it is not spelled out. The pressure measured is exerted over the entire surface of the tire. A larger tire has more area, so less pressure x more area = the same force. This is why a bigger tire at the same pressure will exert more force on the rim as well.
07-25-14, 09:25 AM
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FBinNY:
Thank you, once again, for giving an informed and fact-based response ;o)
As a technical journalist, I've had the privilege of visiting two tire manufacturers (Dunlop in New York and Continental in Hanover). They were fascinating experiences, but, that's another story. I went away from both experiences with great admiration for those company's dedication to facts, dependency upon realistic testing, and engineering competence.
A tire is a practical thing. Yes, it depends upon engineering prowess, but -- a tire's (diesel truck or bicycle) design is evidence/scientifically driven. That is: theory is derived from facts (testing). I have participated in such testing and I can assure you that that the conclusions tire engineers make are based on demonstrable facts.
Continental and Michelin (perhaps more) agree that the 'correct' pressure for a bicycle tire is such that the tire's height is diminished by 15% under load (that is: with your fat-ass on board ;o)). Because of my experience with tire manufacturers, I consider this a reliable starting place for 'proper' tire pressures.
15% compression is for the entire tire height, including the bead. 20% is a more convienient measure as that allows for the 'hidden' tire bead inside the rim.
A typical road bike has a weight distribution of 40% on the front wheel and 60% on the rear, both my working bikes have this distribution. Because I weigh some 225 pounds, I use larger tires; I also run a smaller tire on the front than the rear to stay within the recommended 15% compression under load. This typical load distribution dictates that the front tire needs two-thirds the pressure of the rear to achieve the tested/fact-based recommendations of tire manufacturers.
Joe
Thank you, once again, for giving an informed and fact-based response ;o)
As a technical journalist, I've had the privilege of visiting two tire manufacturers (Dunlop in New York and Continental in Hanover). They were fascinating experiences, but, that's another story. I went away from both experiences with great admiration for those company's dedication to facts, dependency upon realistic testing, and engineering competence.
A tire is a practical thing. Yes, it depends upon engineering prowess, but -- a tire's (diesel truck or bicycle) design is evidence/scientifically driven. That is: theory is derived from facts (testing). I have participated in such testing and I can assure you that that the conclusions tire engineers make are based on demonstrable facts.
Continental and Michelin (perhaps more) agree that the 'correct' pressure for a bicycle tire is such that the tire's height is diminished by 15% under load (that is: with your fat-ass on board ;o)). Because of my experience with tire manufacturers, I consider this a reliable starting place for 'proper' tire pressures.
15% compression is for the entire tire height, including the bead. 20% is a more convienient measure as that allows for the 'hidden' tire bead inside the rim.
A typical road bike has a weight distribution of 40% on the front wheel and 60% on the rear, both my working bikes have this distribution. Because I weigh some 225 pounds, I use larger tires; I also run a smaller tire on the front than the rear to stay within the recommended 15% compression under load. This typical load distribution dictates that the front tire needs two-thirds the pressure of the rear to achieve the tested/fact-based recommendations of tire manufacturers.
Joe
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