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To the physicists here, ? re: tire pressure

Old 07-22-14, 11:22 AM
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To the physicists here, ? re: tire pressure

Never had the opportunity to take physics in school. It's on the bucket list, but will have to wait for retirement in a couple more years.

I've been wondering about this for a long time, and a whole lot more recently, spurred by putting 28mm tires on one of my new frame builds.

My basic question is: why do larger tires, (internal volume?), take less tire pressure?

For example, when I used 23mm tires, I ran them at 120 psi. Switching to 25mm tires, I dropped that to 110 psi. Now with the 28mm tires, it has been recommended to me to go with 85-95 psi, (I was lamenting that I didn't get the comfort I expected, probably due to riding with 110 psi in them). To take it to the extreme, automobile wheels run at 35-32 psi.

In these four examples, each increase in internal tire volume take less air pressure. Why is this?
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Old 07-22-14, 11:26 AM
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My guess is that the larger the cross section of the tire, the more resistance it (the rubber structure itself) offers against deformation. More rubber, more resistance. Oh and I'm originally from RC just north of AV.
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Old 07-22-14, 11:29 AM
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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.
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Old 07-22-14, 11:33 AM
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I thought it was a simple matter of how much pressure is needed to keep the rim away from the riding surface. Thinner tires have less "travel" than wider ones, so more pressure is needed to keep the tire from "bottoming out" when it hits a bump or pothole, if you get what I mean.
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Old 07-22-14, 12:14 PM
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Science and Bicycles 1: Tires and Pressure | Off The Beaten Path
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Old 07-22-14, 12:16 PM
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It's a matter of larger tires having a larger area of support.
Atmospheric pressure is over a ton/ft^2
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Old 07-22-14, 12:22 PM
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Simple stupid answer.....the bigger tire wont pinch flat as easily.
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Old 07-22-14, 01:08 PM
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We all stated and described the same phenomena using different terms. Damned physicists, like economists and politicians, can't even come to an agreement!
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Old 07-22-14, 02:11 PM
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Originally Posted by oldscool View Post
simple stupid answer.....the bigger tire wont pinch flat as easily.
winner!
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Old 07-22-14, 03:02 PM
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I love it when "physicists" are asked questions on BFN. The more "physicists" who respond, the more different answers one gets!

I would rephrase the question a bit:

If 120 or 110 psi is good for a 700x25 or 23, why isn't it good for a 700x38? Especially when research (by physicists?) show that larger tires with more pressure have less rolling resistance, generally.

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Old 07-22-14, 03:02 PM
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I suspect that most riders tend to look at the wrong end of the equation.

Many riders choose skinny tires so they can use more air pressure thinking more air pressure = lower rolling resistance.
Wider tires allow you to use less air pressure, get a cushier ride and still maintain the same rolling resistance.
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Old 07-22-14, 03:33 PM
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Originally Posted by DnvrFox View Post
If 120 or 110 psi is good for a 700x25 or 23, why isn't it good for a 700x38? Especially when research (by physicists?) show that larger tires with more pressure have less rolling resistance, generally.
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.
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Old 07-22-14, 04:20 PM
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This isn't a physics question, it's a geometry question.

The bike rides on air, and is supported by a force equal to the pressure x the area of the contact patch. Since the ground is flat, and the tire is donut shaped, the tire distorts until the contact patch is the right size for the rider's weight and tire pressure.

imagine cutting a slice out of the rim of a donut and looking at the area you've exposed. The larger the donut, the less in from the edge the cut has to be to expose the same area. Likewise the tire, fatter tires make a larger contact patch with less distortion, and so can run at lower pressures for the same load.
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Old 07-22-14, 04:24 PM
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Originally Posted by Shimagnolo View Post
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.
Is that bad or dangerous? Can the rim take that?
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Old 07-22-14, 04:30 PM
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Originally Posted by DnvrFox View Post
Is that bad or dangerous? Can the rim take that?
Only the folks who have done it successfully can answer. The others are all incompacitated or worse.
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Old 07-22-14, 04:30 PM
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Originally Posted by DnvrFox View Post
Is that bad or dangerous? Can the rim take that?
Within reason it's fine. Rim makers estimate the tire width and pressure their clients are likely to use, using the same "recommended tire width" guidelines consumers use. They also monitor what tire companies are rating their tires for, and engineer accordingly allowing for some error on the high side.

Also consider that the side of the rim wears thinner from brake wear over time, so the engineering limit is based on the worn thickness, allowing even more error on new rims.

Except at extremes, hoop stress the tire applies to the rim shouldn't be a worry, but it is something to keep in mind if you like wide tires and high pressures.
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Old 07-22-14, 04:52 PM
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Originally Posted by DnvrFox View Post
I love it when "physicists" are asked questions on BFN. The more "physicists" who respond, the more different answers one gets!

I would rephrase the question a bit:

If 120 or 110 psi is good for a 700x25 or 23, why isn't it good for a 700x38? Especially when research (by physicists?) show that larger tires with more pressure have less rolling resistance, generally.
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.
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Old 07-22-14, 05:25 PM
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Originally Posted by volosong View Post
Never had the opportunity to take physics in school. It's on the bucket list, but will have to wait for retirement in a couple more years.

I've been wondering about this for a long time, and a whole lot more recently, spurred by putting 28mm tires on one of my new frame builds.

My basic question is: why do larger tires, (internal volume?), take less tire pressure?

For example, when I used 23mm tires, I ran them at 120 psi. Switching to 25mm tires, I dropped that to 110 psi. Now with the 28mm tires, it has been recommended to me to go with 85-95 psi, (I was lamenting that I didn't get the comfort I expected, probably due to riding with 110 psi in them). To take it to the extreme, automobile wheels run at 35-32 psi.

In these four examples, each increase in internal tire volume take less air pressure. Why is this?
Oh, why didn't you ask about helmets, Grant Peterson or cleaning chains? Get ready for godknowswhat. Read the side of the tire. Live with it.
Ride your bike and enjoy yourself.
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Old 07-22-14, 05:48 PM
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Originally Posted by FBinNY View Post
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.
Thank you for your response.
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Old 07-22-14, 06:33 PM
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Originally Posted by FBinNY View Post
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.
But if you look at the auto industry, all the "high efficiency" models use narrower tires at higher pressure than the mainstream models. If you are old enough to remember when they had the yearly fuel economy contest ( I think it was Mobile, but could be mistaken) they always used narrow tires at high pressure.
Just as an aside, most people don't push their cars hard enough to know if they handle well, or how good their tires are. What passes for "handling" is steering precision. If you have not slid, you don't know how your car handles. Have you ever used your space saver spare? Did you notice any difference?
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Old 07-22-14, 07:08 PM
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Originally Posted by MikeWMass View Post
But if you look at the auto industry, all the "high efficiency" models use narrower tires at higher pressure than the mainstream models. If you are old enough to remember when they had the yearly fuel economy contest ( I think it was Mobile, but could be mistaken) they always used narrow tires at high pressure.
Just as an aside, most people don't push their cars hard enough to know if they handle well, or how good their tires are. What passes for "handling" is steering precision. If you have not slid, you don't know how your car handles. Have you ever used your space saver spare? Did you notice any difference?
Yes, this is consistent with what I posted. Narrower/higher improves rolling resistance, aka fuel economy. They do this on cars sold on this basis. Who cares if the handling sucks. OTOH, on cars sold based on other performance features, ie sportier cars, or cars where they want to emphasize comfort and traction, it's back to the wider tires at lower pressures. Also consinder that there's a fuel economy benefit to smaller lighter wheels, which is another reason economy cars look like they're using junior wheels.


In the end it's a matter of striking a balance based on one's specific priorities.

Also consider the effect of width. With a wider contact patch the length will be less (for the same area) so there's less distortion of the tire on the ground, and less power lost to hysteresis. But that added width means added weight and air drag, so there's an offset.

Toss all this info into a hat, add your personal bias, and pick a tire that looks like it's work for you. Then dial in the pressure by experimentation over time..


BTW-, consider that there's a fashion/trend component to all this. For years since the seventies, the tires used by the peloton got progressively narrower, and were ridden at higher pressures, eventually getting down to 18mm or so. Since then the pendulum is swinging the other way, and slightly wider tires are in vogue, and more of the pros are listening more to team mechanics and riding at slightly lower pressures, especially in less than ideal conditions.
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Old 07-22-14, 07:16 PM
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Old 07-22-14, 07:53 PM
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The elephant in the room is the fact that the rear tire bears a significantly heavier load than the front, meaning that it should be inflated to a higher pressure. I compensate partially by running about 90PSI front, 100 rear, on "28"mm Contis which are about 26mm across. (I weigh 150 lbs.)
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Old 07-22-14, 08:10 PM
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Originally Posted by John E View Post
The elephant in the room is the fact that the rear tire bears a significantly heavier load than the front, meaning that it should be inflated to a higher pressure. I compensate partially by running about 90PSI front, 100 rear, on "28"mm Contis which are about 26mm across. (I weigh 150 lbs.)
More like a mouse scurrying about. Just about everyone understands that "weight" really refers to each wheel differently, and that rears call for more pressure or width or both.
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Old 07-24-14, 01:48 PM
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