Tourist in MSN

Here is a bit more information on that tire pressure topic I mentioned above.
https://www.adventurecycling.org/def...%5FHeine%2Epdf

Note that the weight on the graph is the weight on the tire, not the weight of the bike. On the graph that is called Wheel Load.

I usually run higher pressures than cited in that article if I have tires with a stiff casing, as that stiff casing at lower pressure feels a bit slow as the casing flex adds resistance. Tires with a more supple casing, it does not make the tire that much faster to pump it up harder so on a supple tire I might run a lower pressure.

Your initial question was on winter tires, my winter tires are Marathon Winters, they are an extremely slow tire but I find all studded tires are slow. See photo. But if I want to stay upright on ice, the studs are needed at the cost of a slower ride.

fantom1 

You didn't read very carefully. You DO put in more gas to maintain the pressure and volume. I said for a CONSTANT pressure and volume, the only variables are temperature and density/moles. You inflate the tires to whatever psi you want. If its colder, that means more gas inside.

If it's still wrong, so be it, but holy smokes, at least comprehend what was written. Seriously...

djb

re Continental Sport Contact, I see that I was mistaken, I am not familiar with these tires. I thought you had these ones:

https://www.mec.ca/en/product/5019-2...x32C-Wire-Tire

what I did see about the sport contact is that the recommended pressure is 85psi, but depending on your weight, you could easily run 75 or less on 32s. Especially from my experience using 45-50psi max. on my 35s carrying often two panniers full of stuff and those pressures being perfectly fine, no squirming or wallowing feeling at all (again, for MY weight)

cyccommute 

If I don't read very carefully it's because you didn't write it very carefully.

Let's start with volume. The volume in a tire is fairly fixed. The weight on the tire is not going to change so the volume doesn't change. Change the temperature and the volume doesn't change. The pressure will but the volume won't. Change the number of moles and, again, the pressure changes but the volume doesn't. The tire can't get bigger.

On to moles of gas. Yes, you can put more gas in the tire. But, honestly, that is going to be a very minor change in the number of moles of gas. Even under pressure, the amount of gas we are talking about is minuscule. A 35mm tire has a volume of, roughly, 1 l. It's a tiny fraction of a mole of gas. Any changes in pressure by putting more gas in the tire is going to be a fraction of a fraction of a mole. It won't make much difference.

But we can lay all the gas law stuff aside because there are other factors that have a greater impact on the ride than a tiny bit of gas does. The stiffness of the tire wall is going to have greater impact. The stiffening of the rubber as a response to cold is going to have a greater impact. The response of the rider to the cold will have a greater impact.

All these are better explanations than going to gas laws. Basic Ockham's razor.

Doug5150

I don't notice much in my own riding, but I have noticed that on some other bicycle Euro-centric forums that they make the distinctions of measured rolling resistance in "summer temperatures" and "winter temperatures". They also usually recommend different tires for both. The reason given for this is because when tire rubber gets cold the rolling resistance tends to change. All of them tend to harden, but some tires harden very little and others harden quite a lot.

Sometimes the lowest rolling-resistance tire for winter is one that doesn't really have very good rolling resistance in the "summer" group, but it just happens to hardly increase at all in cold temperatures where most other tires (that roll with less resistance in warm weather) see a larger increase.

wphamilton

To get to the target pressure he's putting more air in to begin with, because it's cold. Clear enough, though one could take issue with his "less compressible because denser". Compressibility is a function of pressure.

But personally I put in less air when it's cold, because I think I'll get better traction in the conditions we have here along with cold, and because the rubber gets harder.

fantom1 

Here, fixed your criticism of my post in one simple sentence.

Or in a longer sentence: I was right, but in your opinion the change in gas properties is insignificant and does not contribute to any difference in feel.

Fine, I can live with that, that very well may be.

I wrote perfectly well for anyone with a basic reading comprehension and a vocabulary that includes, "constant."

Why is it so hard for you to just say, "sorry, I misread your post"?

rm -rf

I have a multi-purpose bike: either gravel or long all-day solo road rides. I'm about 170 pounds.

29mm width
For road riding, I have some Continental 25mm tires. They were GP-4Seasons, for a little better sidewall cut protection over my usual GP4000S. I recently put on a GP4000S front. With my very wide HED rims, which are 20.5mm internal and 25.5 mm external width, these tires measure 29mm, so I inflate them for that size.

I use 65 psi front and 80 psi rear. On a recent long ride that unexpectedly had a lot of very rough roads, I blipped out some air to be about 60F, 75R. That helped.

39mm width
This fall, I got some expensive Compass Barlow Pass tires, nominally 38mm. They measure 39mm on my rims. They have no tread or knobs, just a minimal file tread pattern. These are very flexible, just like good road tires -- they don't try to be cutproof / flatproof which makes tires stiff and slower.

I want to use them for long distance crushed stone trail rides, expecting that they will float on the surface and be quite efficient. And they should work well on many gravel roads. They just fit in my frame, there's only minimal side clearance at the chainstays.

So I did some road rides with these wide tires. On paved roads, I use 38 psi front, 45 psi rear! (and will try 32F, 40R on gravel.) They are fantastic on broken pavement and on chip-seal. And I don't think they are any slower than my other tires. On group ride downhills, I'm gaining on the road bike riders.

Using my narrow tires, the steering is a little more responsive, and the bike is about 400 grams lighter, of course.

These tires are tubeless ready. Maybe I'll try that someday. I haven't yet, since I do swap tire types on these wheels fairly often, and the sealant would be annoying to deal with.

32 or 35mm width
The 38mm Compass tires (the "extralight" version) are about 400 grams each. My GP4000S 25mm is 190 grams.

Compass also makes 32 mm at 250 grams or 35 mm at 300 grams. If the 35 mm tires fit in your frame, these would be very good for road riding.At my weight: 32mm: I'd try 45-50 psi front, 65-70 psi rear. 35mm: perhaps 42-44 psi front, 54-58 psi rear.
See my tire pressure comment here.

cyccommute 

Which explains nothing and would probably have offended you more than pointing out why you are wrong.

People come to the Bike Forums to ask questions about all kinds of things. A 5 word answer isn't an answer. I know that I go on...and on...and on in my answers to questions but I'd rather give someone too much information than too little. "Because." is an answer but it's a useless answer. Perhaps "Because" followed by 1000 words is an answer that is too long but somewhere in that 1000 words people learn stuff.

Because you aren't "right". Look at your post again. You didn't write it all that clearly. "Constant" in this context could mean the same pressure between the two different tires. If you had said something about adding air to keep the pressure constant, your post would have been clearer.

And I did say that the change in gas properties were insignificant. That was the whole point of my original post.

Okay, I misread your post. But is it so hard for you to say that your post was poorly crafted? You've already said that you, basically, don't understand the principles involved.

fantom1 

1) Wow, you honestly do have a reading comprehension problem. Sorry man. I said nothing of "not understanding" the basic principles involved. Here is my post:

"Its been a while since I even thought about thinking about physics, but PVnrt would indicate that with colder temperatures, and constant volume and pressure, moles must increase (increasing density) and thus the gas mix is less theoretically less compressible for a given pressure, meaning a harder ride.

At least that is my memory of how gases theoretically work; I could be totally wrong, I do hate physics and math..."

2) You've done nothing to show I was wrong, only reinforced the opposite. As another user pointed out, I may have forgotten or confused the reasons for gas compressing, which is fine, my basic end point is still true. You're attempt to be contrary to me has contributed absolutely nothing worthwhile.

3) So lets say its a test question. I write: In a system that maintains constant pressure and volume, while the temperature decreases 20 degrees C, according to PV=nRT, the following is true:

A) The moles of gas must increase.
B) The moles of gas must decrease.
C) The moles of gas remain constant.


You chose C, which is incorrect, and then told me I'm wrong. Seriously, this is like a 6th grade level question. Stop it already.

wphamilton

I don't think it's true. At the same pressure (and same volume as cyccommute mentions), having a greater density will not decrease the gas's compressibility.

FBinNY 

Assuming you take the tire pressure at the same temperature, the cold doesn't affect the air. 80psi at 10F will act the same as 80psi at 90F.

What does change is the rubber, which definitely gets harder at lower temps. Whether you'd feel the difference or not depends on how thick the sidewalls and threads are. On light high performance tires I doubt you could tell. But you might feel it on heavy duty utility bike tires.

fantom1 

Huh, really? Doesn't density increase as compression increases? So if the gas is already a certain density (in this case more dense because of the cold), doesn't that mean it will take more force (pressure from the tire/tube compressing under the weight of the rider, etc.) to compress it, thus being less supple?

wphamilton

Simply: C=1/P (minus a deviation from ideal gas) where C is compression, P pressure. Generally speaking*, that simple.

Not density. It happens that density increases the same as pressure increases IF temperature and volume are the same. The ideal gas law that you quoted.

ie, it takes no more force to inflate at 80psi and 30° than it does at 80psi and 70°.
ie (2) the contact patch is the same size at 80psi and 30° as it is at 80psi and 70°.

*the deviation term is less simple, but depends on the gas and the pressure.

fantom1 

I suppose you know what you are talking about, but I'm looking online and see a lot of things like this: "In addition, the lower the density of the gas the farther apart the molecules must be, since the same number of molecules occupies a larger volume at lower density. We reinforce this conclusion by noting that liquids and solids are virtually incompressible, whereas gases are easily compressed."

So, lets say we have nitrogen. We compress it at a constant temperature and changing volume (increasing pressure) until it is a liquid. Lets say it goes to a liquid state at a made up "10" on a scale of 1-10 for pressure. Now we decrease the temperature 100C and perform the same thing. Doesn't it turn to a liquid at less pressure, say an "8" on this theoretical scale of force/pressure. Basically the old boiling water at a lower temperature with a lower pressure.

And if that is the case, that liquids are less compressible, then I don't understand how increased density wouldn't mean more pressure required to compress it further? Is it always a quantum state change and not linear?

wphamilton

Liquids are not described by the Ideal Gas Law.

If you want to investigate state changes of gas to liquid (it's not quantum) I suggest looking at "triple point" and go from there, but this is off-topic. We're just talking about air in tires.

A change in pressure from a warm garage to freezing cold is as someone mentioned, not very much and furthermore would result in a softer tire not harder. Pressure changes by the ratio of the temperatures in the Kelvin scale (7% lower in this case, from 72° to 32°).

fantom1 

Right, but if its not quantum, then the closer it becomes to a liquid (ie an increase in density), then the less compressible it is, right? That's my whole point. Not talking about liquids, I just used that because it is an extreme which helps illustrate my point/question.

prathmann

At the pressures and temperatures found inside bicycle tires the ideal gas law is a very good model for the behavior of the air inside and therefore compressibility is independent of the density. If you were to compress the air much more to the point where some of the gas components are close to liquefying then you start to see significant deviations from ideal gas behavior - but that's far from the regime inside bike tires.

fantom1 

Makes sense that's its a minuscule difference, but I still don't understand how it can be independent based on the very definite fact that it is denser. If the change from gas to liquid is not quantum, then it is linear or exponential, etc., then there must be a change in compressability dependent on density. Things like wind resistance, which seem to be based on density and the compressability/flow/density of a gas, is very different and notable for different real-world temperatures. Of course this is viscosity and drag, but they all appear to be highly related and at least somewhat proportional. I guess I am missing something obvious here.

djb

hey there Mr Goater, in case you are still reading this responses, I actually checked my tire pressures on my commuter with 35mm tires and they were kind of what I had said earlier:

45 psi rear
37 psi front

me at about 140, one pannier with lunch, clothes, repair stuff, pump, two locks, a bag of stuff like a phone, wallet and other junk like that, plus some extra clothes and mitts. On a bike that is probably 35lbs.

today was about 1 or 2c, so just above 32f, mid 30's ish.

on my typically Montreal potholed road, these pressures were fine--no squirminess, no wallowing, nice suspension effect to an extent but handled bumps and potholes while still riding well-- and I hadnt added air probably since you first put this good question up, so they probably had 5psi more or a bit more.

many years ago I would never have put my tires that soft, and would have thought those numbers were nuts, but like I said, over the years now Ive been bringing the pressures down and I'm convinced now.

I used to have motorcycles, even raced a bit, and on a bicycle put a huge importance on how a bike rides and goes around corners and feels confident riding. In other words, I really pay attention to how tire pressure affects ride and will always put an emphasis on safe and competent ride quality.

these pressures work for me at my weight, and wouldnt recommend them anyone if I was not 100% sure that they are safe.

so at these temps, these pressures give a good ride over these surfaces for these 35mm tires. Other bikes of mine with 40mm and 50mm tires can feel harsher than this bike did today if I overdo the pressures in those tires.....its all about pressures

(and yes, I still find that cold makes tires stiffer, but then I notice it in my car also)

good luck experimenting with pressures

btw, what pressures were you running with both sets of tires ? I dont think you ever mentioned.

WNCGoater

Yes, still reading any responses, though I've kinda lost interest in the physicists' whizzing match above. Anyway, in those 32's I ran 75psi which is max. I always run full pressure in my 25's on the road bike as I've had a couple of pinch flats from running over gravel. I'm 200lbs so not a lightweight. You aren't the only one who has suggested dropping pressure.
I'm going to start by dropping 10 pounds down to 65 and see what happens.


Happy Thanksgiving everyone!

djb

If you have a small gauge, you can even start out a ride at 75, after a while take out 5, ride a while, then take out another 5. That will give you the most immediate feedback to how small changes make the ride different. Oh, and have a pump in case you mess up and take out too much, or dont like how it feels.

to me, the ah ha moment came years ago after reading and hearing of lower pressures equaling a better ride and not slower, I finally started experimenting, and the added huge bonus for me was how I could corner so much faster. When you get the pressure right, your tire still rolls well, but it then has that little give and "suspension effect" when you corner and there is a bump. Just like with downhill skiing, when our legs are working well, we suck up the bump mid corner and maintain good ski to snow contact....or a car or motorcycle suspension that isnt overly stiff or overly sloppy, same thing.

anyway, try diff pressures and go with what feels best. Without knowing your tires, who knows how they are, but next time you are in a bike store, feel the sidewall of diff tires and see the wide range of stiffness. Generally a super tough tire with a super stiff sidewall will ride kinda harsh and blah---less flexing.

Erick L

You also have less adherence at cold temperatures.

FBinNY 

Real gas (air) doesn't exactly adhere to the ideal gas law. However the difference is tiny within the bounds of the temperatures and pressures we're dealing with on bicycles.

Before worrying about stuff like this, you have to consider the materialities. In short any difference that isn't enough to be seen, felt, or measured isn't a difference for practical purposes.

So much debate here on BF is about stuff that isn't material. So, while rubber may harden in the cold, we can consider the air to remain unchanged, except for any temperature related pressure change, which is only about 2% for every 10 degrees.

djb

stop being so goddamn rational buddy, let them argue about microns and thingamabobs and a few percentage of heavier atoms.....

and now to the weather, my ride today was at about -4c, so I imagine my piddly pressures went down a smidge due to the cold, but they were still ok and rode well.
Well, in a practical sense.