Hokay, rare occasion when I ask for help - but it was bound to happen

Without questioning the infinite wisdom of the LFS tire physics model - can someone please answer the following questions (with proof hopefully).

LFS tires are "preheated" already when we exit the pits to let's say 15°C below their "optimum" operating temperature. The color of everything in that stage is dark blue (surface, tire walls, pressure inside the tire). As we go out and do a lap or two the temperatures of the thread ofcourse go up and over the tire's life span they can go to the red zone and then slowly go back to brownish and green and in the end back to blue - when they usually pop. Now, the inside of the tire (the area where the air is) also changes color - goes to blue-ish/green or green during a normal race, but can go to brown/red if you just do a burnout - my understanding is that this color represents the pressure inside the tire (and not the air temperature inside the tire - cause that would be pretty useless).

Question number 1: (by Jonesy) "like, say I put 200kPa in a tire, warm it up to 120degs would give me the same amount of psi in a tire as if I put 180kPa and warm it to 120degs?"

With my limited knowledge of thermodynamics I kinda used a simplified politropic equation to calculate the following example: We have a tire with 200 kPa @ 25°C, we warm it up to 90°C and get 240 kPa in it (these kinda numbers are usually correct in "other sims", I cannot say for real life because I never actually measured a pressure after the tire has been driven/races - but it's safe to say the pressure goes up - the problems come up when we ask by "how much"). We have a second tire with the cold pressure of 220 kPa @ 25°C and we warm it up also to 90°C - how much will the pressure rise? The method I used for this (not taking into consideration how the tire actually produces heat and ignoring the volume etc) is a simple politropic function that states

T2/T1 = (p2/p1)^(n-1/n)

T's - are temperatures in Kelvins
p's - pressures in kPa
n - politropic exponent (not sure about the proper english term)

From the first case (200 kPa @ 25°C -> 240 kPa @ 90°C) I got the n being 12.238 (which is wierd because n is usually from 1-1.4, 1.4 being kappa). But let's go forward anyway, that n was used for the second case to get the p2 or the hot pressure of the tire. The resulting p2 equaled 272 kPa (from 220 kPa @ 25°C to 272 kPa @ 90°C). So the answer to Jonesy's question was: you would get a higher resulting pressure in the first case (the higher cold pressure).

Now back to the LFS realm. One thing I've noticed in LFS is (if the inside colouring is actually the pressure) the lower cold pressure you put into a tire - the higher resulting hot pressure you will get. This is kind of opposite to the example I calculated above - but I think it is correct (and my whole calculation was an EPIC FAIL) because it takes into consideration of how the tire actually produces heat.

Tristan (as you are probably going to be the first one to reply cause ur such a forum fanboy ) - correct me if I'm wrong: as I understand it, the tires' ability to produce heat depends on it's ability to deform - no deformation no heat. Ofcourse the actual friction and sliding (slip angles) of the thread help this quite a bit - but in LFS not as much. Why not as much? The example I can provide is if you take a MRT out on any track, put 150 kPa cold pressure in all the tires (cold in this case is already preheated but who cares) - drive some laps and notice that the color inside the tire (pressure area) remains blue or even goes darker towards black - surface temperature also remains blue or barely greenish. Now cut the cold pressure in half say 70-80 kPa and do some laps. Resulting temperatures and pressures are both higher, inside of the tire is green, sidewalls are green, surface temps are green.

So if my understanding was correct.. When a tire has a lot of cold pressure and is just rock hard, it cannot deform and produce temperature. But then again - if you put in lower cold pressures and you get a higher hot pressure because of it. On the other hand, the overinflated tire should be prone to sliding much more and the surface temperatures should actually be higher, shouldn't they? Just to get back to MRT example for a sec - the cold pressure that you leave the pits with actually drops even more as you are driving and the air is cooling the tire - I would expect it to at least remain constant, but I guess the rate of cooling is higher than the rate with which it can produce temps? That's kinda the question number 2

Finally, question number 3/suggestion: (maybe move this post into Improvement suggestions section - I've only put it here because it will get more views ) Can we please get a pressure reading added into the F9 menu - so just a number inside the color of the "air" that will tell us the exact value of the pressure as the tire goes through it's temperature/life cycle? I don't think this would be such a huge deal to add, especially if we consider real life where we could test this very easily - just inflate a tire - drive - connect the pressure gauge after 2 laps. There are even pressure sensors in newer cars that monitor pressures in real time and report to the driver (for all the people who were about to say "Can u see tire pressures in real cars while driving?" - yes you can.)

P.S. One more thing. The F9 menu inside a FZR looks normal (tire load above tire temp in numbers above the thread temps in color) but in FO8 and BF1 the temperatures in numbers are overlayed on the thread temps in color making the colours and the numbers hard to read - this might just be due to the fact that tire profiles in openwheelers are larger than in GTR's but maybe it can be fixed somehow? If it's too much hassle then forget about it.

Quote from scipy :

One thing I've noticed in LFS is (if the inside colouring is actually the pressure)

Doesn't the the innermost color tell the temperature of the air inside?

EDIT: Ignore Dr. Zoidberg once again.

The colour is the tyre air temperature, not the pressure. Also as far as I know, in the setup you specify hot/optimal/green tyre pressures. So if you set it to 200kPa, when you're put on the track you'll actually have less then 200kPa in the tyres, since optimum tyre temperature is not reached yet.

Quote from scipy :

One thing I've noticed in LFS is (if the inside colouring is actually the pressure) the lower cold pressure you put into a tire - the higher resulting hot pressure you will get.

This for example shows where you go wrong. If you put less air in it simply means that first there is more deformation to cause heat, and second there is much less air inside, so it's easier to heat up (goes green quicker). "Phenomenon" easily explained by: colour = temperature, not pressure.

oh crap. I just took a look at the "advanced setup guide" and yea - it's the temperature inside the tire and not the pressure. Which is quite pointless On the other hand, you say that the tire pressure that we chose in the pits/garage is the one that we end up with when the tire is at optimal temps - but how can this be? The logical thing would be that it's the pressure in the tire @ the temp you leave the pits with.. can someone prove this or the opposite?

I cannot find the post right now, but I have it lingering in the back of my head that Scawen once said the set tyre pressures are for optimum temp, not cold temp.
E: Not true, read below:

Maybe this post helps a bit in understanding the pressure <> heating relation.
http://www.lfsforum.net/showthread.php?p=52917#post52917

E: LOL, well maybe it was the other way round and they are indeed cold pressures. Arrgh
http://www.lfsforum.net/showthread.php?p=596365#post596365

This page may be of some help:

http://hyperphysics.phy-astr.g ... e/kinetic/idegasc.html#c1

As a rough guide according to my calculations a typical road car tyre will have a volume of around 45 ltrs. It's probably best to assume, (for the sake of simplicity) that the pressure increases of the gas caused by temp increases are not going to change the volume, given that the air is "bounded" by a tyre.

Just a note on your calculations: if you use polytropic with n=1.4 you are doing an adiabatic process. This means no heat was added which is not the case. Isochoric (constant volume) is more what a tyre is like.

Ok. Conclusion. Pressure set in the pits is "cold" pressure, and when the tire warms up 20-30°C the pressure obviously rises but not by a lot - is there any way to actually know the value of the pressure at a certain tire temp? I mean other than guestimating or "ideal case" thermodynamics. I would like to know how LFS handles pressure changes - anything other than that is an exercise in futility.

Quote from scipy :

Ok. Conclusion. Pressure set in the pits is "cold" pressure, and when the tire warms up 20-30°C the pressure obviously rises but not by a lot - is there any way to actually know the value of the pressure at a certain tire temp? I mean other than guestimating or "ideal case" thermodynamics. I would like to know how LFS handles pressure changes - anything other than that is an exercise in futility.

you are trying to outsmart yourself.

the pressure you set in the pits is what the tire will have when it's at optimum temp, so it will actually be lower when you first hit the track, and given that the tires have a fixed volume, the calculation of pressure vs temperature is dead simple. no fancy pants required.

From what I have read in this post, there has been no proof of that^,as the giver of the evidence (AndroidXP) seems confused which it is, unless Scawen comes and clarifies.

Well, I can add some real life temp results... Before an autox run, I set my front tires to 42psi. The tires were about air temp, so ~65F. After the run, I checked the pressure and it read 51psi. The tires I would say were about ~110-120F by the feel of my hand on them as I forgot my digital thermometer. So, a 9psi rise for ~45 degrees. 9psi is a pretty large increase imo.

Quote from gezmoor :

This page may be of some help:

http://hyperphysics.phy-astr.g ... e/kinetic/idegasc.html#c1

As a rough guide according to my calculations a typical road car tyre will have a volume of around 45 ltrs. It's probably best to assume, (for the sake of simplicity) that the pressure increases of the gas caused by temp increases are not going to change the volume, given that the air is "bounded" by a tyre.

for normal i think the volume goes up too ? cause the tires goes more round by more pressure , caused of more themperatur ?

PV = nRT http://en.wikipedia.org/wiki/Ideal_gas_law

Quote from legoflamb :

From what I have read in this post, there has been no proof of that^,as the giver of the evidence (AndroidXP) seems confused which it is, unless Scawen comes and clarifies.

You can take that quote of Scawen as the truth. I am sorry if my foggy memory up until that point confused things, you can ignore my earlier post in that thread.

Quote from zsmy :

for normal i think the volume goes up too ? cause the tires goes more round by more pressure , caused of more themperatur ?

PV = nRT http://en.wikipedia.org/wiki/Ideal_gas_law

Yes the volume will increase in real life, because of course the tyre is not a solid boundry and it will flex. The problem is there is no way to determine by how much the volume will increase because we don't know the stiffness of the tyre itself. It's likely to be a relatively small change in volume anyway and therefore wont have a significant affect on the tyre presssure.

I did a quick calculation using the fomula on that site, assuming the pressure as set in LFS is the final working pressure when the tyre is at optimal pressure, (as stated by Scawen in reply to an email of Bobs). I used the R1 Slicks as an example, (70 deg optimal, 50 deg when "cold"), and calculated that the pressure difference between cold and optimal temps was 1.5 psi (or there abouts). I assumed the internal volume to be 44 litres, (which is probably too low given that its a high walled slick), but at the temps and final pressures involved even doubing the volume had zero impact on the initial pressure calculated.

Quote from scipy :

Ok. Conclusion. Pressure set in the pits is "cold" pressure, and when the tire warms up 20-30°C the pressure obviously rises but not by a lot - is there any way to actually know the value of the pressure at a certain tire temp? I mean other than guestimating or "ideal case" thermodynamics. I would like to know how LFS handles pressure changes - anything other than that is an exercise in futility.

With out knowing the exact volume of the tyres, no. But we can make educated guesses, (see post above).

Just to clarify, as quoted in this post, we are indeed setting the cold tyre pressures in the garage.

In my head Bob quoting Scawen somehow became Scawen saying what Bob initially said, so again, we're not setting hot pressures, but the cold ones

Quote :

On the other hand, the overinflated tire should be prone to sliding much more and the surface temperatures should actually be higher, shouldn't they?

In the real world this is correct, over inflation *like the high pressures often used in the game* would cause the tire to slide massively and this would severely overheat the surface of the tire without building heat within the tire.

Overheating the surface while under heating the sidewalls and the absolutely critical bead area between sidewall and surface.

The high pressures often used in the game would destroy the tire completely within just a few laps. Overheated slicks are also effectively useless there after.

Best, Maz

Quote from AndroidXP :

Just to clarify, as quoted in this post, we are indeed setting the cold tyre pressures in the garage.

In my head Bob quoting Scawen somehow became Scawen saying what Bob initially said, so again, we're not setting hot pressures, but the cold ones

Opps I missread that too !

Doesn't affect the calculation however. If the temp change between cold and optimal is 20 deg, (as it is in the case of the R1), the change in pressure between the two remains the same irrespective of which one you are setting in the pits. ie pressure difference between cold and optimal remains approx 1.5 psi.

Quote from gezmoor :

Opps I missread that too !

Doesn't affect the calculation however. If the temp change between cold and optimal is 20 deg, (as it is in the case of the R1), the change in pressure between the two remains the same irrespective of which one you are setting in the pits. ie pressure difference between cold and optimal remains approx 1.5 psi.

You're wrong again.

Cold to warm:
P(cold)*(T(warm)/T(cold)=P(warm).

Warm to cold:
P(warm)*(T(cold)/T(warm)=P(cold).

If look at the two formulas, you'll notice that difference P(warm)-P(cold) is more in the first formula than in second.

Quote from gezmoor :

Yes the volume will increase in real life, because of course the tyre is not a solid boundry and it will flex. The problem is there is no way to determine by how much the volume will increase because we don't know the stiffness of the tyre itself. It's likely to be a relatively small change in volume anyway and therefore wont have a significant affect on the tyre presssure.

I did a quick calculation using the fomula on that site, assuming the pressure as set in LFS is the final working pressure when the tyre is at optimal pressure, (as stated by Scawen in reply to an email of Bobs). I used the R1 Slicks as an example, (70 deg optimal, 50 deg when "cold"), and calculated that the pressure difference between cold and optimal temps was 1.5 psi (or there abouts). I assumed the internal volume to be 44 litres, (which is probably too low given that its a high walled slick), but at the temps and final pressures involved even doubing the volume had zero impact on the initial pressure calculated.

if u only look for the formular

P*V = n*R*T and set this to P*V/T = n*R by n and R are ever constant !!
by T (cold) ~320k and T(warm) ~ 350k + ther is a different at 10% + only by seeing

so P*V must rise up same % to get n*R = ever konstant numbers .

there is already programmed how round the tires are going by changing pressure , that formular we not know , the rest is only physiks

finaly p*v must rise up same T

Quote from afastest :

You're wrong again.

Cold to warm:
P(cold)*(T(warm)/T(cold)=P(warm).

Warm to cold:
P(warm)*(T(cold)/T(warm)=P(cold).

If look at the two formulas, you'll notice that difference P(warm)-P(cold) is more in the first formula than in second.

I'm not wrong. I used the word approximately for a reason. The difference in pressure between the initial and final temps is to all intents and purposes the same, (ie the difference is insignificant), when you set the pressure at the initial or final temp.

Plug in the figures:

If you assume a initial pressure of 25.38 psi cold temp, (I couldn't be bothered to set it to exactly 25 psi - it's not important), final pressure is 26.95 psi. Difference = 1.57 psi.

If you assume a final pressure of 25.38 optimum temp, initial pressure is 23.90 psi. Difference = 1.48 psi.

Not exactly the same, I never said they were. But they are both approximately 1.5psi. Unless you believe you can tell the difference between 0.09 psi in tyre pressures, in which case you are delusional.

Quote from zsmy :

if u only look for the formular

P*V = n*R*T and set this to P*V/T = n*R by n and R are ever constant !!
by T (cold) ~320k and T(warm) ~ 350k + ther is a different at 10% + only by seeing

so P*V must rise up same % to get n*R = ever konstant numbers .

there is already programmed how round the tires are going by changing pressure , that formular we not know , the rest is only physiks

finaly p*v must rise up same T

No PV will not rise by the same as T. The equation isn't an equality it's proportional. You can replace nR, (or even Nk for the other form of the equation), with X, as you correctly state in our scenario they are a constant. But that constant isn't necessarily, (in fact is highly unlikely to be), equal to 1. Therefore PV /= T. PV ~ T . There is a big difference.

Quote from gezmoor :

I'm not wrong. I used the word approximately for a reason. The difference in pressure between the initial and final temps is to all intents and purposes the same, (ie the difference is insignificant), when you set the pressure at the initial or final temp.

You wrote this: "pressure between the two remains the same"
The same is the same, your word approximately is in another place. In this particular case the difference is nearly same though, simply because the difference between hot and cold is quite small. It's 323 and 343 °K. Now, the difference between 1 - (323/343) and (343/323) - 1 is quite small in this case, but those two equations are not the same.

Quote :

No PV will not rise by the same as T. The equation isn't an equality it's proportional. You can replace nR, (or even Nk for the other form of the equation), with X, as you correctly state in our scenario they are a constant. But that constant isn't necessarily, (in fact is highly unlikely to be), equal to 1. Therefore PV /= T. PV ~ T . There is a big difference.

You seem not to understand what proportional means:
http://en.wikipedia.org/wiki/Proportionality_(mathematics)

Slightly OT: I was under the impression that if any ONE surface of the tire got over 200 degrees, the tire would pop. Is this not the case?

Quote from Stang70Fastback :

Slightly OT: I was under the impression that if any ONE surface of the tire got over 200 degrees, the tire would pop. Is this not the case?

No. It's either bugged, or intentionally like that.

If that's not the case, then what exactly are the situations for a failed tire? Just the CENTER getting too hot?