Hello,


I found an article by someone who should know his stuff, it's something I've read by 'real' racers too: Increased tyre pressure gives increased tire temperature, the opposite of what happens in LFS. Please read the full article below before making up your mind.

Quote :

"My name is Alan Meaker, I am the Motorsport Manager for Toyo Tyres (UK) Ltd.

I was at Rockingham on the Saturday 1ST March at the LOT trackday and worked with a couple of cars using our R888 tyre regarding pressures and tread temperatures.

What I would like to do is to offer some advice on tyre temperatures and pressures and how to achieve the best from our tyres when taking part in either trackdays or racing.

The R888 has a semi race construction (very stiff) and a race tread compound. The optimum tread temperature range is between 85C and 95C measured using a probe type pyrometer, and ideally a maximum difference across the tread of 9C. The maximum hot pressure we recommend is 40psi. Camber angles up to 5 degrees are permissible but the final setting will depend on tread temperatures. It is advisable to have as much positive castor as practical as castor induces a beneficial camber change during cornering. I recommend that the tyres be put through 2 heat cycles before hard use.


The pressures you use will initially depend on the weight of the car, too little pressure on a heavy car can lead to over deflection of the tyre and subsequent failure.

Below are some basic settings:

VEHICLE WEIGHT COLD PRESSURE HOT PRESSURE
Very Light < 800kg 17 - 22 psi 22 - 29 psi
Light 800kg - 1000kg 20 - 26 psi 24 - 32 psi
Heavy 1000kg - 1400kg 23 - 27 psi 28 - 40 psi
Very Heavy > 1400kg 27 - 35 psi 37 - 40 psi


As a tyre gets hotter the pressure increases, this is due to the moisture in the air. The cold pressure you set to achieve a desired hot pressure will depend on the conditions on the day i.e. ambient and track temperature, wet or dry. If the day/track is cold you will need to start with a higher cold pressure as the tyre will not get as hot therefore the pressure increase will not be so great.
Hot pressures must be balanced side to side. Once the tyres have cooled you will find that you will have a difference in pressure side to side, if you have been racing on a right hand track you will find the offside pressures will usually be higher than the nearside.


Changing hot inflation pressures by small amounts can be used to fine tune handling.

Reduce Oversteer Reduce rear pressures or increase front pressures
Increase Oversteer Increase rear pressures or reduce front pressures
Reduce Understeer Reduce front pressures or increase rear pressures
Increase Understeer Increase front pressures or reduce rear pressures


Achieving the required tread temperatures will depend again on the conditions on the day i.e. ambient and track temperature, wet or dry.

You often here competitors saying “My tyres started to go of towards the end of the race”, this is usually due to the tread getting to hot.

The tread temperatures are constantly changing through out a race, hotter when cornering and cooler when on the straights and cooling even more when you are slowing to come into the pits. Therefore the temps you record in the pits will be lower than those during the race. So if you record temperatures within the range given above the probability is the temps will be too high during the race.

Increasing your tyre pressures will cause your tread temperatures to increase, more pressure stiffens the tyre’s casing which results in the tread having to do more work resulting in the tread getting hotter. Lowering your pressures will cause them to decrease.


Inevitably changing one thing will affect other things, the whole set up of your car is a compromise between anything that is adjustable.

Some of you are also asking about different compounds. Originally we only had one compound ‘GG’ (medium hard), but we have now introduced some sizes in a ‘SG’ (soft/wet) compound. This was done primarily for sprint/hillclimb (around 60 second runs) where the distances covered are relatively short and you need the tyres to work/heat up very quickly. This is not a compound I would recommend for race/trackday dry use as the tyres will go off very quickly.

"

Based on my first hand experience from taking tyre temps after sprint runs it is the other way around. Heat is generated by the tyre tread and sidewalls moving about. The lower the pressure the more movement and the more heat is generated.

I guess if you go beyond a certain tyre pressure - i.e. so much that the tyre begins to 'crown' (only the middle in contact with the track surface) then that could lead to more heat as a result of more sliding caused by reduced grip because of the smaller contact patch of the over inflated tyre gives.

Air temperature inside the tyre, temperature of the rubber carcass and tread temperature are different things.

LFS F9 display shows the air temperature inside the tyre and the temperature of the rubber carcass. You can see the tread temperature by clicking on some button while the F9 screen is displayed (can't remember which button).

Now if you lower the air pressure inside the carcass it will heat up more. Raising the pressure will stiffen the carcass and it wont heat up so fast. Less deformation.
I'm no expert on tyre physics, but it does sound plausible that the tread has to work more with higher pressure inside the tyre and that in turn would cause more heating. Whether this would make a significant change in the whole carcass temperature I doubt.

Quess I have some testing to do... Now if I only could remember what button to press to get those tread temperatures..

Tyres behave differently at road speeds (even fast road speeds).

In racing - lower pressures = more grip
On road (and track days, unless you are REALLY pushing it) - lower pressures = less grip.

Same with temperatures.

Also, the construction of race tyres is quite different to a road or performance tyre, and this changes how it behaves too.

Press and hold Ctrl-Shift to switch the temp display, the surface temperature appears vs the average which is the default.

Quote from Gentlefoot :

Based on my first hand experience from taking tyre temps after sprint runs it is the other way around. Heat is generated by the tyre tread and sidewalls moving about. The lower the pressure the more movement and the more heat is generated.

Based on nothing, I'm going to toss out another hypothesis. Flame as you see fit:

*drumroll*

Low pressures --> large parts of the tires will flex (including sidewalls) --> the flex is spread over a larger portion of the tire --> little flex in the tyre tread --> lower temperatures on the contact surface

High pressures --> less flex on the sidewalls --> more flex in the tyre tread --> more local heating on the contact surface.

---

Plausible..?

Quote from Gnomie :

Low pressures --> large parts of the tyres will flex (including sidewalls) --> the flex is spread over a larger portion of the tyre --> tyre has enough freedom to deform and deflect --> less friction on the tyre tread --> lower temperatures on the contact surface

High pressures --> less flex on the sidewalls --> tyre is too stiff to move about --> more friction on the tyre tread --> more local heating on the contact surface.

My take on what could cause higher tread temperatures with higher pressures...

Quote from tristancliffe :

Tyres behave differently at road speeds (even fast road speeds).

In racing - lower pressures = more grip
On road (and track days, unless you are REALLY pushing it) - lower pressures = less grip.

Same with temperatures.

Also, the construction of race tyres is quite different to a road or performance tyre, and this changes how it behaves too.

The article is about toyo R888s, a road legal semi slick tyre. I'd call it a race tyre, as it is used in several racing classes, but that's open to discussion I guess. The third line of the article also states that the advise is also applicable for racing.

I find the logics LFS uses in its tyre temperature model, well.. logic, but I'm guessing it's not as black and white as LFS makes us to believe. I'm not interested in flaming LFS for this, I am however thinking about doing some real life club racing and I'ld like to seperate facts from BS. (And there's a lot of BS out there )

If it's road legal then it's probably more road than race.

The Toyo man's advice goes against all racing advice about pressure, temperature and grip correlating, so I'd be careful what you believe. But if you're racing in reality you can at least try things out and see what happens. You'll probably find a pressure and camber that you always run, both of which will be almost exactly what the tyre manufacturer suggests as a starting point. Without lots of sensors, track time and, ultimately, money there is little more one can do.

My guess is that you can't make any generalizations as tyre properties differ a lot depending on the use. Examples:

Road Tyre, hard compound, high walls, tread.
Road performance tyre, hard compound, low walls, tread.
Single Seater Racing Tyres, soft compound, high walls, slick.
F1 Tyres, soft compound, high walls, tread.
Touring Car tyres, soft compound, low walls, slick.

These differences in spec will definately influence where the heat goes. And even if all tyres were the same, I think both high and low pressures will cause overheating at the extreme ends of the scale.

A discussion on the same topic was brought up a while ago.

http://www.lfsforum.net/showthread.php?t=46541

I don't know if LFS tyres are "correct" or not but my feeling is that the difference between how slicks heat and cool and how treaded tyres behave in LFS is too similar.

I also agree with J.B. in that there is a very large range of tyre constructions out there in rl (rubber compounds, tread patterns and carcass constructions) so it stands to reason that there is a strong possibility that different tyres can behave in quite dramatically different ways to each other

So the question that comes to my mind is this: Why do things get hot when flexed? Do all materials behave the same when flexed? For example, does metal and rubber create the same amount of heat when flexed the same amount?

My suspicion about tire heating is that the flex of the sidewalls contributes very little to heat generation in the tire. I suspect the great majority of heat is generated by the friction on the tread.

All I know is that the hot pressure of the tire is an important setting to keep the tires from getting greasy by the end of a session. If I let the pressure build above a certain level, the tires will get greasy by the end of the session. If I let the pressure get too low, the car feels squishy and imprecise; it will have a tendency to not go where you point it.

So, with my very limited experience with just a couple of different types of performance street tires says that pressures above a certain threshold will cause overheating tire surfaces more quickly than tires at the right pressure. Low pressure tires don't seem to overheat, but then again, they are hard to drive aggressively because they just don't react like they should.

Quote from Hallen :

My suspicion about tire heating is that the flex of the sidewalls contributes very little to heat generation in the tire. I suspect the great majority of heat is generated by the friction on the tread.

I think in the same line but with a slightly different take, in that I suspect its the flex in the tread blocks that create the majority of heat. That goes some way to explaining why high pressure would generate more heat in a performance road tyre as well as there would be less tread contact area thus high forces acting on each tread block where with a lower pressure there would be a larger contact patch with the same forces spread over more tread blocks. But that is also why I think there should be a larger difference between the way slicks heat and cool when compared to treaded tyres, it would also suggest why slicks can generally handle softer rubber compounds

Quote from Hallen :

So the question that comes to my mind is this: Why do things get hot when flexed?

Friction between the molecules of the matter.

Quote :

Do all materials behave the same when flexed?

Yes, all heat up. But the amount differs.

Quote :

For example, does metal and rubber create the same amount of heat when flexed the same amount?

Depends on what you mean by "amount". If both are rods of similar dimensions and are bent to the same angle and back the same amount of times then the metal will heat up more. But if you put the same amount of energy into the bending process of both of them then it is a whole different story.

You can try this at home with a piece of copper rod and a small rubber hose.
But be carefull not to burn your fingers on the rod because it can get pretty hot if you bend it fast enought.

Quote from Glenn67 :

I think in the same line but with a slightly different take, in that I suspect its the flex in the tread blocks that create the majority of heat.

That is what I have read too; that hysteresis is the primary heating factor. Hence why a sticky slick tyre, which may offer double the grip of a lesser tyre (through surface friction) will not have double the rolling resistance (assuming all other factors equal, which isn't the case, but oh well).

Quote :

As a tyre gets hotter the pressure increases, this is due to the moisture in the air.

Wow, that sentence ends in a great piece of fail... someone needs to point him to the Ideal gas law.

Also, rubber does not behave like metals and other more "normal" solids in regards to heat and compression:

http://scifun.chem.wisc.edu/HOMEEXPTS/rubberband.html

and

http://en.wikipedia.org/wiki/Natural_rubber#Elasticity

Quote from Rotareneg :

Wow, that sentence ends in a great piece of fail... someone needs to point him to the Ideal gas law.

Actually it's not entirely incorrect. Whilst it's not the moisture per se that is causing the pressure increase the humidity of the air does have an affect on how much the pressure increases due to a temperature increase in the air.

It comes down to the value of R in the equation you link to.
There are two forms of Gas constant, Universal (Ru) and Specific (Rs), and they have the following relationship:

Rs = Ru / M

Where M is is the molecular weight of the gas in question.

If you look up the values of R for water vapour (Rw) and dry air (Ra) you get

Rw = 461.5
Ra = 287

Now obviously air isn't pure water vapour, but then neither is it ever dry (as per the definition ie containing no water vapour at all), so real air is always somewhere in between. The actual value will depend on the Relative Humidity (RH) of the air in question. So although in practice it's only a small variation the RH of air will affect the pressure of the air at a given temperature.

Quote :

Also, rubber does not behave like metals and other more "normal" solids in regards to heat and compression:

http://scifun.chem.wisc.edu/HOMEEXPTS/rubberband.html

and

http://en.wikipedia.org/wiki/Natural_rubber#Elasticity

Not quite sure of your point here. It's patently clear that the tread pattern on a tyre has a direct affect on the temperature generation within the tyre. This has been proven by emprical data taken by tyre manufacturers and they put a lot of research and effort in to the design of tread block size/pattern because of it. Whether this is due to the intrinsic properties of the rubber itself or some other mechanism is moot to the point the author in the quote is making. It's a well understood fact that the more the tread blocks move around they quicker they heat up, which is what the quoted was stating.

About the moisture.

If some of the water is in liquid form when the tyre is cold and as the tyre warms up it evaporates. Then the pressure is increased even more.

Even if the temperature wont reach the boiling point of water in said pressure the water liquid and vapour equilibrium state does change according to temperature.

Whether there is enough moisture in a tyre to achieve this I don't know.

Quote from Glenn67 :

I don't know if LFS tyres are "correct" or not but my feeling is that the difference between how slicks heat and cool and how treaded tyres behave in LFS is too similar.

The primary difference between road tyres and race slicks is the size of the temperature range in which they operate effectively. As soon as you are 10 degrees lower than optimum temp with a slick you lose massive amounts of grip whereas road tyres still offer good grip even when completely cold.

So currently in LFS the road tyres are probably influenced by temperature more than they should be then? There has been other discussions about road tyres loosing too much grip when they are hot compared to rl as well that and the window of usable pressures is too large - i.e. doesn't seem to be enough penaltiy to grip when under or overinflated

Quote from gezmoor :

Actually it's not entirely incorrect. Whilst it's not the moisture per se that is causing the pressure increase the humidity of the air does have an affect on how much the pressure increases due to a temperature increase in the air.

Humidity may be a factor in tires filled with ambient air, however tires are sometimes filled with pre-compressed nitrogen. The point of using nitrogen instead of ambient air is to do away with the trace gasses and moisture, but I'm not sure whether it is pure or not or if there is moisture in the compression process.