## Friction coefficient and temperature/pressure

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Normally, we consider the tire friction with a dry road being 0.8. This is the general coefficient that can be "applied" to any car, any tire, any temperature/pressure condition. But we all know that if the tire is 80°C hot, it will stick better to the road, because the frictional coefficient changes.

So now I'm wondering if anybody has information about the modification of that coefficient depending on the tire/ground temperature or the tire pressure ? Does that kind of information exist ?

Olen Sveitsista mutta mun sydän on Suomea :)
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Joined: 8 Jan 2011

80C temperature will not necessarily make a tire "grip" better. For some tires, that level of heat helps (and perhaps only for a brief time). For others, it hurts.

Suffice to say that the mechanical properties of rubber compounds found in tires can be highly dependent upon temperature, temperature history, strain, strain rate, and strain history.

Yes, there is some public domain information on how some of these relationships work - but finding the hard numbers for such stuff is likely impossible unless you do your own testing or have an NDA with a supplier. An example starting point may be looking at the Williams Landel Ferry model, with some information found here and here.
Grip is a four letter word.

2 is the new #1.
Jersey Tom

Joined: 29 May 2006
Location: Huntersville, NC

And here I thought with how much everyone loves to talk about tire temp as the end-all-be-all of performance, this would be a more popular topic!
Grip is a four letter word.

2 is the new #1.
Jersey Tom

Joined: 29 May 2006
Location: Huntersville, NC

JT has already said most of the important things, when it comes to tyres IMHO, the main thing to remember IMHO, is that you cannot (or should not) see things in isolation (like CoF vs. temperature), because it's just a two dimensional snapshot of a multi-dimensional "problem'/problem.

As JT said, it is unlikely, that you will come across any specific numbers for your given tyre, unless you have a close collaboration with the manufacturer, or can do your own tests.
You will find some free information's about "CoF vs. temperature" for different "rubber" compounds in the internet,
such as this one:

but as you see, this is not only influenced by temperature, you see that the rubber compound (here described as different values of TG (Glass Transition Temperature), the vertical load and the sliding speed/velocity all come into it.

But as most people on here, will like some "shiny graphics", here is one from Michelin, showing some values, but again as you see, it's an surface response plot, including in this case sliding velocity (which means it's for a given tyre, and other parameters such as vertical load/contact pressure etc etc. are assumed to be constant)
Therefore, it's not really helpful for you, as you cannot/should not extrapolate this to your own tyre(s), it's just a "nice illustration"

try google for "The pneumatic tire" +ebook+pdf etc, it should bring up a free E-book from the nhtsa.
Inside you will find some useful info (IMHO) concerning some of your questions.
I'm afraid there is no "simple" answer to them, but I'm sure that the book will enhance your understanding
of the topic.
gato azul

Joined: 2 Feb 2012

the theory of tyre temperature correlation to performance is countered by the sight of Formula 1 cars leaving the pits with perfectly preheated tyres but still struggling for grip for at least half a lap....

I wonder if it were not better to have tyre "chewing" machines (run on a Drum and apply vertical load and possibly a bit of slip angle) in the pits to briong them up to their operating window and come out of the pits with the tyre already switched on....is this allowable at all?
marcush.

Joined: 9 Mar 2004

marcush. wrote:the theory of tyre temperature correlation to performance is countered by the sight of Formula 1 cars leaving the pits with perfectly preheated tyres but still struggling for grip for at least half a lap....

I wonder if it were not better to have tyre "chewing" machines (run on a Drum and apply vertical load and possibly a bit of slip angle) in the pits to briong them up to their operating window and come out of the pits with the tyre already switched on....is this allowable at all?

Assuming that does it. Or if you really think they just have to be worn in a bit, scuff them in practice.
Grip is a four letter word.

2 is the new #1.
Jersey Tom

Joined: 29 May 2006
Location: Huntersville, NC

Jersey Tom wrote:
marcush. wrote:the theory of tyre temperature correlation to performance is countered by the sight of Formula 1 cars leaving the pits with perfectly preheated tyres but still struggling for grip for at least half a lap....

I wonder if it were not better to have tyre "chewing" machines (run on a Drum and apply vertical load and possibly a bit of slip angle) in the pits to briong them up to their operating window and come out of the pits with the tyre already switched on....is this allowable at all?

Assuming that does it. Or if you really think they just have to be worn in a bit, scuff them in practice.

we know what it does when scuffing them ,but you still need them to get to work don´t we ? So as you can´t have a second car bringing your tyres up to working conditions ...what else? have a chemical agent dipping ?
sure the myth of temperature is coming from the sensation one does experience when going out on slicks for the first time and having that sensation of immense grip when the tyre is getting into its operatng window....and afterwards you can sense the tyre is hot-so the heat is the reason for performance or something along those lines...another area where effect and cause can easily get mixed up..the temperature is not the reason for the grip it´s the other way round in a ways...
And only if you are able to get rid of the excessive heat (as work is done) you will achieve an equilibrium and the tyre will last .
marcush.

Joined: 9 Mar 2004

Tires are multi-dimensional regarding performance parameters. I certainly don’t understand all –or many- of them. But that the tire continues to vulcanize with heat introduces another factor to consider as performance varies with heat. With many race tires, and certainly with Kart tires, the tire wears an ogee curve in the contact surface. This suggests that with use the tire assume a profile that, when working hard, provides a full contact surface as it is deformed under turning strain.

Heat is probable both a cause and an effect.
olefud

Joined: 12 Mar 2011

marcush. wrote:So as you can´t have a second car bringing your tyres up to working conditions ...what else?

Get proper tires to begin with.
Grip is a four letter word.

2 is the new #1.
Jersey Tom

Joined: 29 May 2006
Location: Huntersville, NC

marcush. wrote:the theory of tyre temperature correlation to performance is countered by the sight of Formula 1 cars leaving the pits with perfectly preheated tyres but still struggling for grip for at least half a lap....

I don't think they are "perfectly" preheated. Even if they were the right temperature when they took them out of the blankets, they're going to cool down quite a bit while the pitcrew waits, puts them on, and then the car trundles down the pitlane at 80kph or whatever. It takes some work to get them back up.

So why not make the blankets even hotter? I'm not 100% sure, but there might be a rule for safety reasons (accidentally touching a tire at 100 degrees C would be ... well, like touching boiling water). You also might not want to overcook the tires before their time, effectively putting them through an unnecessary heat cycle.
munks

Joined: 20 May 2011

It is my understanding that to really get a tire up to temperature, you need to heat the entire thing all the way through, and get heat into the actual carcass of the tire. Blankets only heat up the surface.
Lycoming

Joined: 25 Aug 2011

Lycoming wrote:It is my understanding that to really get a tire up to temperature, you need to heat the entire thing all the way through, and get heat into the actual carcass of the tire. Blankets only heat up the surface.

no they will heat all the way through they just need enough time in the blanket.
"The question isn't who is going to let me; it's who is going to stop me."
flynfrog

Joined: 23 Mar 2006

using micro waves to preheat tyres is a pretty quick and effective way.
but like most things working well, it was banned in F1 and other major racing series,
Audi used it in LeMans with great success, before it was banned.

Stuart Hepworth’s reputation as a tyre industry innovator is impressive. He developed and patented microwave and infra-red based alternatives to the electric blankets motorsport pit crews use to keep tyres warm. F1 teams Renault and McLaren tested and approved them, but the FIA – in trying to keep team costs down – banned the system. Stuart has also worked with numerous Le Mans 24 Hour Race teams, including recent winners Audi and Peugeot, on tyre heating technology.

gato azul

Joined: 2 Feb 2012

Interesting information there, thanks a lot ! I'm not advanced on the calculation point of view, but at least I can explain why !

My question was a bit naive, since you cannot know the EXACT rubber mixture used by the manufacturer. If all of them would provide the composition that if a company (like TüV for example) would test all of them using the same method, I am sure we could get a formula defining the friction coefficient depending on the temperature.

What would be interesting is to evaluate which components in the mixture has the most influence on the grip parameter. Durability would also have to be considered, but I can imagine the curve is not exponential so in the end it is always a compromise.

A shame this discussion is going to a dead end but that micro-waves method is interesting nevertheless ! Who knows what's next !
Olen Sveitsista mutta mun sydän on Suomea :)
>> Follow my "Subaru Impreza Pikes Peak" project! <<

Joined: 8 Jan 2011

MadMatt wrote:My question was a bit naive, since you cannot know the EXACT rubber mixture used by the manufacturer. If all of them would provide the composition that if a company (like TüV for example) would test all of them using the same method, I am sure we could get a formula defining the friction coefficient depending on the temperature.

Unfortunately it is a bit more involved than that, even if the manufacturers did give away the booksheets for all their compounds (which they will never do).

Characterizing compound traction like that is pretty challenging work. Better left for companies with R&D budgets in the hundreds of millions!
Grip is a four letter word.

2 is the new #1.
Jersey Tom

Joined: 29 May 2006
Location: Huntersville, NC

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