# Possible bugs in TGM tyre model or TGM/ttool documentation

Discussion in 'Bug Reports' started by Knut Omdal Tveito, Sep 11, 2014.

1. ### Knut Omdal TveitoRegistered

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Mar 16, 2012
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WLF transform calculation
In the TGM Quickstart guide the following is presented:
log10(aT) = -8.86 ((Tq - Tg - 50) / (51.5 + Tq -Tg))
Sliding friction coefficient is given as a function of:
log10(aT*V) = log10(aT) + log10(V) ------[Eq.1]

However from the ttool and the telemetry I have found the following to be the implemented:
Sliding friction coefficient is given as a function of:
LOG10( EXP( log10(aT)[Eq.1] + LN(V) ) )=( log10(aT)[Eq.1] )/LN(10) + log10(V) -----[Eq.2]

It seems like either the TGM Quickstart guide does not include this detail of logarithm base or the tyre model itself has a bug. Would be great if ISI could clarify as it has a significant impact on peak temperatures etc...

In the TGM Quickstart guide these curves are presented as friction as a function of log10(aT*V). Based on curves like this in the literature I would assume the total is:
mu_total ="SlidingBaseCoefficient" * MAX( mu_adhesion , mu_micro , mu_macro )

But from the telemetry I have found the following:
mu_total ="SlidingBaseCoefficient" * MAX( 0.25*mu_adhesion , 0.5*mu_micro , 0.25*mu_macro )

These multiplication factors are not obvious to me, and when I enter the friction data in the TGM file I expected the exact numbers to be used and not scaled like this. Again, I'm not sure if it's a bug, but it would be nice if ISI could clarify the Quickstart guide so that people don't make the same mistake as me.

I hope You find the time to answer.

Knut

2. ### Terence GroeningRegistered

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I don't have time to look at the first one right now, but I know the second answer off the top of my head: those ratios comes from the "Roughness" profile for the terrain. Note that the SkipBarber tire files have a comment mentioning this. Ideally, one could use the "Roughness" profiles to describe the different types of aggregate used in asphalt, in addition to differentiating concrete surfaces.

3. ### Terence GroeningRegistered

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A quick idea: I think those Log10's are supposed to be natural logs in the documentation, but again I don't have time to go through all that right now.

4. ### Knut Omdal TveitoRegistered

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Thanks for the reply, much appreciated! I will look into the terrain.

For the logarithm, I have to point out that aT and V have a different logarithm base, and that sounds like a bug to me as they usually appear as a product in the logarithm expression. As a result the relative influence of temperature and sliding velocity is not in accordance with litterature. The way it is now the problem can be overcome by changing C1 iun WLF (-8.86).

5. ### DhuraraRegistered

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This thread is very instructive for a modder who is interested in the tyre model of rFactor 2

Unfortunately I don't have enough skill and knowledge to validate the model.
But the discussion between knowledgeable persons will offer us valuable insight into the model

6. ### RaintyreRegistered

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Jul 28, 2013
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I also would like to ask about WLF equation. From tyre documentation WLF equation would be:

log10(aT) = - 8.86 ((Tq - Tg - C1) / (C2 + Tq -Tg))

However, in most of sources i found WLF equation form is:

log10(aT) = - K * (T - Tg) / (C2 + T - Tg)

So, is documentation right? Can you confirm what is the exact equation to apply for 'WLFParameters' line?

Thanks !!