The standard darkline from rfactor in the corners is already programmed to give about 5% more grip than the normal asphalt.
But indeed you are right, the runoff area's should have their grip greatly reduced compared to the track itself.
You should rename the material you use at the runoff to runoff.
Than add this part below to your TDF file.
After the line materials at the end you can add more material names that should have the same property.
Here is a example that should work. Just past it in your TDF file.
// Runoff
[FEEDBACK]
Dry=RoadDustGrip Wet=RoadWetGrip Resistance=0.0 BumpAmp=0.1 BumpWavelen=0.0 Legal=true Spring=0.0 Damper=0.0 CollFrict=0.4 Sparks=1 Scraping=1 Sound=dry
Reaction=tiresmoke Tex=smokewhite.tga Max=1024 Scale=(1.0,1.0,1.0) Growth=(4.0,3.0,2.0) ASDEnvelope=(2.0,0.2,3.5) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Reaction=skid Tex=skidhard.tga Max=2500 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=wetskid Tex=skidwet.tga Max=1024 Duration=0.40 Pixel=NoReduceDetail Particle=Plane+Deformable+SingleSided DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
//Reaction=spray Tex=rainspray.tga Max=1024 Scale=(0.3,0.05,0.6) Growth=(0.3,0.3,0.6) GrowthVel=(0.08,0.09,0.30) Power=0.41 RampSpeed=80.0 OffsetVel=0.10 ASDEnvelope=(0.05,0.07,0.90) DestBlend=InvSrcAlpha SrcBlend=SrcAlpha
Materials=runoff,
This should really give the car the behaviour it's going over a dirty piece of tarmac.
Where all the dust, sand, debris and rubber marbles end up.
Good luck with it!
