So basically you just need to mod the ODEF file. Basic ODEF file:
"Track.mesh
1.0, 1.0, 1.0
beginmesh mesh
Track.mesh
endmesh
end"
If you wanted to make it slipery do this:
"Track.mesh
1.0, 1.0, 1.0
beginmesh
mesh Track.mesh
stdfriction snow
endmesh
end"
Snow is like a light mud, Ice is like a really slippery track, Grass is pretty basic, and Sand is a little slick. I don't know any other type of traction option, but those work pretty well.