I know I've made various suggestions about this in the past, please review those if you'd like as well. :)
We just got done turning about 400 laps on a track and it was awesome how the track and line evolved over the course of the laps. A few things came up as we were driving.
The track naturally starts off smooth, and then the soft soil and soil layers erode quickly, after maybe 50 laps, and by the time the compact soil layer starts to show through in most areas of the racing line the track is just about too rough to drive on. (Relevant bits of the THT file below.) So at that point what was talked about is having the soil erode slower so smaller heights could be used when defining the layers. The drawback to this is that a cushion or lip will not form at the edge of the groove or compact soil layer. At this point we soldiered on driving the very rough line in order to get the track to smooth out a bit as more and more compact soil showed through. Eventually it got smooth enough to be decently "raceable" at which point we started to work our right rear tire onto the cushion and grippier soil/soft soil that was at the outer edge of the compact soil layer. Lap times were a little faster or equal to times that could be turned by running the compact soil line at this point. Maybe around 150-200 laps total usage on the track. So we kept running with the right rear tire out on the cushion for another 100 laps or so pushing the cushion higher and higher as we went, eventually pushing it high enough that the wall started to become a problem and the laptimes equaled back off as the was no more grippy soil left to drive on. This was probably around the 300-350 lap mark. Then we started to square off the corner, entering high and exiting low, doing our best to keep the tires in the all important soil/soft soil layers. The corner exits stated to get rough quickly and once again the quick way around became to just run the normal groove/racing line on the mostly smoothed off compact soil now.
So about the actual suggestion. Clay surface types that erode extremely slow, and an all important base clay layer that takes on rubber. In my head it works out to like base clay, clay, and clay1. That way we can start off with a track that looks fresh and then erodes to a lighter dryer layer and eventually to a dark or rubbered up layer that is slicker but eventually takes on rubber and gains a small amount of grip back. As far as erosion rates go we though somewhere around 1000 laps (total laps, 10 cars x 10laps = 100 laps) would be approximately the 150-200 lap description above. 2500 laps would be the 300-350 lap description above. So erosion for these clay surfaces would have to be somewhere around 1/10th of what the current rates are, if not more in the case of the soft soil layer which wears very quickly. As you can see below, we're working with about 12.5cm or 5" of erode before reaching the compact soil. As far as the height of the cushion goes this is mostly accurate, but we're getting there way too quick, and I am hypothesizing that that is the reason for the track getting far too rough in the early laps. Take a look below and see if you think I am using anything incorrectly, or if I can do anything differently to get a more favorable result.
I've also noticed that certain cars, or tires wear the track much more quickly than others. What all goes into calculation the erode rate and could a line be added to the tyre file to have some control over this? Just to make sure that a sprint car doesn't work the track in in 100 laps, but a midget takes 1000.
So for reference here are my textures and some other info....
Soft soil (freshly prepped track, most grip, 1.06, 5cm)
Soil (water blown off, medium grip, 0.95, 7.5cm)
Compact soil (grooved track, least grip, 0.85, this layer should take rubber)
samples_x = 1025
samples_z = 1025
data = heightmap1025.raw
size_x = 350
size_z = 350
scale = 4.43
num_material_layers = 4
material = compact soil
material = soil
; mask = mask_track.tga
thickness = 0.075
material = soft soil
mask = mask_track.tga
thickness = 0.05
material = grass
mask = mask_grass.tga
thickness = 0.025