[CMOTD]

The track is covered between club nights and it is only after we have run vintage with ortmanns that we get the grip problem. Maybe because vintage cars, particularly 1/24 scale tend to be much heavier than modern cars they wear the tyres faster and produce more dust. Most vintage cars also run much wider tyres than modern cars so more tyre area to produce dust. 
 I have tested modern cars with other urethanes during club nights and no one has complained of loss of grip when they followed me onto a lane which led us to conclude that Ortmanns on vintage cars are causing the problem.

Thinking about it, there have been at least three different formulations of Ortmann tyres over the years so it is possible that one of them could have caused some problems. The original version was the best but the EU banned certain components of it. The second version was not very good and produced blue dust when trued. I am unsure about the latest version. Any idea how old the ones you are using are?