[autoavia]

The problem with urethanes is that some produce dust which, on a routed wood track, temporarily destroys the grip for rubber tyres.

 At SSRC we run vintage race evenings once or twice a month on a Friday. Most of the vintage racers run Ortmann tyres because they have one of the widest ranges of vintage tyre types available.  On the following Tuesday club night when we run modern hard body cars with rubber tyres (usually F22) we find that initially there is no grip and the tyres pick up a lot of dust from the track. At first the grip vanishes after a few laps but if everyone cleans their tyres frequently during practice it improves over the course of the evening. 

 Ortmann use some kind of filler powder in the tyre mix. The dry powder is clearly visible in any air pockets in the tyre which come to the surface during tyre truing and we suspect that it is the powder which gets laid on the track as the tyres wear during racing. 
 So on our wood track there is definitely a problem with mixing Ortmann and rubber tyres but it is comparatively easy to resolve.
 I have run some other urethanes on modern cars during testing as we have been looking at using urethanes as a control tyre for some classes and there doesn't seem to have been any problem with loss of grip for the next person using rubber tyres on the same lane.