But the reason race cars/bikes use slicks is because on a dry, very smooth circuit they will provide maximum grip since more rubber is in contact with the track. Drag is not really an issue as the increase in mechanical grip far outweighs any aero advantage. On anything but smooth concrete a slick on a trials bike doesn't seem like too good an idea! Particularly on natural. However, having said that, I've seen how Stan and Ali were riding with the front Try-All with all the knobbles cut off and again with Vincent's one so there's obviously more to it than meets the eye...
Bike looks seriously sweet Stan! Some interesting mods going on there...