Looking for Lower Grip Tires

Shoreline Model Raceways has been working on new cars, actually buses, for our weekly IROC races. On some nights we run cars supplied by the track owner, which can be more unusual, like Fly trucks or Artin NASCAR stockers towing trailers. In other cases specially prepared club cars are used, until now those have been either Scalextric TransAms or Racer DPs. Looking for something different (!) we decided on buses. The buses have been built, but the problem has been finding the right tires for them. The idea was to have enough grip so that there won't actually be any wheel spin, but the buses should have enough slide so they are more challenging to drive. If the buses have too much grip the tend to flip over rather suddenly. Our tracks are all wood, so the buses cannot be tuned using magnets. We normally run at 10 volts, but that can be changed if necessary. All of our cars have silicone tires, but we will use whatever works on the buses. Silicone tires of any sort have too much grip, the same goes for urethane and most rubber tires. We have one pair of rubber tires that work great, those might be old Slot.its, I will have to check on that. Older rubber tires do tend to harden up and loose grip.
If you have any ideas about what we might use please post something. The guy in charge of building the buses recalled that someone had started a thread about simulated ice/snow racing, but I was not able to find that.

If you have any ideas about what we might use please post something. The guy in charge of building the buses recalled that someone had started a thread about simulated ice/snow racing, but I was not able to find that.
