RC racing cars that use rubber tires pretty much always use some sort of foam inserts inside the tire to help the tire keep its shape. Because RC tires are not airtight or inflatable, the tire needs some support to keep its shape. Without the inserts, tires tend to "fold over" in high speed turns, and indeed even in straights they would compress excessively, so the inserts help them keep their shape and keep contact with the track.
There are two major types of foam inserts used in RC cars, closed-cell and open-cell, and they have different affects on your car's handling.
- Maintain tire shape better than open-cell, especially for soft-compound tires
- More durable than open-cell
- Expand and contract less than open-cell
- Prevent water from soaking in
- Often used on smooth, high-grip tracks
- Allow more tire flex which can improve traction (in moderation)
- Less durable than closed-cell
- Can absorb water
- Often used on bumpy, low-grip tracks
Also, see some of the articles below for more tricks and tips on tuning with tire inserts.