Just thought I would poke in here on the discussion of tires, weight and traction.

A lot of the literature that is being quoted is from books discussing road going cars (full size even). Now, the general principal that tires are 'load-sensitive' is true, but I think RC cars definitely behave in a more odd manner.

However, the analogy I like to use when explaining how weight can affect lateral grip is using a mass on the end of a thin string. The string being the tire and the mass being spun around is like the mass of the chassis. Now, for a given tire, it can only do so much work (even as load continues to increase) and this is the limit for when the tire begins to slide and induces understeer or oversteer (depending on many other parameters). Likewise, the string can only do so much work before it breaks (similar to breaking traction) as the mass is being spun around faster and faster. The lateral g's that the mass is pulling is analogous to a car going around a corner. So, if the string breaks at a particular rpm for the mass then if you decreased the mass you would be able to then spin the mass faster before the string breaks. Implying in the same manner that a lighter car is able to corner faster because it requires more lateral acceleration for a lesser mass to break traction.

No, this a good theoretical example that is better at describing touring cars (both full scale and R/C), but since off-road surfaces are typically more inconsistent other variables.

Hope that helps give another perspective on how to think about tires!