[Spaceywilly]

I too have been reading wikipedia to find the answer I found this wikipedia article that explains what's going on

http://en.wikipedia.org/wiki/Tire_load_sensitivity

Tire load sensitivity describes the behaviour of tires under load. Conventional pneumatic tires do not behave as classical friction theory would suggest. The load sensitivity of most real tires in their typical operating range is such that the coefficient of friction decreases as the vertical load, Fz, increases.
Coulomb friction theory says that the maximum horizontal force developed should be proportional to the vertical load on the tire. In practice, the maximum horizontal force Fy that can be generated is proportional, roughly, to the vertical load Fz raised to the power of somewhere between 0.7 and 0.9, typically.
Production car tires typically develop this maximum lateral force, or cornering force, at a slip angle of 6-10 degrees, although this angle increases as the vertical load on the tire increases. [1] Formula 1 car tires may reach a peak sideforce at 3 degrees [2]

Now I have to go read some more and understand what they're talking about

more good stuff here
http://en.wikipedia.org/wiki/Cornering_force

and here
http://en.wikipedia.org/wiki/Weight_transfer

In the case of tires, although traction increases with load, it increases less than linearly, and coefficient of friction decreases with load. A larger contact area reduces the load per unit area, resulting in more grip. There is a point of diminishing returns, such as weight and aerodynamic drag. It's also true that a larger tire dissapates heat and with a lighter load involved, it wears less. Another reason for a larger contact area is to compensate for debris or track imperfections interfering with the tire and pavement contact.

Note that load sensitivity is commonly used to adjust the understeer or oversteer of car. When a car turns, the downforce on the outside tires is increased and the downforce on the inside tires is decreased. The body of a car also rolls a bit, and the supension can be used to unequally distribute the load between the front and rear tires. If the front end is relatively stiffer, then more of the downforce is exerted on the outside front tire, and the relative grip is reduced because of tire load sensitivy. A stiffer front end causes the front end to lose some grip in turns, resulting in understeer. A relatively stiffer rear ends results in oversteer. Street cars are generally setup with understeer, while race cars are setup with a small amount of oversteer.

http://www.physicsforums.com/showthread.php?t=330790