For the second experiment, try weighing down your tyre with some ballast. I used a couple of dumbbells slung through the centre of the wheel with rope to give me a total weight of 90 pounds. Now, I had to push with 150 pounds of force to move the tyre sideways on concrete. Still about 1.70G. We observe the fundamental law of adhesion: the force required to slide a tyre is proportional to the weight supported by the tyre. When your tyre is on the car, weighed down with the car, you cannot push it sideways simply because you can't push hard enough.

The force required to slide a tyre is called the adhesive limit of the tyre, or sometimes the stiction, which is a slang combination of "stick" and "friction." This law, in mathematical form, is where F is the force with which the tyre resists sliding; µ is the coefficient of static friction or coefficient of adhesion; and W is the weight or vertical load on the tyre contact patch. Both F and W have the units of force (remember that weight is the force of gravity), so µ is just a number, a proportionality constant. This equation states that the sideways force a tyre can withstand before sliding is less than or equal to µ times W. Thus, µW is the maximum sideways force the tyre can withstand and is equal to the stiction. We often like to speak of the sideways acceleration the car can achieve, and we can convert the stiction force into acceleration in gees by dividing by W, the weight of the car µ can thus be measured in gees.

The coefficient of static friction is not exactly a constant. Under driving conditions, many effects come into play that reduce the stiction of a good autocross tyre to somewhere around 1.10G. These effects are deflection of the tyre, suspension movement, temperature, inflation pressure, and so on. But the proportionality law still holds reasonably true under these conditions. Now you can see that if you are cornering, braking, or accelerating at the limit, which means at the adhesive limit of the tyres, any weight transfer will cause the tyres unloaded by the weight transfer to pass from sticking into sliding.

Actually, the transition from sticking 'mode' to sliding mode should not be very abrupt in a well-designed tyre. When one speaks of a "forgiving" tyre, one means a tyre that breaks away slowly as it gets more and more force or less and less weight, giving the driver time to correct. Old, hard tyres are, generally speaking, less forgiving than new, soft tyres. Low-profile tyres are less forgiving than high-profile tyres. Slicks are less forgiving than DOT tyres. But these are very broad generalities and tyres must be judged individually, usually by getting some word-of-mouth recommendations or just by trying them out in an autocross. Some tyres are so unforgiving that they break away virtually without warning, leading to driver dramatics usually resulting in a spin. Forgiving tyres are much easier to control and much more fun to drive with.