Friction circle
The friction circle shows a tire's total grip limit, shared between braking, accelerating and cornering, as a circle on a g-g plot.
Also called: traction circle, g-g diagram, friction ellipse
A tire has a finite amount of grip. You can spend it on braking, on cornering, on accelerating, or on a mix. Plot your car's acceleration with braking and acceleration up and down and cornering left and right, and the limit forms a rough circle, really more of an ellipse. Any point inside the edge is unused grip.
The key insight is that the grip is shared. If you are braking at 100 percent, there is little left for turning. That is why a car turns poorly if you stomp the brakes mid-corner, and why trail braking works: as you ease off the brake, you hand the grip over to cornering, tracing the edge of the circle rather than cutting across the middle.
Novice data usually looks like a plus sign: braking, then turning, then accelerating, with gaps in between. Faster drivers fill in the diagonals with smooth combined inputs. Exceed the circle at the front and you get understeer; exceed it at the rear and you get oversteer.
Using the full circle, especially in the transitions, is where much of the lap time between novice and expert drivers is found.
Each tire can only make so much force. Brake hard and there's little left for turning. Blend one into the other (trail braking) and you stay near the edge of the circle the whole time.