In racing, it is possible to exceed the grip available on all four tires and still keep your right foot planted. That occurs when a driver enters a four wheel drift
. A four wheel drift occurs when the car is traveling on a controlled path but has temporarily exceeded all the available grip
from his tires. Normally this occurs on a type of high speed corner known as a sweeper. Imagine a car negotiating an eighty degree bend at approximately 90MPH (140kph). Like at Turn 2 at Nelson Ledges Road Course
. The car is following its trajectory but is sliding ever so slightly to the outside of the corner. Inside the car the driver will feel the car get 'lighter' almost as if it had 'stepped up' on a piece of ice. But he or she need not lift
. What has happened is that the car has passed beyond its tires available slip angle
. But the car is pointed correctly and has barely exceeded the limits of grip. If the car threatens to understeer
off the edge of the track, a slight lift of the throttle transfers enough weight
forward to increase the contact patch on the front tires, returning steering. So a four wheel drift is controllable using the throttle with additional throttle making the car loose, but mostly to tighten the car when needed with a small lift.
What makes a four wheel drift is a property of two things, slip angle, which is the ability of the tire to pull in a direction different from which it is rolling, and inertia
which transfers weight to the outside of a car during a corner. The car wants to go straight when a steering input is applied. When the front tires turn, they deform and the traction path is not entirely straight but somewhat across the tire. The car is pulled along. if the driver has chosen his steering, braking and throttle inputs properly and has performed them smoothly
, the car will negotiate the curve with its nose pointed in the right direction as inertia continues to force it somewhat along its former path. It is moving forward but drifting slightly to the outside, along its former path. At that point grip has been exceeded but the car remains under control.
When you do it right, to enter a four wheel drift is a lot of fun and can be very, very fast. Ideally, the tires will regain grip just before you run out of asphalt
. I can tell you from personal experience that it's fun! The car must be balanced with all four tires just above their slip angle envelope
. In many types of corners a four-wheel drift is the best way to get around the course.