Threshold braking

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Threshold braking or limit braking is a technique wherein the driver adjusts control of the brake system in an attempt to maximize the braking force of the vehicle. Done properly, this reduces the time and travel distance required to stop the vehicle to optimal amounts, or when racing, allows the driver to delay braking when entering a corner. The optimal amount of braking force is developed on a wheel at the point when the wheel just begins to slip (optimal slip will be between 5-15%). Braking beyond this point, as the tire begins to slide, the friction force between the tire and driving surface transitions from the static friction range into the kinetic friction range. Threshold braking avoids sliding and tries to keep the tire's percentage slip at the optimal value, the value that produces the maximum frictional force.

The friction force (i.e. braking force) developed between the tire and driving surface is proportional to the load on the tire times the coefficient of friction. The coefficient of friction is a complex combination of factors, the most important of which is the combination of materials involved, namely, the tire (usually rubber) and the surface (asphalt, dirt, ice). When the wheels are slipping significantly (kinetic friction) the coefficient of friction is typically substantially less than when the wheels are not slipping (static friction) thereby reducing the braking force. The peak coefficient of friction occurs between static and dynamic endpoints, at the optimal slip percentage. The coefficient of friction is also a function of temperature, humidity, and applied load.