Threshold braking
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Threshold braking or limit braking is a driving technique most commonly used in motor racing, but also practiced in road vehicles to slow a vehicle at the optimum rate using the brakes.
The technique involves the driver controlling the brake pedal (or lever) pressure to maximize the braking force developed by the tires. The optimal amount of braking force is developed at the point when the wheel just begins to slip. Braking beyond this point causes the tire to slide and the friction adhesion between the tire and driving surface is reduced. The aim of threshold braking is to keep the amount of tire slip at the optimal amount, the value that produces the maximum frictional, and thus braking force.
When wheels are slipping significantly (kinetic friction), the amount of friction available for braking is typically substantially less than when the wheels are not slipping (static friction), thereby reducing the braking force. Peak friction occurs between the static and dynamic endpoints, and this is the point that threshold braking tries to maintain.
Because available friction at a given moment depends on many factors including road surface material, temperature, tire rubber compound and wear, threshold braking is nearly impossible to consistently achieve during normal driving. In vehicles not equipped with ABS the following technique can be used to approximate threshold braking:
- Press the brake pedal about half way in, as if you were braking normally.
- Smoothly increase brake pedal pressure until wheels just start to slip and threaten to lock up. On dry, hard pavement you will start to hear a low-level squealing sound. On loose or slippery surfaces you will start to hear a low-level swashing sound. On all but the highest grip surfaces the front wheels will lock first by design in order to avoid oversteer.
- Reduce brake pedal pressure by a small amount, enough to diminish the squealing or swashing sound and allow the almost-slipping wheels increase their traction again.
- If more braking is desired, increase brake pedal pressure until the wheels threatens to lock up again, then release pressure, and so on.
- All all times, avoid actual locking of the wheels. Note that on slippery pavement if you actually lock up the brakes you will have to lift your foot almost entirely to regain wheel rotation.
A human driver can perform one press-release cycle per second with practice. The ultimate goal is to achieve constant brake pressure and tire traction across the entire braking process.
However, if the surface is bumpy or uneven, a threshold braking traction of constant value might not be achievable, so varying brake pressure as the vehicle rolls over each obstacle becomes the default performance action.
In a situation where a car needs to turn or swerve to avoid a collision but is traveling too fast the above technique allows the driver to retain steering control of the car while decelerating (though when turning brake pressure must be necessarily less). Simply standing on the brake pedal would lock up the front wheels with no possibility of changing the car's direction. Formula One drivers actually try to distribute tire traction between the braking process and the steering process in one smooth varying continuous whenever they negotiate turns.
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